Biological Psychology, 2, 1975, 237-307.

SELECTIVE

ATTENTION

HUlMANS-A

CRITICAL

@ North-Holland Publishing Company

AND EVOKED

POTENTIALS

IN

REVIEW + t

Institute of Psychology, Unicwsity of Helsinki, Finland

Accepted for publication 23 September 1974

Human evoked-potential research on the neurophysiological substrate of selective attention is reviewed. Most of these studies report enhanced amplitudes of potentials evoked by attended (task-relevant, meaningful, important, etc.) stimuli the results of which are generally regarded as providing an electrophysiological correlate for selective attention. In accepting such claims, there appears to be two major procedural problems generally not satisfactorily solved in these studies: (1) the inability to reliably separate the specific and non-specific physiological changes ~oncon~itant with selective attention from each other: and (2) inadequacy of peripheral sensory control possibly inducing contaminating changes already at the level of the proximal stimulus. Problem (1) originates from, and the importance of (2) is emphasized by, the temporal stimulus structure of experimental tasks in these studies which allows the sub,ject to predict above the chance level the relevant events and, thus, to differentially prepare himself for these in advance (increased non-specific arousal and selective peripheral sensory orientation, the latter often made possible by insuficient control, have possibly been among these changes). Those studies to which these two (and other) remarks do not apply at all or only to an insignificant degree have generally shown no selective evoked-potential changes (or these changes have occurred only with a long latency (‘P3’ or *P300’)making their interpretation especially uncertain). There is one exception for this general notion, the reasons for and significance of which are dealt with in detail. Finally, the difficulties and inherent limitations of inferring brain events from scalp-recorded evokedpotential data, especially with respect to the important selective-filter hypothesis of selective attention, are extensively discussed and, in the light of these difficulties, some trends for future research proposed.

1. Introduction Selective attention is a psychological and behavioural phenomenon the existence of which is generally admitted on the grounds of everyone’s personal experiences and is also verified by numerous experiments, but the physiological *Address for correspondence: R.N8Bt&1en, Instituteof Psychology, University of Helsinki, Ritarikatu 5A, 00170 Helsinki 17, Finland. TAlthough the present review is entilled to deal with (certain psychological correlates of) se/ectiz>e attention, no work is excluded for using some other term to describe the (related) psychological and behavioural phenomena such as importance, task-relevance, interest value, meaningfulness or significance of stimuli, which expressions, by referring to the object of (selective) attention, impty basically the same process. It appears that this variety in terminology has led to some confusion and artificial classifications and borderlines. 237

mechanisms of which are still largely unknown (Horn, 1965). Simple neurophysiological explanations according to which selective attention is based on the analogous selective blocking of afferent sensory impulses of unattended sensory modalities, even stimuli, have been given by many authors (e.g. Adrian, 1954; Hernandez-Peon, 1961; Hernandez-Peon, Scherrer and Jouvet, 1956; Lindsley, 1960, 1961). These investigators have mainly proposed the existence of some kind of efferent control function for the brain by means of which it selectively regulates its own input via the afferent sensory pathways. The related experimental work started with anima!s (for reviews, see Horn, 1965; NaHtanen, 196’7; Thompson and Bettinger, 1970; Verberne, 1968). The pioneering work was conducted by Hern~ndez-Penn and his colleagues in the 1950s who, however, did not demonstrate that the reduction of the amplitude of the evoked potential (EP) to the irrelevant stimulus really was a selective phenomenon and not only a non-specific result of, for example, reticular activation concomitant with increased attention (Jane, Smirnov and Jasper, 1962; NHHtanen, 1967). There are at least two main reasons for the general tendency from animal towards human subjects in the beginning of 1960s (however, see, for example, Thompson and Bettinger, 1970; Hyvlrinen, Poranen and Jokinen, 1974; and Khachaturian and Gluck, 1969, for recent animal work with high promise). (1) The development of averaging techniques and devices which became convenient and not too expensive laboratory tools allowing the detection of some stimulus-specific brain events even from scalp recordings. (The signal/ noise ratio of EPs recorded from the scalp is very small compared to that of EPs recorded directly frotn the brain by implanting techniques.) (2) The great difficulties encountered in using animal subjects. Such difficulties were, for example, the assessment of the relevance of different stimuli, the impossibility of measuring EPs to certain biologically relevant stimuli for their uncontrollable stimulus characteristics, the impossibility of using instructions or other means to flexibly change the degree of relevance of physically same stimuli, and the control of movements, especially the eye movements (Naatanen, 1967, pp. 41-43). 2. Experiments with evoked potentials to stimuli of different relevance in separate runs 2.1. EPs to simpIe and meaningfess stimuli Pioneering human experiments on the issue were conducted, among others, by Hernandez-Peon and Donoso, recording from the depths of the occipital lobe (Hernandez-Peon, 1966), by Jouvet and Courjon (1958), recording from the optic radiations, by Larsson (1960) and by Garcia-Austt with his colleagues (Garcia-Austt, Bogacz and Vanzulli, 1964; Bogacz, Vanzufli and GarciaAustt, 1962). Generally, an enhancement of the size of the EP was reported which was interpreted as reflecting selective attention.

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The experimental paradigm employed by these investigators naturally resembled that used in the related animal experiments-these human works signify the step from the animal to the human laboratories in the search of brain-event correlates of selective attention; generally, the subject was exposed to a series of monotonous stimuli and it was tried to manipulate attention by distraction and different instructions (counting, etc.). In the following, only the study by Garcia-Austt et al. (1964) will be examined in detail, but the description and the points raised are also generally relevant to the other related contemporary studies. The subject was exposed to a series offlashes and the EPs were recorded from the leads 0, -F, and 0, - Fz (‘lo-20 system’, Jasper, 1958). Distraction, or ‘interference’ as they want to call it, by other stimuli of a different (pure tones or clicks) or of the same sensory modality (low-intensity peripheral flashes with a different frequency or random presentation, with the instruction not to stop gazing at the test flashes) generally elicited in the non-habituated subject an overall decrease in the size of the EPs to the fovea1 flashes. Tasks involving mental calculation had a similar effect. However, if the subject was already habituated to the flash stimuli, the effect of the distraction by other stimuli on the EP was reversed. The amplitude enhancement was also produced by ‘the forced focusing of attention towards the stimulus’, induced by, e.g. the instruction to count the stimuli. This effect was stronger when the flash-EP was habituated. These experimental results were claimed to indicate a correlate for selective attention in the amplitude of the EP. For these and similar results there are, however, alternative explanations : (I) Peripheral

receptor conditions

It is possible that the subject fixated better to the flashes when the instruction directed attention to them (Oswald, 1959). This remark seems to be especially important because no fixation cross was used and during some parts of the experiment the subject’s eyes were closed. Rietveld, Tordoir and Hagenouw (1966) have shown that the ‘eyeball artifact’ diminishes in size when attention is focused on the visual stimuli in an experimental situation in which clicks and flashes are delivered in an alternating order at 1 set interstimulus intervals (ISIS). Also the diameter of the pupil may have been larger when the flashes were attended to. It was shown by Hess and Polt (1960) that the ‘interest value’ of the stimuli increases the diameter of the pupil both in cats and in humans. (The ‘interest value’ of the flashes probably is increased by directing attention to them by means of the instruction to count them or to perform a discrimination task on them, and attenuated when a mental calculation task or distraction by other stimuli is introduced.) A positive relationship between the amount of light falling on the retina and the EP amplitude has been established by many

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investigations (see, e.g. Horn, 1965; see, however, Schechter and Buchsbaum, 1973). Chapman and Bragdon (1964) demonstrated with humans a strong relationship between the luminance of a diffuse visual stimulus, such as that used by Garcia-Austt et al. (1964), and the amplitude of the EP (2.5 cm below C, and 0,; bipolar recording) within a rather extensive intensity range. If Garcia-Austt et al. had used the artificial pupil this uncontrolled peripheral factor would have been eliminated. Generally, it would appear that the peripheral control of vision in experiments on selective attention is especially difticult (see Weerts and Lang, 1973) and that other modalities should preferably be used in such experiments. (2) Stimulus conditions Flash-EPs were compared between conditions in which the flashes were presented either alone or among distraction stimuli of the same or a different modality. When, for example, interspersed distraction flashes were delivered to the periphery of the retina, the total frequency of the flashes given was higher with the consequence that the amplitude of the EPs to the foveally presented flashes was likely to be attenuated for many reasons (e.g. for inhibitory interactions or for the refractory period (see Horn, 1965, p. 174) or for certain peripheral changes, e.g. decreased accommodation and convergence to the fovea1 flashes). The results of Horn and Blundell (l959), Horn (1960) and Bihari and Bental (cited by Horn, 1965), according to which visual searching behaviour reduces the amplitude of flash-EPs in cats, are also of relevance in this connection. When, again, distraction was introduced by presenting auditory stimuli, the attenuation of the amplitude of the flash-EPs may result from cross-modal (purely physiological) influences (Davis, Osterhammel, Wier and Gjerdingen, 1972; Allison, 1962; Fruhstorfer, 1971): the total frequency of the stimuli was higher during the auditory distraction (see also Horn, 1965). It is also possible that the auditory distraction induced changes in the peripheral conditions of the visual system (e.g. decreased accommodation and convergence to the flashes and a decrease in the diameter of the pupil) which may have caused the attenuation of the amplitude of the flash-EPs. (3) The diflhentiul preparation Karlin (1970) and Naatanen (I 967, 1969a, 1970a) have extensively discussed the confounding role of non-specific changes taking place in the organism during selective attention, which changes are often erroneously interpreted as indicative of the selectivity of the attentive state. Several experiments have shown that EP amplitudes are usually enhanced with increasing arousal or vigilance in man (e.g. Fruhstorfer, Hakkinen and Bergstrom, 1967; Haider, Spong and Lindsley, 1964; Isgur and Trehub, 1971; Sakai, Gindy and Dustman 1966; Eason, Aiken, White and Lichtenstein, 1964). In the

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last-mentioned study, a clear enhancement of the EPs to probe flashes was observed to accompany different kinds of mental or physical effort compared to potentials evoked by the same flashes during relaxation. This was an excellent demonstration of the non-specific enhancement under arousing conditions; it is natural to assume that during relaxation the flashes were more attended by the subject than during an intensive concentration on, e.g. adding by ‘13s’ or thinking about a visual scene or a melody. The authors write that ‘whether or not shifts in attention independently of level of activation also affect the evoked potential patterns in any systematic manner still remains to be demonstrated’ (Eason et al., 1964, p. 893). The paradigm of Garcia-Austt et al. is not particularly suitable for studying the possible EP correlates of selective attention for the inherent impossibility of separating the specific and non-specific effects of attention from each other. For example, when the subject was instructed to count the flashes and an enhancement in the size of the EP was revealed, this may be due to selective attention to the flashes, to increased general attentiveness or to both. A valid test for these alternative interpretations would be provided by the simultaneous recording of the EPs elicited by the irrelevant stimuli (see section 3): if they are not enhanced to the same degree as the EPs to the relevant stimuli, only then the enhancement of the size of the EPs to the relevant stimuli can be considered a correlate of selective attention. rn such an experiment, the state of the organism at the moment of presentation of the irrelevant stimuli must not on average differ from that at the moment of presentation of the relevant stimuli-a methodological problem also in the majority of more recent works as we shall see. The possible role of non-specific factors in inducing EP differences between attention and non-attention conditions was also emphasized by the results of Rietveld et al. (1966) who observed a remarkably stronger alpha activity under the non-attention condition compared to that under the attention condition. Moreover, Velasco, Velasco, Machado and Olvera (1973) demonstrated clear background EEG, ECG (electrocardiogram) and GSP (galvanic skin potential) differences between their ‘novelty’, ‘attention’, ‘habituation’ and ‘distraction’ conditions involving electrical stimulation of the left median nerve at the wrist. Under the first condition, the subject had no knowledge about the experiment and was instructed to remain quiet with his eyes closed. Under the habituation condition several days later, the subject was encouraged to co-operate and remain quiet with his eyes closed and to ignore the shock stimuli. Immediately after ‘habituation’, he was asked to respond to each shock by pressing a lever with his right hand as fast as possible (‘attention’). Under the distraction condition, he was asked to ignore the shocks and to detect and report the pain threshold to pinching of his fingers or toes. Later EP components attained maximal amplitude during ‘novelty’, were large during ‘attention’ and much smaller during ‘habituation’ and ‘distrac-

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tion’. No apparent variations were observed in the amplitudes of the early components. The EEG frequency was approximately 8 cps during both ‘novelty’ and ‘attention’ conditions and decreased during ‘habituation’ and greatly increased during ‘distraction’. Parallel changes were observed in both ECG frequency and GSP fluctuations through all these conditions. There is no way to decide whether the EP changes observed were caused by nonspecific differences in the state of the organism between conditions (of which differences this study yielded valuable detailed information) or by different degrees of shock-selective attention manipulated by means of the four conditions. Therefore, it is not possible to agree with the authors when, referring to the reduced EPs during ‘distraction’, they suggest that these ‘may be explained by a gating mechanism operating at lower relay stations blocking the input of the non-attended modality (Donchin and Lindsley, 1966; HernandezPeon et al., 1956; Galambos and Sheatz, 1962; Garcia-Austt et al., 1964; Spong et al. 1965; Donchin and Cohen, 1967)’ (p. 108). Furthermore, this EP attenuation demonstrating dissociation between ‘general alertness’ and the size of late EP components can be due to increased frequency of somatosensory stimuli under the distraction condition (see Horn, 1965, p. 174; Allison, 1962) : pain stimuli were intermittently presented, initiating from 1 to 2 set before the shock and lasting from 2 to 3 set thereafter. The result of the ‘attention’ condition that larger EPs were associated with faster motor responses was one of the two facts observed that made the authors think that variations in late somatic EP components are ‘due to changes in the level of selective attention, independently to the state of general alertness’ (p. I OS). This experimental result, well established earlier and now corroborated by Velasco et al., has not, however, generally been regarded as strong evidence for EP correlates of selective attention because of the many alternative interpretations for such a result. Faster RTs and larger EPs can both, for example, indicate increased cortical non-specific excitability (see e.g. Magoun, 1964, p. 91). (4) Background EEG dlrerences The background EEG differences might also have other influences on EPs than those related to the previously described ‘activation’ and preparation effects. For example, many workers have cautioned that regularly occurring waves in the EEG can be included as late components of afterdischarge in the EP and Barlow has demonstrated that sometimes the variance of an EP can contain the variance of the background EEG; most writers emphasize this danger for alpha activity (Tecce, 1970). Moreover, Ciganek reported a higher auditory EP with an alpha than with a beta background, Rodin and Luby observed a decrease in visual EP with progressive EEG synchronization, Kooi and Bagchi found some components of the visual EP higher in subjects having higher alpha amplitude, Dustman and Beck reported that subjects with faster

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alpha frequency had faster afterdischarge frequencies in the visual EP, and Gaarder with his associates concluded that alpha is part but not all of the source of alpha-like components of the visual EP (Tecce, 1970). On the basis of the studies reviewed by him, Tecce (1970) suggests that the EP is enhanced when the background EEG is high voltage and synchronous. In this connection, however, he mentions some studies, e.g. Chapman and Bragdon (1964), in which the visual EP ‘is not closely related to background EEG’ (p. 353). The picture thus far appears somewhat confusing: in (3) above, preparation was explained to enhance EPs and, on the other hand, it is well known that preparation is associated with EEG desynchronization, whereas in the foregoing, evidence supporting the attenuating effect of desynchronization on EPs is referred to. The explanation for this controversy might be found in the agent which induces the desynchronization (see also Naatanen, 1973): if desynchronization is caused by preparation, EPs might be enhanced, whereas some other agents, or ‘spontaneous’ desynchronization, might well be associated with reduced EP amplitudes, There seems to be nothing inherently impossible in assuming that certain aspects of the background EEG-EP relationship are critically dependent on other factors (i.e. spurious). Later studies with paradigms in which the same stimulus was delivered under conditions with different instructions (and in which, consequently, possible differences in the background state, e.g. in preparation, might have caused the EP differences reported) were conducted by, among others, Rietveld et al. (1966), Gross, Begleiter, Tobin and Kissin (1965), Ciganek (1967), Shevrin and Rennick (1967), Small and Small (1970), Groves and Eason (1969), and Keating and Ruhm (1971). The latter authors, for example, showed that discriminating and counting tasks produce an increase in EP amplitude relative to either a ‘quiet’ or a reading condition. On the other hand, Small and Small detected no EP differences between conditions in which the subject was either required to attend to the stimuli without response or to push a button after each click, in order to terminate a train of clicks. (Under each condition 50-100 flash-click stimulus pairs were delivered with an interval of 0.5 set between the paired stimuli.) Instead, a clear contingent negative variation (CNV) developed under the latter condition between the paired stimuli and was sustained well after the click in the data of several subjects. No CNV-EP relationship was established. In their attempt to assess the relative contributions of arousal and attentional factors affecting the amplitude of the visual EP, Groves and Eason (1969) showed that the latter was enhanced and the RT shortened under conditions involving reacting to each flash within a specified period of time in order to avoid a shock. The reference conditions were: (1) passively watching the flashes; (2) reacting to each flash; and (3) reacting to each flash while receiving an occasional unavoidable shock. This result was interpreted to ‘support the

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contention that attending to a sudden sensory stimulus leads to an increase in the amplitude of the visual evoked cortical potential’ (p. 398). This result might, however, also be due to increased arousal and motivation associated with the avoidable-shock condition-something could be gained by the subject by increased efforts. Additionally, it is well established that faster reactions are associated with enhanced EP amplitudes (see, e.g. Donchin and Lindsley, 1966) and this effect can be mediated by many other factors besides selective attention. Most of the conditions of the interesting study by Guerrero-Figueroa and Heath (1964) with recordings from cortical and subcortical structures in man with implanted electrodes also resemble paradigms of the aforementioned investigations. 2.2. EPs to complex or afective visual stimuli Lifshitz (1966) found interesting differences between EPs to different kinds of informationally complex pictorial stimuli (indifferent scenic, repulsive medical and nude female slides). These responses were also different from those to projected words, colours, or geometric patterns. As the stimuli of each category were presented in a row rather than mixed with stimuli of other categories, it is impossible to decide whether the differences observed were due to pre-stimulus differences in the state of the subject or to differential processing of different stimuli. Randomized presentation was apparently used under some of the conditions in which EPs to slides made non-associational through defocusing were compared to EPs to the same slides when focused. However, as the author himself points out, the EP differences obtained under the latter conditions might be due either to physical stimulus differences or to differences in meaningfulness. Focused pictorial slides were followed by slow, late, CNVkind of negativity which might indicate attention to pictorial contents and associated mental processes. Begleiter, Gross and Kissin (1967) demonstrated interesting differences between visual EPs to three different line figures, devoid of meaning, after a conditioning procedure in which one of the figures was associated with words with positive meanings, one with words with neutral meanings, and one with words with negative meanings. Unfortunately, after the conditioning procedure each visual stimulus was presented 70 times in a row (IS1 1.5 set) and, consequently, the EP differences obtained might be caused by pre-stimulus ditlerences in the subject associated with different states of conditioned affectiveness rather than by differential processing of stimuli with different affective meanings. Had these three visual stimuli been delivered in a random order within a run, the differences obtained could have been ascribed to differential processing peu SL’.(See also Begleiter, Gross, Porjesz and Kissin (I 969) who, using a similar procedure, studied the effect of the subject’s degree of awareness of the conditioning procedure on the relationship established.)

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Cohen and Walter (1966) showed that if a visual stimulus is interesting or meaningful it tends to be preceded by a higher CNV than a less interesting or less meaningful stimulus under conditions in which the kind and timing of the stimulus were predictable (a warning signal was used). The EP to the interesting or meaningful stimulus was also enhanced (and followed by a large, slow positive wave) which may be due to differential preparation for (the CNV difference as a possible indicator), or to differential processing of, interesting and non-interesting stimuli. In another, similar, setting with similar EP effects (their fig. 5), they did not use a warning signal, but the subject apparently was able to know the general nature of the next stimulus: ‘The exposures were repeated but were so short that the pictures could be recognized only with difficulty and only after several trials’ (p. 195). 2.3. EPs to wrbal stimuli There are also EP experiments with verbal stimuli which naturally are of relevance to the central theme of the present review. In the following, studies are dealt with in which possible selective and non-selective effects of attention, stimulus meaningfulness, etc. are confounded because of the presentation of stimuli belonging to different relevance categories in separate experimental situations. In the first part of their experiment, Roth, Kopell and Bertozzi (1970) compared vertex EPs to acoustically presented sentences under attend and non-attend instructions. Within-subject wave form (but not across-subject amplitude) differences were found which might also be reflections of differences in the state of organism at the respective stimulus moments: before the sentence was spoken, the subject knew whether to pay attention or not. For this experimental condition it might be very difficult or impossible to change the paradigm to separate the possible specific and non-specific effects from each other without essentially interfering with the basic idea of this experiment; on the other hand, the mere repetition of several of the related experiments by using a randomized design would harmlessly unconfound these two kinds of effects : for example, in the study by Young and Horner (I 971) in which EPs to affective and non-affective verbal stimuli given in their own lists were compared. Interestingly, it was found that the interpretation given by the subject to the intent of the study was keenly related to the effect of affectiveness of words on EPs. The authors themselves were well aware of the aforementioned interpretational difficulties : ‘Since both the affective and non-affective words were presented sequentially (i.e. in separate lists), those subjects who gave reduced IVY-P, peak amplitudes for the affective stimuli possibly realized at some early point in the affective list that all of the words had been emotional. They would expect, therefore, the next word to be emotional, and it would be this ectancexpy, or anticipation, that could create the decrease of the N1 -PI

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peak amplitude’ (pp. 299-300). A somewhat similar remark was made by Matsumiya, Tagliasco, Lombroso and Goodglass (1972) with regard to their interesting results involving interhemispheric asymmetries of different magnitudes of EPs to speech and sound effect stimuli. The authors summarize their results by stating that the amount of asymmetry was related to the meaningfulness of the stimulus rather than to the mere use of verbal versus non-verbal auditory stimuli, but raise the question whether the asymmetry was caused by a prior linguistic set or by post-signal discriminatory events. They point to the relatively short latency (100 msec from stimulus onset) of the wave with peak asymmetry, ‘W wave’, and state that ‘the W wave occurs when less than 25 “/o of the stimulus word has been heard. Thus, the W wave is probably not a reflection of the meaning intrinsic in the message, but rather of the mental set which the particular conditions imposed on the subject’ (p. 791). It would be very interesting to know whether different amounts of asymmetry would exist between EPs to the kinds of stimuli used when delivered randomly. 2.4. EPs to ‘anchor’ stimuli of adaptation-level studies An elegant, different kind of approach to the attention-EP problem was provided by Sarris and Haider (1970) who measured vertex EPs to anchor tones of differing frequency distances to the series tones to be judged. The pitch of the latter stimuli was varied from 500 to 600 Hz and each was preceded by an anchor kept constant throughout the experimental session. The anchor values used varied from 60 to 10 000 Hz. After each stimulus pair, the subject judged the series stimulus according to a nine-step rating scale. According to Sarris’s well-established ‘similarity classification model’, the common contrast effect found in adaptation-level studies gradually disappears when anchors become psychologically extreme. Specifically, the model predicts that an ‘extreme’ anchor does not serve even as a partial point of reference for the judgments of the series stimuli. On these grounds the authors were tempted to speculate that an extreme anchor loses its ‘attention’ value. They predicted that the anchor pitch-EP relationship will follow an inverted U function: an anchor physically equal to the midpoint of the series was expected to produce a maxima1 EP amplitude since here the anchor would ‘maximally stabilize Ss’ judgmental behaviour ; with increasing distance of the anchor from both ends of the series, the CEP measure should gradually decrease because of the anchor’s diminishing cognitive relevance’ (p. 113). The results came out as predicted. The anchor-adaptation-level relationship followed the cubic trend; i.e. with an extreme anchor a without-anchor adaptation-level value was approached. The anchor-EP (NI - PJ relationship followed the hypothesized inverted U trend. (In a control experiment, no reliable EP differences were found with the different anchor frequencies.) On the basis of these results and of those from a similar GSR (galvanic skin response) work (Sarris, Tews and Schonpflug, 1971), in which the extreme

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anchors produced maximal GSRs, it appeared to the authors reasonable and attractive to assume that ‘CEP measures a cognitive “information-integration” aspect, whereas GSR measures an “arousal” aspect of attention (at least under the experimental conditions reported here). But presumably this important distinction does not hold generally (see, e.g. Natitanen, 1967)’ (p. 114). An alternative interpretation for the cognitive one given for the inverted U trend of the EP amplitude would involve the differential attitude of the subjects to the anchor stimuli : when these could be of some help in the judging task or simply were similar to the series stimuli soon to be judged, they presumably prepared for, and concentrated on, listening to them. This was apparently supported by the spontaneous comments made occasionally by subjects at the end of session. (The anchor was constant under each condition and, apparently, not completely temporally unpredictable; the subject was, however, instructed neither to judge nor to attend to the anchors.) Thus, also general differences in the state of the organism might have caused, or contributed to, the EP phenomenon observed. These experiments are important enough to be repeated, using different anchors in a randomized order within the same experimental condition. The present prediction would be that EP differences between the different anchors would vanish or occur with increased latencies.

3. Experiments with evoked potentials to stimuli of different relevance in the same condition 3.1. Experiments providing some basis for anticipation of relevant and irrelevant stimuli

A much more advanced experimental paradigm was employed by Spong, Haider and Lindsley (1965; see also Haider, 1967). When the subject was performing the visual vigilance task, attending to the flashes and ignoring the clicks, he received bright flashes requiring no response, and occasional dim flashes which required the pressing of a key. The clicks which alternated with the flashes (IS1 1 set) were all of the same intensity and were to be ignored. In the auditory vigilance task, he was required to press the key in response to occasional weak clicks interspersed among the more numerous louder clicks. Flashes alternating with the clicks during the auditory vigilance task were all of the same intensity and were to be ignored. It was found that when the subject was attending to flashes, these elicited much larger (occipital) EPs than the same flashes did when he was attending to the clicks. Similarly, when the subject was attending to the clicks, the amplitude of the temporal EPs to clicks was greater than when he was attending to flashes. Unfortunately, as pointed out by Naatlnen (1967, pp. 45546), the enhanced amplitudes of the EPs to the relevant stimuli may result from differentially

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enhanced non-specific or general alertness or ‘activation’, as an anticipatory and preparatory reaction to a relevant stimulus immediately prior to and at its presentation. These differential reactions to relevant stimuli were made possible by the alternating order of presentation of the stimuli, which allowed the subject to know the kind of stimulus that would come next. He proposed that ‘alertness’ and ‘activation’ will be high when the subject knows that a relevant stimulus will be immediately presented, and will be low between the relevant stimuli when the irrelevant stimuli are presented. Basically similar paradigms to that of Spong et al. (1965) were used, among others, by Satterfield and Cheatum (1964) Satterfield (1965) Davis (1964), Chapman and Bragdon (1964), Chapman (1965) and Sheatz and Chapman (1969). Most of these studies regard their results as providing an EP correlate for selectiveness of attention, for stimulus relevance or related psychological phenomena, and in the literature these works have frequently been cited as evidence for such relationships. A similar remark as that made with respect to the Spong et al. study can, however, be also made with respect to these studies. (For a review of these works and the details of this criticism, see Karlin, 1970, and Naatanen, 1967.) Also Guerrero-Figueroa and Heath (1964) had a corresponding condition in their aforementioned study. The EP enhancement, interpreted as reflecting selective attention, might be due to non-selective differences in the state of the organism between the moments of the delivery of the relevant and irrelevant stimuli as the order and timing of the stimuli appears to have been predictable. Like many of the abovementioned workers, Debecker and Desmedt (1966) also delivered their relevant and irrelevant stimuli (clicks and shocks with alternated relevance) in an alternating order and at regular intervals. The late positive component was enhanced in vertex EPs to relevant stimuli, whereas no earlier effects were observed. The late effect was dependent on the ISI. Donchin and Cohen (I 967) further developed the methodology of the related studies by replacing the alternating manner of presentation of the relevant and irrelevant stimuli with a paradigm in which the subject could never be certain as to the category of the next stimulus appearing. They presented flashes at irregular intervals from 2 to 3 set and, timed independently of these, background reversals at irregular intervals from 3 to 4 set on the same retinal location. Thus, the test flash was superimposed on either of the background figures, one or the other of which was always present. When the subject’s attention was directed to the flashes, these elicited larger EPs than when attention was directed to the background reversals and vice versa, this enhancement occurring mainly in the late positive component. These results were regarded as providing a physiological correlate for intramodal selective attention within the visual modality even under conditions in which the relevant stimuli could not be anticipated.

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In his comment, NBHtanen (1969a, 1969b; for the reply, see Donchin and Cohen, 1969a, 1969b) referred to the possibility that the subject at least sometimes knew approximately when relevant stimuli would be presented (see also the evaluation of the method used by Eason, Harter and White, 1969, later in the present review). A possible consequence would be that the enhancement reported could also be due to (non-specific) differences in the state of the organism between the moments of occurrence of the two kinds of stimuli. (Karlin (1970) also made a similar remark and extended it to a later similar study by Smith, Donchin, Cohen and Starr (1970)) Related differences in peripheral receptor conditions may also have existed before the relevant and irrelevant stimuli (Naatanen, 1969a). Kopell, Wittner and Warrick (1969) replicated the Donchin and Cohen study with slight modifications and obtained the EP enhancement to relevant flashes but not to relevant background reversals. The remark related to the above-chance-level predictability of the relevant and irrelevant stimuli applies also to this investigation. Later Donchin and Smith (1970) presented excellent data showing clear (vertex) CNVs selectively preceding the relevant stimuli in an experimental situation which differed from that used by Donchin and Cohen (1967) only in that this time the ISI was constant (for Aashes 2 set and for background reversals 2.25 set) rather than irregular, with a small variation. A similar finding concerning the occipital CNV was also reported. They also made the important proposition that CNV resolution and P,,,, a large late positive EP component, might be ‘two aspects of the same process’. Naatanen (1967) conducted three experiments to separate the specific and non-specific effects of attention on EPs from each other. The first was aimed at studying EPs to stimuli of an irrelevant modality during attention directed to another modality. ‘It might be that all kinds of stimuli, even of a completely irrelevant sense modality, elicit evoked potentials with enhanced amplitudes, $presenM during attention directed to one sense modality’ (p. 60). Attention to vision was attempted by introducing periodically a combination of a warning flash and an irregularly presented (1, 2 or 3 set after S,) imperative flash requiring a rapid key press. The amplitudes of the vertex and temporal EPs to clicks, presented randomly either 1 or 2 set after Sr (in 39”/;; of the trials; always before S,), were signi~cantly larger than those of the EPs to physically similar clicks presented randomly 3, 4 or 5 set after S, (in 44% of the trials), when the subject was relaxed again. The duration of the main EP components was measured to control the degree of time-lockedness of the single EPs (see Sakai et al., 1966) and generally no signiftcant differences between the two click categories were found, thus ensuring that the larger amplitudes were not due only to a smaller variation in the latencies of the single responses. Clicks were understood to be more irrelevant when the visual task was introduced by the warning flash than during relaxation between the visual reaction

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trials. (This logic is in accordance with that of distraction studies conducted with animals first by HernBndez-Pe6n and his group.) Karlin (1970) has claimed, however, that the result obtained is ‘not conclusive because it assumes that the subject could follow the instructions literally and disregard equally both inside and outside clicks’ (p. 123), continuing that the subject might have tended to use the inside clicks ‘as warning signals in addition to or instead of the intended warning signal’ (p. 123). He also referred to some evidence that the click might be more effective as a warning signal because of its modality. Hence, he doubted the real irrelevance of the inside clicks. Karlin is possibly right in claiming that the inside clicks were not irrelevant, but probably not for the reason given by him: the use of inside clicks as additional warning signals’. First of all, these were given only in 39’4 of the trials and irregularly in temporal relation (with intervals of 1 or 2 set) to both S1 and Sz. Additionally, even the maximal foreperiod was rather short (3 set) which, together with the rectangular distribution of foreperiods used, probably made the subject’s time uncertainty as to the S, moment (see Klemmer, 1956) already rather low (see Nickerson, 1967, and NGt%nen, 1970b, 197 1). Furthermore, an S, flash could also be delivered at a possible moment of an inside click; in fact, the delivery of S, was more probable than that of a click both at 1 set and at 2 set after S,. Thus, there is no objective reason for the subject first to wait for a click and only then to start to increase preparedness. As to the attempt to use the click as a warning signal after hearing it, the aforemade points make it somewhat improbable and the fact that key presses to inside clicks were rather frequent (not reported in that monograph) even more so. This observation suggests that the subject really was scanning the situation in terms of the visual modality (‘flash’ or ‘no-flash’; see section 4)‘. This observation, however, also points to a possible way how the inside clicks were relevant: the process of perception of a discrete stimulus against an ‘empty’ background possibly takes place so that what is first perceived is that something is happening3 and only thereafter the modalityand stimulusspecific features of the stimulus emerge (see Smith, 1963). In this case each

‘For the reasons given in the following, the similarity of NZitlnen’s (1967) first experiment with the experiments of Davis (1964) and Sheatz and Chapman (1969) is probably not as

notable as Karlin (1970) describes it. ZMoreover, if the EPs to ‘inside’ clicks were enhanced because of being used warning signals, as Karlin proposed, then the EP to those inside clicks delivered S, should have been larger than, or at least as large as, the EP to inside clicks S1. When differences were observed, however, the latter EPs were larger which

as additional at 1 set after at 2 set after also supports

the non-specific enhancement explanation. 3Thisexperiencemight be related to some of the first evoked effects of a stimulus detected at the scalp only a few milliseconds post-stimulus which very early components were assumed to be generated by the effects of the stimulus on the reticular formation (see Jewett, Roman0 and Williston, 1970). It might be that such reticular effects are associated with a ‘nonspecific’ experience of a stimulus while more specific features of subjective stimulus effects are associated with influences on the centres along the lemniscal pathway.

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stimulus, at least those delivered during a period with some degree of subjective probability of the occurrence of the relevant stimulus, cannot be experienced irrelevant until the point in stimulus processing is reached in which a m&match, with respect to the modality or some stimulus-specific feature, occurs. The general implication would then be that it is impossible to deliver really irrelevant stimuli during anticipation of, or preparation for, the occurrence of the relevant stimulus : if the stimulus cannot be irrelevant to the subject a priori, it can be that to him only a posteriori (after discrimination). In his third experiment, Naltanen (1967, see also 1970a) demonstrated that when relevant and irrelevant stimuli (strong and weak clicks with alternated relevance between blocks) are presented in an alternating order and at regular (1 set) intervals, the CNV (see fig. 1) and the flattening of the amplitude of the background EEG (taken from the period 267-O msec pre-stimulus) selectively precede the relevant stimuli. These pre-stimulus changes took place mainly only with those subjects who showed an enhancement of the (vertex and temporal) EPs to the relevant stimuli (three out of five subjects). Some indication was obtained for a stronger relationship between the differential EEG flattening and the EP enhancement than between the differential CNV and the latter. The EEG flattening indeed

t REL S

IRR S

REL S

Fig. 1. The upper curve represents an averaged vertex (C,) record of one subject during the 2 set interval between two successive relevant stimuli (weak clicks) intervened by an irrelevant stimulus (strong click), with the bandwidth setting of l-70 cps. The lower curve represents the same averaged vertex record with the difference that the bandwidth was 0.3-70 cps. N = 45. (From NaiPtanen, 1967. Reproduced with the permission of the Finnish Academy of Science and Letters.)

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was a non-specific phenomenon : for example, the occipital EEG tended to be of significantly lower amplitude before the relevant than before the irrelevant auditory stimuli, i.e. when the subject presumably paid attention to auditory stimuli than when he did not do so. For some subjects, the task with the weak clicks as the relevant stimuli was the more difficult task: for the rest the other task. In keeping with the differential-preparation interpretation, the more difficult task produced a more pronounced EP enhancement as well as larger CNVs to the relevant stimuli. It was concluded that if the task offers some basis for anticipation of the relevant events, the subject ‘works periodically, maximizing his alertness and activation at the anticipated presentation of the relevant stimuli, to make the detection or discrimination required as efficient as possible. If the task is easy, cues for improved performance are not utilized as unnecessary’ (p. 166). Naatanen (1970a) also replicated the Spong et al. experiment (1965) to ascertain the existence of the cyclic pre-stimulus arousal also under their conditions, using, however, only one subject. A highly significant decrease of about I I “/, of the vertex EEG preceding the relevant stimuli was found in relation to that preceding the irrelevant stimuli. The corresponding (statistically significant) percentages for the occipital and temporal data were about 10 and 4, respectively. Eason et al. (1969) also claimed to have found an EP correlate for selective attention within the visual modality. They presented flashes to the right and left visual fields of the subject, alternating their relevance by means of an RT or counting task which involved either the right or left flashes. A very clear enhancement of the EPs to the relevant flashes was obtained. (A single amplitude score on the basis of three amplitude measures from each condition from each subject was taken-this procedure does not take into account the possible component-specific variation.) These results are not, however, unequivocal for the following two reasons. (1) It is possible that the subject did not fixate his gaze the whole time to the fixation point positioned between the visual fields, but on average more in the direction of the visual field in which the relevant stimuli appeared. (2) There was some objective basis for the subject to predict the nature of the next stimulus above the chance level : two photostimulators were independently programmed to present flashes concomitantly but never simultaneously within each visual field. The within-field flash interval randomly varied from 2 to 8 set with the limitation that the time between two consecutive flashes (relevant and irrelevant taken together) was never less than I sec. It is possible that during the course of the experiment the subject learned that for a short period after a relevant stimulus another relevant stimulus is not likely to be delivered (Karlin, 1970; Naatanen, 1969a, 1971) and that the more time has elapsed since the preceding relevant stimulus, the higher the probability of the immediate delivery of a relevant stimulus will be. In particular a rc~tang~ku/

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distribution of the different foreperiod durations (used by Eason et al.) is usually followed by a learning process such as described above4. Its outcome is in turn reflected by preparation cycles during the course of the run, with preparation ‘peaks’ occurring mainly during the latter part of long foreperiods .. . (see Nickerson, 1967, and Naatanen, 1970b, 1971). From the previously described kind of relationship of such cycles with the attended stimulus series and from the independence of the two stimulus series, it naturally follows (if such cycles indeed exist) that the preparation on the average was higher at the moment of the delivery of the relevant stimuli than of the irrelevant stimuli. Thus, the selective enhancement reported by Eason et al. (1969) was possibly due to differences in the peripheral receptor conditions and/or in preparation--the former appearing as at least occasional shifts in the direction of the gaze to the side of the relevant stimuli, the latter as cycles of preparedness temporally more related to the relevant than the irrelevant stimuli. If these factors could be ruled out, there still exists the following alternative ‘non-specific’ explanation to the ‘specific’ one given by the authors: by superimposing the EPs to the corresponding relevant and irrelevant stimuli in their figures, we find that the traces start to deviate from each other only after some 130 msec from the stimulus onset. By that moment the brain might have been able to discriminate between the relevant and irrelevant stimulus as this can simply be made on the basis of the stimulus location; see section 4. The consequence might be that the later EP components to the relevant and irrelevant stimuli were elicited during differing states of the organism and that this resulted in the enhancement of those components to the relevant stimuli. (The median RT was some 200 msec for many subjects, the EMG response starting some 4An experiment on this point was conducted by the author with the assistance of A. Merisalo and 1. Linnankoski. Four squared paper tapes of 50 ‘stimuli’ (crosses) were prepared with a midline. Crosses were drawn on its both sides with irregular intervals (1 set = 1 square) obtained by simulating the randomization procedure of Eason and his collaborators. On the average, a cross was succeeded by a cross on the other side of the midline in about 80 y0 of the cases. Each tape was slowly drawn from behind a screen into the sight of the subject and stopped for a short moment after the appearance of each cross. At that moment the subject predicted the side on which the next cross would appear. The subjects gave their (mainly alternating) predictions quickly so that the above-chance-level predictions are not attributable to the present method allowing time for reasoning. The first subject was instructed about the rules which were followed in making the tapes, the four others were only instructed to give a prediction after each cross. The mean percentages cf correct predictions for the four independently made tapes for subjects l-5 were 72.5, 76.5, 55, 68.5 and 70, respectively. (Subject 5 was a boy of the age of six years.) There is no doubt that the highly distinguished subjects of the Eason et al. experiment among the.n R.E., R.H. and C.W. were able to perform at this level or better. Later, the probability of alternation was also simulated by a computer program by P. Laurinen. This probability was found to vary between 0.71 (in all cases in which stimuli of both sides wotild coincide in lime, the one being on the same side as the preceding stimulus was selected) and 0.87 (the other stimulus was selected). The number of observations amounted to 3979,

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50msec earlier (Ritter, Simson and Vaughan, 1972), onset of a motor potential preceding the latter by 30-60 msec (Vaughan and Costa, 1968)-the discrimination must have taken place much earlier than at 130 msec post-stimulus.) This reactive-arousal explanation resembles that given by Karlin (1970); the latter, however, refers to the possible sudden drop of non-specific arousal caused by the relevant stimulus. But the direction of such a non-specific stimulus-locked change might depend on what the stimulus requires the subject to do: if it signifies ‘still harder times’, e.g. increased efforts (on a shortterm basis), the change would rather be an increase in arousal. For example, in a constant-foreperiod RT task with very short ISIS, the CNV onset can take place already within 200 msec of the stimulus onset (Posner and Wilkinson, 1969; Gaillard and Naatanen, 1973), i.e. during the latency range of the principal EP components. Furthermore, it has been demonstrated that if SZ introduces a difficult task of several seconds (with no additional stimuli) the pre-S, CNV is sustained and can peak well after S, (Donald, 1970; Jarvilehto and Fruhstorfer, 1970; see also fig. 3) and Karlin, Martz and Mordkoff (1970) have shown that when S2 requires (a simple RT task) or can require (a disjunctive RT task) a fast motor response, N1, Pzand N2 components are shifted in the negative direction (compared to a control condition). It might be also possible to find better indicators for the non-specific change proposed than the CNV (see Lansing, Schwartz and Lindsley, 1959, and Naatanen, 1970a). In particular the Karlin et al. study points to the possibility that in some situations the enhancement of N, and N2 components to relevant stimuli might simply be due to their summation with the CNV rapidly rising for the task the stimulus introduces. (See also Debecker and Desmedt, 1971, reviewed later.) Using a somewhat similar stimulus paradigm to that of Eason et al. (1969), ijhman and Lader (1972) also reported an enhancement of certain EP amplitudes attributed to selective attention. As they compared EPs to similar relevant and irrelevant auditory stimuli, possible differences in peripheral receptor conditions cannot be referred to as a possible explanation for the finding as was done with respect to the visual task of Eason et al. (1969). However, also in this auditory task, probably it was possible for the subject to roughly approximate the periods during which a relevant stimulus would and would not be presented as the inter-click interval (ICI) varied from 8 to 12 set under the long-ICI condition and from 2.4 to 3.6 set under the short-1CI condition (rectangular distributions). When the clicks were irrelevant the subject was instructed to respond to visual stimuli occurring with the same mean IS1 as the clicks of the same condition but with a somewhat larger range. A possible result would be that the enhancement was due to an on the average higher preparedness at the moment of delivery of relevant than irrelevant clicks. (For details ofthis argument, see the foregoing discussion on Eason et al., 1969.) This possibility was, however, considered by the authors who also measured skin

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resistance before the stimuli and found no related difference. But it is possible that some other ‘activation’ measure would have shown a difference, perhaps some more central measure, such as the CNV. A very interesting feature in their results was the component-specific variation of the size of the EP: PI - Nl and N, - P, amplitudes, but not P, - N2, were increased by attention and by longer ICIs and showed more marked and consistent habituation than the latter. It was also of considerable interest that the EPs to the attended clicks showed faster habituation. To sum up this subsection: when some basis is provided for the subject to predict the order of the relevant and irrelevant stimuli above chance level, the EP enhancement to relevant stimuli could be also due to general dzflerences in the state of the organism at the moment of presentation of the relevant and irrelevant stimuli. When they alternated at regular intervals, a clear CNV differentially precedes the relevant ones (Donchin and Smith, 1970; Wilkinson and Ashby, 1974; Corby and Kopell, 1973; Naatanen, 1967, 1970a), and irregular intervals are no guarantee against the development of the CNV (see Blowers, Ongley and Shaw, 1973). (Also with irregular intervals the CNV may or may not-depending on the predictable/unpredictable order of stimulihave a higher mean amplitude before the relevant than before the irrelevant stimuli.) A related differential flattening of the background EEG amplitude was also demonstrated (Naatanen, 1967, 1970a). This kind of predictability can also lead to peripheral sensory differences between the moments of presentation of the relevant and irrelevant stimuli (see, e.g. Oswald, 1959; Naatanen, 1969a). 3.2. Experiments with no basisfor anticipation of relevant and irrelevant stimuli Naatanen’s (1967) second experiment was similar to that of Spong et al. (1965), with the difference that the order of the relevant and irrelevant stimuli was randomized and, instead of the 1 set constant HI, randomized ISIS of 1, 2 and 3 set were used. Because there was no basis for the subject to predict above the chance level which kind of stimulus would come next, the possible differential-preparation artifact was eliminated and the effect of the task relevanceper se of the stimuli on the EPs could be observed. No significant differences in the vertex and temporal EPs between relevant and irrelevant clicks were found, whereas the relevant flashes evoked vertex and occipital potentials with enhanced P, or P, (see Sutton, 1969) components. The latter effect was ascribed to possible peripheral sensory changes (loss of ocular accommodation and convergence; see Oswald, 1959) which occur when the subject loses his interest in the visual field (clicks relevant). ‘The crucial point . . . is that relevance did not enhance the click-potentials, whose peripheral factors are much more reliably controlled. . . . If more central factors had been operating for the differences in flash-potentials, these should have cause differences also between the potentials evoked by the relevant and irrele-

vant clicks. It is very difficult to imagine that the sensory pathways and centers of vision, but not of audition, would have this property’ (pp. I I661 17). Pointing to Naatanen’s Experiment 3 (weak and strong clicks were alternated) which ‘demonstrated enhancement with relevant stimuli but only with weak clicks’ (p. 123) and to his attribution of this difference to the subject’s impression of task difficulty, Karlin (1970) claimed that ‘if the latter explanation is correct then it also needs to be ruled out as a factor in Experiment 2 expecially since the click intensities would seem more comparable to the strong clicks than to the weak clicks of Experiment 3’ (p. 124). It is true that the task difficulty was rather similar between Experiment 2 and the strong-click task of Experiment 3, but some indications of EP enhancement was also obtained in the latter task. As to the vertex EPs, the variance analysis performed gave a significant (p < 0.05) enhancement of the P, - N, component (Naatanen, 1967, p. 204) and the Ni - P, component demonstrated a distinct tendency in the same direction (on the average, it was 16.7 x) larger in the EPs to the relevant stimuli; the same figure for the weak-click task being 26.9%); pp. 121-122). Furthermore, the temporal data showed a significant (p < 0.01) enhancement of the N, -P, amplitude in both tasks (p. 124). Referring to this experiment, Lindsley has raised the important question ‘whether it is possible to assume and maintain an attentive set toward given stimuli when it is impossible to organize and regularize one’s internal neural systems of response because of irregularity of stimuli presentation’ (Lindsley, 1969, p. 33). Although the impressions of the subjects favour the view that selective attention and perception is possible also under such conditions, such selectiveness might be clearly impaired in comparison with conditions with regular stimulus presentation. It appears that this point has received sufficient attention neither in experimental planning nor in interpreting results by Naatanen and others who have used similar stimulus paradigms (see section 4). See also Lindsley and Wicke (1974, pp. 48-49). In his elegant replication with some methodological improvements of Naatiinen’s (1967) previously reviewed second experiment, Hartley (1970) obtained mainly corroborating results. Visual and auditory stimuli were randomized and ISls were even longer, varying randomly from 2 to 6 set with a mean of 4 sec. Wilkinson and Lee’s (1972) experiment differed from Nhatanen’s Experiment 2 in two important respects: the ISIS were much shorter and only the auditory modality was involved. The use of shorter ISIS can be considered an important methodological improvement for two reasons: (1) the slow rate of stimuli makes it possible for the subjects to handle the information on both channels should they wish to do so, and which they might do even if experimental instructions do not require it (Hartley, 1970); and (2) the objective and subjective probability of the immediate delivery of a relevant stimulus is higher (see Lindsley’s remark quoted above).

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Following a warning signal, brief runs of about 25 tones were presented at random ISIS varying from 30 to 1760 msec. The tones were of three different frequencies and the task was to count the tones of a certain frequency. As the run was short it was assumed (but not measured because of limitations of the apparatus) that the CNV was present and, since the stimuli were unpredictable, of equal amplitude for both the relevant (counted) and irrelevant (uncounted) stimuli. Nevertheless N1 - P2 amplitude was about 10% larger in the EPs to the relevant stimuli. This difference did not correlate with the level of performance but another measure did, a large positive wave extending from about 100-150 to 600 msec post-stimulus. As only occasional transient Psoo components could be revealed this slow positivity was interpreted to be CNV resolution occurring only to relevant stimuli. It was proposed that the enhancement of the N, - P2 amplitude was caused by the selective summation of this amplitude with CNV resolution to the relevant stimuli, not by an increase in the size of the EP itself (see also Wilkinson, 1973). It is important to notice here that the ‘relevant’ stimuli in this experiment were the target stimuli to be detected and counted and not of the type of standard relevant stimuli (relevant because of either their similar timing or physical similarity to the targets). Accordingly, their results could be interpreted to signify that the CNV resolution under these kinds of experimental situations takes place selectively to the targets, i.e. is associated with the positive outcome of the search (or of stimulus processing) for a target rather than with the search itself (or that the resolution to the targets is much stronger than to the other stimuli). Further correlational evidence relating the slow positive post-stimulus wave to both prior CNV and apparent effect of selective attention on the Iv1 -P, and Psoo amplitudes was provided by Wilkinson and Ashby (1974) who in part replicated and extended Naatanen’s (1967) second and third experiments. In the predictable condition, high (1500 Hz) and low (400 Hz) tones alternated at 1 set intervals; in the unpredictable setting the same tones appeared randomly at 1 set intervals. In some runs the high tone was designated relevant and in the others the low, the signal stimuli embedded with the relevant stimuli being similar stimuli with a slightly prolonged duration. The Nr - Pz amplitude showed enhancement only in the predictable setting. Under this condition the CNV was observed to differentially precede the relevant stimuli, corroborating the results of Naatanen (1967) and Hartley (1970). (Note the longer ISIS and no N1 - P2 enhancement in the unpredictable settings of Wilkinson and Ashby, of Naatanen and of Hartley versus shorter ISIS in the unpredictable setting of Wilkinson and Lee with NI -P, enhancement.) There was also a significant difference (given by one, but not the other statistical procedure used) in P3,,,, (positivity of traces relative to the EEG level at stimulus onset at a point 300 msec post-stimulus) between the EPs to the relevant and irrelevant stimuli in the predictable but not in the unpredictable setting. (These as well as the other amplitude measures were relatively low in this study, due

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probably to the easiness of the discrimination task; see, e.g. NEMnen, 1967, pp. 163-166.) Again, no discrete P3 was detected but there were signs of a much slower positive wave extending from about 200 to 500 msec post-stimulus: measures PdoO and PSoO, taken in a similar manner as PXoO, demonstrated a considerable agreement with the latter in reflecting difference between the relevant and irrelevant stimuli in the predictable runs. This slow wave was thought to be the CNV resolution. With predictable presentation strong positive correlations were observed between CNV, iV1- P2 and Y,,, in terms of both absolute level and, particularly, relevant/irrelevant difference. Holding CNV constant statisticaily, and to a lesser extent PSoO, reduced these correlations. The important suggestion was that ‘CNV resolution in the post-stimulus trace reflects selective attention paid to the stimulus, and may be responsible, through summation, for claims that N1 - P2, and sometimes P,,,, does so’ (p. 167). Unfortunately, PI - Nl amplitude was not measured, apparently for its vagueness in the traces. Neither did Wilkinson and Lee measure it. This amplitude would be not at a11or much less than ,!V1- P2 modulated by the CNV resolution, owing to its shorter latency; if an enhancement had been observed, it could not have been attributed to summation with the CNV resolution for the positive direction of the amplitude concerned5. Another unreported feature of interest was the possible CNV in the unpredictable setting6. If there was aCNV-which in this case was of about equal ‘In NBBtlnen’s (1967, Experiment III) rather similar predictable condition also PI - N1 was measured. This amplitude showed no enhancement (while N1 - P;: did), thus supporting Wilkinson’s hypothesis. 6A CNV possibly developed also under the unpredictable condition as (1) the probability that the next stimulus will be a relevant one was 0.50. Under this condition clear CNVs which were even larger than those under the certain condition have been recorded (see, e.g. Hillyard and Galambos, 7967, JBrvilehto and Mlntysalo, 1973, Karrer, Kohn and Ivins, 1973, and Tueting and Sutton, 1973a), and (2) the ISI was a constant of 1 set known to produce very highCNVamplitudes(see,e.g.Tecce, 1972,and Gaillard and NLHtFmen, 1973). If there was a CNV, it probably was smaller than that preceding the relevant stimuli under the predictable conditions because under the latter case the frequency of single CNVs would probably be approximately a half of that under the unpredictable condition (this suggestion is based on the number of stimuli known in advance by the subject to be relevant and possibly relevant, respectively). There are certain reasons why the suggestion that the relevant stimuli are selectively followed by the CNV resolution might perhaps have been better testable under the unpredictablecondition (if the CNV existed) than under the predictable condition: (I) under the former the CNV at the moment of the relevant and irrelevant stimuli was about equal and (2) probably larger than that preceding the irrelevant stimuli (0.6 WV; arbitrary baseline) under the predictable runs. If the claim for the selective CNV resolution mainly rests as far as the irrelevant stimuli areconcerned on observations involving the degree of resolution of a CNV with an average amplitude of only 0.6 pV, more evidence is probably needed (for additional evidence, see, however, Wilkinson, 1973, and McCallum and Walter, 1968). There are, however, some inherent difficulties in testing the selective-resolution hypothesis under unpredictable conditions: (I) the real irrelevance of the ‘irrelevant’ stimuli can be questioned as Wilkinson and Ashby did (see p. 259 of this review); and (2) the determination of the baseline for the measurements of the CNV amplitude.

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amplitude before the relevant and irrelevant stimuli for their unpredictable order-it might be argued that selective CNV resolution should also have taken place under this condition. This was not the case as there were no significant amplitude differences between the EPs to the relevant and irrelevant stimuli. The authors explain this lack of difference by the actual relevance of the socalled irrelevant stimuli under the unpredictable setting: ‘Both stimuli are effectively relevant as far as the EP is concerned merely because the onset of either one presents task-relevant information. With alternating presentation no task-relevant information is passed when a predictably irrelevant stimulus occurs at a predictable point of time’ (p. 173). This explanation, however, seems to be somewhat at variance with Wilkinson and Lee (1972), according to whom the CNV resolution takes place to the positive outcome of the search (stimulus processing) for a target rather than to the search itself. However, the data from the Wilkinson and Lee and the Wilkinson and Ashby (unpredictable condition) studies do not necessarily conflict, although selective CNV resolution occurred to the ‘relevant’ stimuli in the former but not in the latter study. This difference might be due to the different manner that the ‘relevant’ stimuli were relevant in these experiments-i.e. in the former the targets, in the latter the ‘standard relevant’ stimuli were the ‘relevant’ stimuli to which EPs were averaged. (In the latter study, no EPs to targets were averaged.) Moreover, in the former the ISI greatly varied, in the latter it was constant; the relative frequency of the ‘relevant’ stimuli also differed between the experiments’. ‘The difference between the views of Wilkinson (and his group) and of NLLtanen (1967,1969, 1970a) about the reasons for EP enhancement to relevant stimuli under predictable conditions could be characterized as follows: for Wilkinson, the EP enhancement (as appearing in records) to relevant stimuli is caused by the summation of an invariant Ni - Pz amplitude (and perhaps PJ component) with temporally coincident CNV resolution; for Nailtanen, a ‘real’ enhancement is involved, caused by non-selectively increased reactiveness of the organism before relevant stimuli which increased reactiveness is indicated, e.g. by the CNV and the flattening of the amplitude of the background EEG. Although there is plenty of evidence for the positive relationship between ‘arousal” and the size of the EP (e.g. Sakai et al., 1966; Fruhstorfer and Bergstrom, 1970; Isgur and Trehub, 1971) as well as for the existence of increased ‘arousal’ selectively before relevant stimuli (e.g. Donchin and Smith, 1970; Wilkinson and Ashby, 1974; Corby and Kopell, 1973; Nlatanen, 1967,1969,1970a), the explanatory power, weighing all the evidence together, of the selective-resolution view of Wilkinson apparently is stronger. The possibility is not excluded, however, that both these factors together operate to produce the effect concerned: although Wilkinson’s view takes into account both the pre-stimulus amplitude of the CNV and the ability of the stimulus to resolute it (as well as the time course of the resolution) as determinants of the size of the effect, this does not exclude the other possible significance of the pre-stimulus ‘arousa!’ proposed to the effect concerned: the EPs might additionally be enhanced by the increased ‘arousal’. Naltanen has based his explanation solely on the (non-specific) pre-stimulus state, whereas Wilkinson’s explanation is based both on (non-specific) pre- and post-stimulus events; the selectiveness of the latter events with respect to stimulus relevance implies, of course, a preceding discrimination process. Such a discrimination process is implied also by Karlin’s (1970) view according to which a late positive component is either produced by, or enhanced through summation with, the CNV return occurring selectively to relevant stimuli. For this

A deviating feature in the Wilkinson and Ashby study was the prolongation of some ‘relevant’ stimuli (from 10 to 40 msec) which thus became targets, instead of introducing a small frequency difference--the ‘standard’ relevant and irrelevant stimuli differed in frequency. If the subject utilized only the stimulus duration in his search for the targets, then both ‘relevant’ and ‘irrelevant’ stimuli in fact were equally relevant (or irrelevant) under the unpredictable runs (see also Sutton, 1969). This possibility is increased by a possible carry-over effect from the interspersed predictable runs in which the subject apparently almost solely worked on the basis of stimulus duration, the frequency difference between the alternating stimuli probably acting only as a kind of reminder to ensure that the duration-discrimination activity was directed to the right set of stimuli*. Corby and Kopell’s (1973) design was very similar to that of Wilkinson and Ashby, the main differences involving: (1) the visual modality; (2) the longer ISI which was 2.05 set; and (3) a fast motor response was required to each ‘relevant’ stimulus. The authors reported an enhanced late positive component to the relevant stimuli under both predictable and unpredictable setting, ‘positive displacement of the late positive component’. In evaluating this result, there is no doubt about the unpredictability of the order of the stimuli under the ‘unpredictable’ condition, whereas the use of fast motor response to the relevant stimuli is somewhat problematic: possibly the effect observed could at least partially be ascribed to the motor response to the relevant stimuli. More specifically, the ‘Pz’ of the motor potential might have summated with the EP to the relevant stimulus amplifying late positivity and shortening its latency (see also McAdam and Rubin, 1971, and Hillyard, 1973); its mean latency was only 233 msec. Then, however, a latency difference for this component in the EPs to the relevant stimuli should have been observed between the predictable and unpredictable settings, owing to the generally longer RTs of the latter kind of situation, which EP latency difference was not observed. However, no RTs were reported. Furthermore, a latency difference for this component should have also been observed between the relevant and irrelevant stimuli, but no information on this particular point was provided. selective return he offered two explanations: (1) subjective probability of a relevant stimulus immediately after a relevant stimulus is very low; and (2) momentary relaxation for task completion. It is important to observe here that all these views can, in principle, also explain (or be extended to explain) a selective-attention effect under unpredictable conditions (provided that its latency is long enough to allow an intervening discrimination in terms of stimulus relevance). However, the selective-resolution view cannot account for an EP enhancement in the negative direction (such as N1 enhancement of Hillyard et al., 1973b, discussed later), whereas a differential arousal reaction, where feasible, to the outcome of the discrimination process, or of some of its phases, could, in principle, be used to explain enhancement in both directions without being in conflict with known facts. *The complications related to assessing the effective relevance of the stimuli in the selective-attention experiments are in more general terms considered in section 4.

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The possible part of the effect observed not related to motor response could probably be best explained by selective CNV resolution to the relevant stimuli ; see also fig. 2b in their report (cf. Wilkinson and Ashby, 1974). In another recent experiment by Kopell and his colleagues (Ford, Roth, Dirks and Kopell, 1973), an interesting paradigm was followed. Two kinds of auditory stimuli and two kinds of visual stimuli were delivered in a random sequence and one of the four stimuli at a time was made relevant by instructing the subject to press a button as soon as possible to this stimulus. There was no difference in the iV2component between the EPs to the relevant and irrelevant stimulus of the relevant modality, whereas this component was smaller in EPs to the stimuli of the irrelevant modality. On the other hand, theP, component was larger to the relevant than to the irrelevant stimulus of the relevant modality and smallest in EPs to the stimuli of the irrelevant modality. These observations involved both modalities used. As to these findings within the visual modality, peripheral factors might have greatly affected the EPs: the subjects were instructed to fixate on an asterisk on the centre of the visual field, but no control was reported against the possible loss of fixation under conditions in which an auditory stimulus was relevant. On the other hand, similar auditory findings cannot be questioned on the basis of possible differences at the level of the proximal stimulus. An interesting feature in these results is that the earliest EP difference between a relevant and an irrelevant stimulus occurred later when the discrimination between these two stimuli apparently took more time (see above). Note, however, that even the earliest effect was rather late as the peak latency for NZ of the auditory EPs varied between 190 and 270 msec and of the visual EPs between 170 and 280 msec. Consequently, if the authors are right in writing that their N, ‘may reflect sensory gating based on modality parameters, or it may reflect a preliminary decision regarding the stimulus relevance, based on modality parameters’ (p. 466), such a reflection is very late compared to what it was assumed to reflect and might involve (specific or non-specific) postdecision processes (see p. 272). Finally, as a fast motor response to the relevant stimuli was also required in this work, the late positivity in EPs to these stimuli might have been enhanced for this reason. In their modification of the Wilkinson and tee experiment (1972), Hillyard, Hink, Schwent and Picton (1973b) also used very short ISIS to force their subjects to ignore irrelevant stimuli. Vertex EPs were recorded from subjects who listened to a series of tone-pips in one ear and ignored concurrent tonepipsin the opposite ear. In Experiment 1, a sequence of tone-pips (800 Hz) with randomized ISIS between 2.50 and 1250 msec was delivered to the left ear, while an independent series of 1500 Hz tone-pips of similar intensity, duration and random interval structure was presented to the right ear. About one-tenth of the tone-pips in each ear had a slightly higher frequency than that of standard tone-pips. A commendable methodological detail was an interspersed reading

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task with the instruction to disregard all tones: this was intended to serve as a ‘buffer’ to reduce carry-over effects between the conditions, The highly important result was that N, to the standard stimuli delivered to the attended ear was remarkably enhanced while Pz showed no related effect. (Under similar conditions Wilkinson and Lee reported an enhanced ~~ - p, amplitude to the relevant stimuli but did not measure N,, as mentioned in the foregoing.) In this stimulus paradigm there still is some basis for the differentialpreparation artifact (of which possibility the authors were aware and for the elimination of which the second experiment was conducted) : during the course of the experiment the subject possibly learned that for a very short period after each relevant stimulus another relevant stimulus will never occur, and that the more time that elapses since the relevant stimulus, the higher the probability of the immediate delivery of another relevant stimulus will be. (For details of this claim, see the foregoing discussion (pp. 252-253) on a rather similar stimulus paradigm of Eason et al., 1969.) In Experiment 2, tone-pips were delivered to the right and left ears according to a single sequence rather than to two independent overlapping sequences as in Experiment 1. The IS1 was randomized between 100 and 800 msec and each tone was delivered either to the right (800 IIz) or left (1500 I&) ear with equal probability. Now there was no possible basis for the differentialpreparation artifact. Experiment 2 was otherwise identical to Experiment 1 except for the omission of the reading condition. The results closely paralleled those of Experiment 1 although the effect observed was somewhat smaller this time. There are at least three factors in Experiment I which could account for this difference: (I) the possible differential-preparation artifact; (2) the longer mean ISI; and (3) the ‘buffer’ condition. Additionally, the EPs to the signal stimuli in the attended ear were separately averaged in both experiments, which indeed should be done in each related study where possible. A late positive ‘P3’ wave, peaking at a latency of 250400 msec, which did not exist in responses to standard tones, was reported’. Careful examination of their fig. 2 (reproduced here as fig. 2) reveals a large, relatively unpeaked positive wave which apparently should not be called ‘P3’ but ‘P300’ (see Wilkinson and Ashby, 1974). When also considering the clear enhancement of PZ in many of the signal EPs shown, and, on the other hand, the insensitiveness reported of this component in reflecting differences between the relevant and irreievant standard stimuli as we11 as the riding of the N2 component on a more positive baseline in signal traces, the most parsimonious “Also Yagi and Ohtani (1973), in an experiment with binaurally delivered auditory stimuli, found that the late positive component (peak latency 350 msec) in EPs to signals was greatly enhanced in comparison to EPs to non-signals under conditions jn whjch signals were not predictable. Also an earlier positive component with a peak latency of 180 msec was enhanced in signal EPs. (No earlier enhancement was detected.) Additionally, a negative component with a latency of 250 msec was found in the fatter EPs.

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signr1.. . . .. . . . . . . Standard

Right ear stimulus attend-right

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t”

5w

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Fig. 2. Vertex EPs to attended right-ear stimuli (left side) and to attended left-ear stimuli (right side), given separately for relevant standard stimuli and for signal stimuli. (Reproduced from Hillyard et al., 1973b (Science, 1973, 182, 177-180). Copyright 1973 by the American Association for the Advancement of Science.) for the EP differences between the signal and standard stimuli to the attended ear would lie in selective CNV resolution occurring after the signals (see especially the records from Subject D.W.). An additional feature of the signal EPs, perhaps supporting such an interpretation, is the last 501.50 msec of these records which suggest a clear CNV development selectively occurring after the signals: in four of the six traces shown, each point at the latency range 400-500 msec is more negative for the signal stimuli than for the standard stimuli (with reference to the respective levels at the stimulus moment). Apparently no CNV was revealed, in general, in these experiments, owing probably to the fast rate of stimulus presentation and to the difficulty of the task, keeping the subject continuously aroused; under such circumstances the CNV development after the signals was perhaps made possible by the more positive ‘baseline’ and by the short rest period which PsO,, might signify under such conditions (see Karlin, 1970). It might be possible that P,,, in signal traces is not at all related to their being signals but rather to their rare occurrence, which is characteristic of signals in explanation

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selective-attention studies. This proposition means to say that the deviating EP to signals as compared to that to ‘relevant’ standard stimuli is due to physical difference rather than to the former being signals. (This remark seems to apply to other related studies as well.) (It is especially to be noted here that the amplitudes of EPs both to the relevant and irrelevant standard stimuli are possibly selectively habituated for their high frequency of presentation under these kinds of experimental conditions.) This claim would be in line at least with the orientating-response view of PZoO(Ritter, Vaughan and Costa, 1968; Roth and Kopell, 1973; Friedman, Hakerem, Sutton and Fleiss, 1973)‘O. This proposition is lent credence by their observation that sometimes P, existed also to the ‘signals’ to the unattended ear. These two alternatives might perhaps be put into a critical test by introducing into an experimental situation, such as that of Hillyard et al.‘s second experiment, still another kind of stimulus (not to be responded to) deviating physically as much as the signal from the ‘relevant’ standard stimulus and presented as rarely and as unexpectedly as the signal’l. The authors suggested that ‘Nr and P, are signs of fundamentally different selective attention processes, corresponding closely to the “stimulus-set” and “response-set” modes of attention, respectively, described by Broadbent (1970). A stimulus set preferentially admits all sensory input to an attended channel (stimuli having in common a simple sensory attribute such as pitch, position in space, receptor surface or the like) for further perceptual analysis, while blocking or attenuating inputs arriving over irrelevant channels (for example, the unattended ear) at an early stage of processing. Response set is a subsequent processing stage in which sensory information is compared against memorized “templates” or “models” . . . for selected stimuli which are not distinguishable simply by virtue of belonging to a particular sensory input channel; a response set acts to facilitate the recognition of these specific task-relevant signals’ (p. 179). One check for the dual-stage view of stimulus processing proposed would have been to correlate the magnitude of the N, effect of a run with the amplitude of P, to the ‘signals’ delivered to the unattended ear. The prediction would be that involving a negative correlation : the more effective the stimulus-set, the less (or the easier) the work left for the response-set mode of attention. Wilkinson and Spence (1973), in a study aimed at clarifying in more detail the reasons for CNV resolution, also distinguished two stages of attention or decision in the EEG in their experimental situation: ‘First, the coarse, sensory recognition of the stimulus as one of a generally relevant class, e.g., a tone of some sort in the present setting (CNV resolution). Second, a finer analysis to determine future action, e.g., a high or low tone (redevelopment of CNV, i.e., point of divergence)’ (p. 507). (See also Harter and Salmon, 1972.) “See also Karlin and Martz (1973). “For example, if standard is of 1500 Hz and signal of 1560 Hz, then this additional would be of 1440 Hz.

stimulus

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As to P300 selectively following the signal stimuli, Hillyard and colleagues might be right in interpreting this as a reflection of the selective recognition of the signals (provided that the effect is not due only to the rarity or physical deviance of these stimuli). As the authors themselves observe, this reflection can be pretty remote; such late waves might be signs of a non-selective reactive change of state which follows recognition of the targets (see Karlin, 1970). In the foregoing, the possibility of slow selective resolution of negativity is brought up; the data seem to lend some support to this interpretation. By no paid to the stimuli means can PxOOin these data reflect selective attention delivered to the ‘attended’ ear as no PjOO was detected after the standard stimuli to that ear. The previously reviewed study of Hillyard et al. (1973b) is the only one among those investigations reviewed so far yroviding no basis for the subject to predict the order of the relevant and irrelevant stimuli in which a selective enhancement of a relatively early component (much before the late positivity) is reported. (The significance of this finding will be extensively dealt with in section 4). In achieving this result, the authors themselves emphasized the following features of their design: (I) the relevant and irrelevant stimuli differed from one another both in spatial localization and pitch attributes, making them easily distinguishable (see Ng&inen, 1967, pp. 17-19); (2) the fast rate of presentation of the stimuli; and (3) the difficulty of the task, which ensured that the subject really had to be selectively attentive in order to cope with the rask requirements. Ntigtgnen (1967, second experiment) as well as Hartley (1970) clearly differ from Hillyard et al. with respect to (2), whereas the Wilkinson and Ashby study seems to differ in all the three aspects. Wilkinson and Lee (1972), on the other hand, meet all the other criteria except for (1). As to (2), the mean ISI in the latter study was only from about 200 to 300 msec longer, but it might be possible that even this slight difference was enough for the CNV (and its resolution) to appear, which apparently did not generally take place in the former”. The important difference, relating to (I), between these studies was, however, the binaural delivery of each stimulus in the latter as compared to the presentation of the relevant and irrelevant stimuli to different ears in the former study in which it was probably much easier for the subject to be selectively attentive. This difference, however, may have also another significance: the possibility is not totally excluded that in their study the N1 enhancement was produced by a difference in the possible tonic contraction of the middle-ear muscles between the ears-the attentive state apparently was very intensive (e.g. Galambos, 1960; Hugelin, Dumont and Paillas, 1960; Horn, 1965, pp. 165-166 and 204;

‘*This IS1 difference might be related to the fact that whereas the CNV resolution in the Wilkinson and Lee study, presumably, took place after relevant stimuli, it only occurred to signals in the Hillyard et al. study, if at all.

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see, however, Moushegian, Rupert, Marsh and Galambos, son and Bettinger, 1970). The question remains whether

1960, and Thompthere exists selective

middle-ear muscle contraction in line with the laterality of auditory attention, i.e. had Hillyard and his colleagues cut the middle-ear muscles of their subjects or, preferably, short-circuited the middle-ear muscle controls through the use of a bone conductor to deliver stimuli to the basilar membrane (Horn, 1965, p. 175), would the Ni enhancement still have been observed (see also the study of Picton, Hillyard, Galambos and Schiff, 1971, reviewed in pp. 267268)13. On the other hand, on the central side we also can find no clear limit above which the locus of the effect cannot be: the results do not bear in any direct way on the question of how EPs in the main sensory pathways vary during selective attention as only vertex EPs were recorded. These results might perhaps show this better if it had been possible also to record the primary EPs of the temporal area (see section 4). In addition to leaving no time for attending to the irrelevant stimuli, point (2) might be of importance also in the sense of Lindsley’s (1969; see also

r3Quite recently, Schwent and Hillyard (Evoked potential correlates of selective attention with multi-channel auditory inputs, unpublished manuscript) have obtained data which they claimed to exclude unilateral middle-ear muscle contraction as a possible explanation for a selective (early) EP enhancement such as that obtained by Hillyard et al. (1973b). Tone-pips were presented with a random sequence at a rapid rate (averaging one tone every 225 msec) which came from four different sound sources or sensory ‘channels’, each having a different pitch and coming from separate, apparent spatial position (spaced equidistant across the head). Within a ‘channel’ of tones, some tones randomly were of a slightly higher pitch (‘targets’). The subject was to attend to one channel at a time and to count the targets in that channel. The N, component of the vertex EP elicited by a channel of stimuli when attended was considerably enhanced in comparison with the average of the three elicited by that channel when the other three channels were attended. The authors concluded that this N, enhancement could not be attributed to peripheral mechanisms acting on sensory transmission and that this N, enhancement reflects a ‘finely tuned’ selective attention to one channel of stimuli among several concurrent and competing channels. The evidence against the possibility that unilateral middle-ear muscle contraction existed in the subjects of this study is rather convincing; this does not, however, indicate that this phenomenon did not exist under the experimental conditions of Hillyard et al. (1973b). It might well be that the delivery of the auditory stimuli in the former study with four different (equidistant) apparent spatial positions (obtained by adjusting the loudness balance of the tone-pips in the right and left earphones) involved too slight ‘location’ differences between the stimuli for the organism to put the peripheral system concerned into action. (In such a case, the successful task performance completely rested on the other means of the organism to cope with such situations.) Hillyard et al. (1973b) used only two, and very easily separable, spatial locations (the feft and right ears of the subject) which way of stimulus delivery is much more likely to call the suggested lateral control system into operation. As to the interpretation of the finding reported by Schwent and Hillyard, it would be very important to know the earliest latency of the effect (for explanation, see section 4). Unfortunately, no sufficient information on this point was provided; it was only mentioned that, for two representative subjects, N, peaked between 80 and 130 msec after tone onset. Anyway, their effect appears to have a somewhat longer latency than that reported by Hillyard et al. (1973b). For implications, see especially the discussion on the latter study on p. 286.

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Lindsley and Wicke, 1974) remark with respect to Naatanen’s (1967) second experiment conducted with a randomized order of presentation of the relevant and irrelevant stimuli. Lindsley emphasized that it might be impossible to be really selectively attentive if there is considerable temporal uncertainty as to the occurrence of the relevant stimulus. Although the order of the relevant and irrelevant stimuli was randomized also by Hillyard and his colleagues, the ISIS were so short that the selective anticipatory attention to the relevant stimulus and the prior concentration on the difficult discrimination task were apparently very soon rewarded and reinforced by the delivery of such a stimulus and, consequently, this probably helped to maintain these forms of behaviour (Buckner and McGrath, 1963). This was presumably also the case with respect to the work of Wilkinson and Lee (1972), who apparently were the first to apply a sufficiently fast rate of stimulus presentation to EP experiments on selective attention; in the related non-physiological studies a fast rate of stimulus presentation seems to be a common feature. Hillyard and his colleagues (Picton, Hillyard, Galambos and Schiff, 1971) have more data of direct relevance to some questions concerning experimental control and interpretation of results raised in the foregoing. In this study they recorded click-evoked electrical responses of the human cochlear nerve from the external ear canal concurrently with the vertex EPs from the scalp. By introducing different auditory discrimination tasks they were able to observe a clear enhancement of the vertex EP but detected no change in the cochlearnerve response. The authors suggest on the basis of this finding and the earlier related reports that ‘attention is mediated not by selective gating of inputs at the periphery but by specialized processing of relevant stimuli at higher levels of the sensory system’ (p. 353). In the two first experiments, comparisons were made between potentials while the subject, instructed to ignore the clicks, was reading and while he was trying to detect occasional weaker clicks. The vertex EP enhancement can be due to an overall difference in the state of the organism between the conditions -one involved a performance task, the other did not-which difference was, perhaps, increased just before and at the moments of stimuli which were presented at regular intervals. The authors point to the saccadic eye movements during reading as a possible source for the effect observed, as certain eye movements were shown by Ebersole and Galambos (1969) to reduce cortical responses to clicks in cats. The third experiment is of more direct relevance to the present discussion : both ears received stimuli of which those given to one ear or to the other were relevant at a time. There might be three alternative interpretations to the vertex EP enhancement in addition to that given by the authors. (1) As the ISI range within the stimulus sequence for each ear was narrow, varying from 1.2 to 1.6 set, and as the two stimulus sequences were independent, there was a basis for the existence of the differential-preparation artifact (for details of this

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argument, see pp. 252-253 for discussion on Eason et al., 1969). (2) As the subjects were required to attend to the clicks in one ear and to write down the order of single and double clicks in that ear, they apparently were more often writing during an irrelevant stimulus than during a relevant one. This might be related, for example, via hand and eye movements (Ebersole and Galambos, 1969), to the amplitude attenuation observed in the vertex EP to the irrelevant stimuli. (3) Only one ear was used for recording the cochlear response (from a needle electrode inserted beneath the skin of the superior wall of the exterior auditory meatus under local anaesthesia). Such a unilateral arrangement might have caused the subject to put more effort into the task when this ear was relevant than when the other ear was relevant. (EPs to the stimuli delivered to the other ear were not investigated which may also have biased the experimentersubject used.) Thus, the specialized-processing interpretation given is far from being unequivocal. As to the unaffected cochlear nerve response within each of these experiments, it would be premature to conclude on this basis that differences in these responses did not exist between the attended and unattended ears in Hillyard et al.‘s second experiment. (The lack of this difference would mean that no major differences in the contraction of the middle-ear muscles existed between the ears.) This would be premature for at least the following reasons: (I) the ISI in each experiment of Picton et al. (1971) was much longer and was either constant or varied only within a very limited range; (2) in Picton et al.‘s Experiment 3 there was some objective basis for the subject to predict the order of the relevant and irrelevant stimuli-in their two other experiments all the stimuli were either relevant or irrelevant at a time--whereas no such basis was provided by Hillyard and his colleagues (second experiment); (3) the discrimination tasks of the former investigation, at least that involved in its third experiment, were easier than that of Hillyard et al.; (4) the continuous writing-down of the order of the single and double clicks in one ear in the third experiment of Picton et al. might have formed a considerable source of distraction in the middle of rapidly impinging stimuli dispairing selective attention and, perhaps, changing the subject’s set or working strategy, for example, from pre-stimulus anticipatory attention to post-stimulus analysis of the stimulus in the short-term memory (perhaps simply for being sometimes caught by the next relevant stimulus while still engaged in writing activities); and (5) only one subject was used. It is time to return to the previously reviewed Velasco et al. (1973) study whose second ‘attention’ condition was not subject to the differential-preparation artifact and whose results are of great interest in this connection : shocks to left or right forearms were presented in a random schedule with an average IS1 of 5 sec. The subject was instructed to press the lever with his right hand only when he felt a shock applied to his left arm and to ignore a shock applied to the right. Following this experiment he was instructed to press with his right hand

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for right arm shocks and to ignore the left arm ones. The amplitude of the late EP components was reported to be significantly larger to the attended stimuli. The earliest component showing enhancement apparently had a latency of some 90 msec. A PaoO kind of component appeared to both types of stimuli and there was no significant difference in the amplitude of that component between the two stimulus categories. This component had a similar amplitude over the vertex and left and right somatosensory scalp regions. Strangely, a very early (‘P2a’) component showed amplitude decrease to attended stimuli at 0.05 significance level which, however, is not taken seriously here unless the phenomenon is repeated. This is because of the peculiar direction of the change and the low level of statistical significance. The experimental setup and the result obtained somewhat resemble those by Hillyard et al. (1973b)---could this result be regarded as somatosensory equivalent to the rather convincing auditory finding involving relatively early selective attention effects on the EP by the latter investigators? The present answer is negative for the following alternative interpretations. The first thing to be considered in this comparison is the direction of the enhancementpositive in the Velasco et al. study. This leads in to Wilkinson and Karlin with their selective CNV-resolution and reactive-arousal explanations, respectively -the positive-going amplitudes might have been enhanced by a slow positive shift. The relatively short latency, approximately 90 msec, of the earliest enhanced component is no guarantee against this possibility as the discrimination as to which hand was shocked is an extremely easy and fast performance, especially because of the high shock intensity; it is possible that this discrimination preceded the earliest enhanced component by a great margin. It might also be possible that there were some differences in the posture or muscle tension between the attended and non-attended hands-in fact, paying attention to one hand may cause such peripheral changes which in turn may have some effects on EPs. No adequate control for this possibility was reported. Moreover, each relevant stimulus was instructed to be responded to by a maximally fast lever press. This may be of significance to the question concerning the real cause of the enhancement as motor potentials such as are usually associated with rapid hand movements may have summated with shock EPs to the attended stimuli, and the positive components of the former might have caused, or contributed to, the enhancement. Finally, the order of the two conditions was not counterbalanced with the consequence that the EPs to attended stimuli to each hand were recorded earlier than the EPs to nonattended stimuli. For this reason, the latter EPs might have been more attenuated by habituation. The study of Picton and Hillyard (1974) is also of direct relevance to the present discussion. In their basic paradigm, ‘standard’ clicks were presented to the subject’s right ear at a rate of 1/sec. Every 5-30 set the intensity of a single click (‘signal’) was lowered to the extent that it was possible for the subject

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to detect between 80 and 95 “/;:of the signals. During the ‘attend’ condition, he was asked to count the signals, during the ‘ignore’ condition to read a book and to disregard the ongoing auditory stimuli. This paradigm represents a clear regression from that used by Hillyard et al. (1973b), but the vertex recording of the very early EP components (see also Picton, Hillyard, Krautz and Galambos, 1974; Jewett, Roman0 and Williston, 1970) is of great interest. Fifteen distinct components of the auditory EP over the vertex could be consistently recognized in all subjects. Of these components of the EP to ‘standard’ clicks, only N1 and P, were substantially increased during the ‘attend’ condition. Based on their subjects’ reports and on the fact that the N, -P, changes occurred without significant change in the N, component shown by Picton et al. (1974) to be very sensitive to changes in ‘non-specific arousal level’, the authors suggested that the P, and N, enhancements observed ‘are indicative more of selective attention than of arousal or alertness’. This evidence for the equality of ‘non-specific arousal level’ is not convincing, especially in the light of the fact that one of the conditions involved a difficult task, the level of performance in which was measured, while the other introduced no task at all. Moreover, although the difference in the N, component was not statistically significant, the mean for the ‘attend’ condition exceeded that of the ‘ignore’ condition by one-third. Additionally, the demonstration referred to of the sensitivity of N, to changes in ‘non-specific arousal level’ involved a comparison only between sleep and wakefulness conditions. EPs to signal clicks were also recorded. The only reported difference to those to standard clicks was the appearance of the P, component. This was distributed more posteriorly on the scalp than the N1 and P, components. There was no difference in any signal EP component prior to N1 between the ‘attend’ and ‘ignore’ conditions. (This latter observation was yielded by a modification of the basic paradigm to include enough signals for such a demanding EP analysis.) This kind of detailed early EP component analysis would be a very desirable addition to an experimental paradigm like that of Hillyard et al. (1973b) in which strong evidence for channel-selective enhancement of the N, component was brought up. In weighing this result as possible evidence for the selective filtering hypothesis, it would be very important to know whether an enhancement of any EP component had already occurred prior to N, (see section 4). In Schechter and Buchsbaum’s (1973) experiment, four different tone intensities and four different light intensities were used in a pseudo-random order in each stimulus sequence which was the same in every run. They attempted to create six levels of attentional set by introducing the following tasks: (1) no instructions; (2) light intensity vigilance; (3) tone intensity vigilance; (4) light counting; (5) tone counting; and (6) distraction by mental arithmetic. Conditions (2) (4), (5) (3) (6) respectively, were regarded as

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forming a series of attentional states ranging from high to low attention to visual stimuli. A steady decrease in P, - NI (‘Ploo - Nlho’) amplitude (across intensities) was observed with diminishing attention (p < 0.01). AS to the P2,,,,)’ amplitude, a similar effect can be regarded as uncertain N, -P, (‘NM (the values for the five conditions were 19.87, 19.58, 18.38, 17.64 and 15.27 uV, respectively). The 0.05 significance was possibly reached for the low value of the distraction condition. It is, however, questionable, whether it was completely justified to include the distraction value to the statistical analyses performed. It is possible that the subject while subtracting sevens serially from 2000 did not properly fixate to the flashes (see Oswald, 1959). And as he was required to report verbally the result from each subtraction, the EPs were also possibly attentuated for these verbal activities. The degree of general arousal under this condition might also have deviated from that under the others-all the stimuli were irrelevant in the former in contrast to the latter-and possibly induced differences in EPs. That remark involving the inclusion of the distraction value into statistical analyses performed equally well applies also to P, - N,, but the statistical significance of the ‘attention effect’ in these data was by no means dependent on the presence of the distraction value in the analyses performed. On the other hand, it is possible that ocular accommodation and convergence (Oswald, 1959) were better, the more visual attention the task demanded and that the attention effect on P, - N, observed was in fact due to peripheral visual factors. (This remark is pertinent also to N, - P2.) No adequate control for this possibility was reported. The attention effect might also have been enhanced by the order of the conditions followed-apparently those conditions demanding more visual attention were on an average earlier in succession and, hence, suffered less from habituation effects, The importance of this point is shown by the observation that both amplitudes reached their highest values under the No Instruction Condition which was given first. As to the tone EPs, the amplitude values for conditions (3), (5), (4), (2) and (6) (the order of descending auditory attention according to the authors) for the Pi - N, amplitude were: 9.84, 8.23, 7.76, 8.73 and 5.89 uV, respectively, and for Ni -P,: 8.51, 9.90, 9.22, 9.05 and 6.81 uV, respectively. Both attention effects were reported to be significant at the 0.05 level. Attaining this significance level, however, was apparently critically dependent on the inclusion of the distraction values into the statistical analyses. Its justification can be questioned for the possible arousal differences between this and the other conditions and for the reporting activities (see the foregoing). In conclusion, in their methodologically interesting paper, Schechter and Buchsbaum (I 973) have not shown unequivocally EP enhancement related to selective attention. The reader is advised to familiarize himself with the interesting condition-intensity (and subject-intensity) interactions on the EP

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amplitudes found. For example, when visual EPs of a certain condition were generally large, the intensity effect tended to be small and vice versa. Debecker and Desmedt (1971) delivered binaural clicks at a regular interval (0.2-1.0 set) kept constant throughout a run. Each run included 50% signal clicks and 50% non-signal clicks of lower intensity delivered at random, in addition to occasional weak electrical shocks administered to the skin of one finger. Under one condition the signal clicks, under the other the shock stimuli were to be counted silently by the subject. Vertex EPs L3 signal clicks during the auditory and somatosensory tasks were compared. A ‘decision component’, a slow negative component with a peak latency of about I20 msec, was observed in response to signal clicks during the auditory task when the task was difficult, i.e. the intensity difference between the two click categories was small and the ICI was not shorter than 0.5 sec. When the intensity difference was large, the decision component was elicited only with very short (generally ~0.5 set) ICIs. No other EP differences between the tasks were reported and cannot be detected in the traces shown. On the basis of the traces shown, it seems possible that the decision component observed is a rapidly developing CNV (see Rebert and Knott, 1970, Posner and Wilkinson, 1969, and Gaillard and NBMnen, 1973) rather than an EP component. The CNV under these conditions might reflect discrimination activities. (Unfortunately, traces of non-signal clicks were not shown or described.) It is of considerable interest that no P, or PXoOkind of component to the signals was reported and cannot be seen in the traces shown. Neither amplitude values nor statistics performed on these were reported. P,-correlates of selectiw attention’“. rn addition to the single study by Hillyard et al. (1973b) reporting a statistically significant selective-attention effect on a relatively early EP component without providing the subject with any objective basis to predict the order of stimuli above the chance level and also being of high methodological standard in other respects, there are experiments with a similarly random design reporting only a late enhancement of the EPs to the relevant stimuli (e.g. Harter and Salmon, 1972). In the present author’s opinion, such changes cannot, because of their long latency, generally be codes (see Uttal, 1965) of decision processes related to the perception of relevance of stimulus; rather they belong to post-decision processes (see Hillyard et al., 1973b; Karlin, 1970; Wilkinson and Ashby, 1974). Such late changes are not codes of selective attention paid to stimulus itself but rather are related to its sequelae. Among others, the long and variable latency and generally large wave form observed in many of these studies point to a non-specific reactive change of state (see also Karlin, 1970). Note particularly the high correlaGons between the amplitude of the CNV and P,,, (and PhOOand P,,,,) 14No systematic study-by-study review of these works reporting which were most evident in a late positive component is attempted

attention-related changes in this connection.

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of Wilkinson and Ashby (1974) and their criticism of the earlier studies on this relationship; on the other hand, there appears to be no necessity or reason to equate the non-specific change of state concerned with a DC change or CNV resolution, at least as long as we do not satisfactorily understand the significance of the CNV (or more generally, of slow-potential shifts) in the behaving organism and its relationship with other physiological phenomena. In fact, even the apparently most intimate relationship between the CNV and the DC level is unmapped for the technical difficulties encountered in such an endeavour (see Gaillard and Naatanen, 1974). More systematically, at least the following evidence exists for the claim that what has been called P, or P3,,,, might in many studies in fact indicate a non-specific change of state (see also Karlin’s (1970) excellent treatment of the issue). and behavioural (1) P,oo occurs too late compared to the psychological phenomena whose direct correlate it has been claimed to be (see, e.g. Ritter, Simson and Vaughan, 1972); also the duration (width) of this component seems often to be too long to fit well to such an interpretation (see, e.g. Ritter et al., 1972). (2) The extensive latency range of the peak of Psoowhich according to Regan (1972, p. 144) is 200-550 msec, and the often relatively unpeaked wave form; in fact, often no discrete peak is found (see Jenness, 1972a, 1972b, Wilkinson and Lee, 1972, Wilkinson and Spence, 1973, Wilkinson and Ashby, 1974). The component-like appearance of Psoo with a peak and descending and ascending limbs observed in many studies is not necessarily an evidence against the CNV-resolution explanation of Pjoo (see Karlin, 1970, and Wilkinson and Spence, 1973); for example, the latter authors proposed on the basis of their data that some P, (Pxoo) waves in the literature may merely be ‘points where a resolving CNV gives way to the development of a further CNV in preparation for the next stimulus’. More generally, there is no reason why a spatially extensive change in the organism could not be transient (see later). (3) There is some evidence for a topographical and quantitative relationship between CNV and P,,,: e.g. their topographical areas appear to be affected in a similar manner by experimental manipulations (Donchin, Johnson, Hernong and Kutas, 1973a), but there also exist data showing topographical differences between the phenomena concerned (Hillyard, Courchesne, Krause and Picton, 1973). Such differences do not, however, appear as crucial evidence against the conception of Psoo as a resolution phenomenon of negativity because (1) there are many slow-potential generators in the brain (see, e.g. Donchin, Otto, Gerbrandt and Pribram, 1971, and Rebert, 1972) whose activity might be released to different proportions of the total ongoing activity of a generator, and (2) in situations showing dissociation, either Psoo or CNV, or both, may have summated with other kinds of waves with spatial distributions different from those of P,oo and CNV.

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P,,, was observed by Wilkinson and Ashby (1974) to correlate strongly with the pre-stimulus CNV amplitude, as also did P,,, and P,,,. This correlation was stronger than that between pre-stimulus CNV and N1 -P, amplitudes. For evidence for dissociation between P 3OOand pre-stimulus CNV amplitudes, see Donald and Goff (1971) and, for an alternative interpretation of their results, Wilkinson and Spence (1973) and Wilkinson and Ashby (1974). Some of the claimed dissociations might be explained by the fact that the P,oO amplitude seems to depend on both the pre-stimulus CNV amplitude and the ability of the stimulus to resolve it, as shown by Wilkinson and his group (see Wilkinson and Ashby, 1974). (See also Karlin, 1970.) If a P,,, occurring with no detectable CNV is referred to as evidence for their dissociation, the problem is how to know that a CNV of a very long duration (beyond the recording capabilities of the used time constants of recording systems or simply not visible for the shortness of the pre-stimulus trace) did not exist pre-stimulus; more generally, it is possible that at any moment during an experiment there always is ‘space’ for rapidly increasing positivity in the recorded brain structures, i.e. no floor efect exists-the baselineIs is in our records, not in the organism. This means to say that the CNV (as it is usually measured, i.e. with reference to the pre-S, baseline) represents only the latest change in the total degree of negativity of the brain area recorded. Therefore, even though the P,,, amplitude in fact would be closely related to the total degree of negativity of the same location, there necessarily is no correlation between the CNV and P300 amplitudes. This is because the former amplitude seems to be inversely related to the initial degree of negativity (see Gaillard and NBgtZnen, 1974 for a demonstration of the validity of the law of initial value with respect to the CNV amplitude). Thus, different experimental conditions, and perhaps different phases in them, are presumably associated with different degrees of tonic negativity (see Jenness, 197213,Khachaturian and Gluck, 1969, and Haider, 1970) which might, together with the superimposed shorterduration change related to stimulus contingencies (CNV) and with the ability of the stimulus to resolve negativity, determine the amplitude of the positive resolution. If this were the case and the CNV were negatively correlated with the basic negativity, a correlation between the CNV and P,OO amplitudes should not necessarily be expected. The latter points are strongly supported by the data from a signal-detection . . experiment (Naatanen, Gaillard and Mgntysalo, 1974) shown by fig. 3 in which S, was followed by slow positivity with no CNV observable in the pre-

l5In the sense that this concept is used in much recent EP and CNV work. It would be another story as to why the pre-trial degree of negativity would apparently soon be reached post-trial, accomplishing the component-like appearance of PjOO with an ascending limb (negativity upward). This is possibly due to the tendency of the organism to maintain a certain basic degree of negativity during a task, which degree apparently varies according to the general level of the task requirements.

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S1 traces. It is interesting to note that this slow positivity in the traces shown, as well as in the rest of the data, is maximal over the parietal area as also P 3oo is often known to be (see, e.g. Ritter et al., 1972). (Also Sz seems to be followed by slow positivity maximal over the parietal.) The traces on the right refer to a condition in which the probability of S, (a weak auditory signal correctly detected in about 90 “/, of cases) was 0.70; the traces on the left to a condition with a probability of 1.O. S1 was a dim visual signal well above the threshold and the subject was instructed to indicate the detection of S, by pressing one of the two buttons a couple of seconds after S,. (S, - S, interval randomly varied between 8 and 11 sec.) If there were a long-latency positive EP component to S,, with a generator structure not identical, or partially identical, with the generator of the CNV developing before S2, then it could not, after its period of occurrence, seem to ‘prevent’ the CNV from rising well above the S1 baseline, i.e. provide the CNV with an effective baseline. In plenty of earlier works, S, was observed to produce a large P,oo component whose amplitude seemed to be inversely related to, or exert an influence on, the amplitude of the CNV developing during the S1 - S, interval16. The most parsimonious explanation for such observations in the light of the data shown in fig. 3 would be that both the positive and negative processes are generated by at least partially identical generator structures. (See also the steady increase of negativity after the phase of the maximal positivity in these as well as in plenty of earlier data-there is no component appearance for this slow positivity in fig. 3.) Thus the P,,, positivity to S, might be a temporary resolution of tonic negativity prevailing during the experimental condition (or an enhancement of the P, component of the EP by this resolution). The preliminary analyses of these data support this proposition: there appears to be a positive relationship between the (tonic) negativity at the S1 moment and the amplitude of the slow positivity (NZGnen et al., 1974). The reader is referred to the excellent systematic review of Tueting and Sutton (1973b) on the CNV-P,,, relationship and to ‘The relationship between P,,, and the CNV’, a correspondence conducted in preparation for the Bristol CNV conference (prepared for circulation by Donchin (1973)). See also paragraph (8), below. (4) As pointed out by Hirsch (1971) and Wilkinson and Lee (1972), some data refer to the possibility that alpha-return time-locked to the stimulus could be easily mixed with P,,, (or could enhance the amplitude of that component). If Karlin (1970) is right in assuming that the detection of a target is ‘% the discussions held in connection with the excellent demonstrations of the Bristol CNV congress in August 1973 the question was raised whether the amplitude of the CNV developing during the S, - S, interval should be measured with reference to the pre-S, baseline or to the level of the peak off,,,. Again, if P,,, and CNV had separate generator structures, then the amplitude of the latter as measured, for example, I set post-S, should not be critically dependent on the amplitude of the former.

P: 1.0

PD

PZ0.T

loo0

msec.

Fig. 3. Data of two subjects (EN and PO) from four EEC channels as indicated in the left from a signal-detection experiment (N%itlnen et al., 1974). The traces in the left refer to a condition involving an & probability of 1.0, the traces in the right to a condition with an S2 probability of 0.7. ISI = f sec.

0,

Pz

c*

fz

EN

3

P 5

a”,

2 8;

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associated with momentary relaxation which in turn is known to release alpha, then this would be one way to explain the well-established experimental fact that PsoO is associated with such a detection. (5) The scalp topography of PzoO is much larger than that of the other stimulus-locked components and rather similar for both auditory and visual stimulation (Vaughan, 1969). (6) The scalp topography of P, evoked in different tasks is rather similar (Hillyard et al., 1973a). In that study, there were, however, some discrepancies among the topographies in some subjects, mainly involving an anterior displacement of the ‘no-go’ P3 relative to the others. According to the authors, such disparities might be caused by ‘the addition of more or less CNV fall-off, motor potential, or “orientation response” (Loveless, this symposium) to an underlying “unitary P3” ‘. As the alternative, they bring up the possibility that ‘The P,s in the various tasks arise from similarly disposed but distinctive brain systems’. (7) There is evidence that P,,, could be a cerebral component of the orientating response (Ritter et al., 1968; Roth and Kopell, 1973; Friedman et al., 1973). According to the lastmentioned study, there is good correspondence between P,,, and the pupillary dilation response. The possibility is not, of course, excluded that PJoO forms a specific part of the orientating response. (8) Pzoo might, at least sometimes, form a part of an also temporally larger response pattern (see also paragraph (3) above and fig. 3). An increased late positivity is often associated with an attenuated or positively displaced Nz component (e.g. Friedman et al., 1973; NBBtanen, 1967, figs. 14-18 and 40-55; see, however, Harter and Salmon, 1972), sometimes also with an enhanced P2 (Friedman et al., 1973; Adams and Benson, 1973) or P, component (Friedman et al., 1973). Such earlier changes, together with changes in very late positivity such as indicated by the measures PhOOand PsOO in the Wilkinson and Ashby (1974) study, suggest that the P 3OOcomponent under many conditions might form only a part of a longer-duration positive response (or resolution) whose maximal amplitude might be reached around 300 msec post-stimulus. (This timing of the maximum and the EP-component kind of appearance of the PjOO might be partially artificial as the earlier slow positivity might to some extent be hidden by overlapping EP components, the very late positivity by too short post-stimulus traces, by response and by ocular and other movement artifacts, and both the early and very late positivity by averaging single slow positive waves with greatly varying (see Ritter et al., 1972) onset and offset latencies.) This kind of interpretation is also supported by the fact that in many studies it was di~cult, or impossible, to separate the P2 and P3 components (see, e.g. Squires, Hillyard and Lindsay, 1973). This seems to be the case especially when the N, component is missing, being thus unable to cut the slow positivity into two components.

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(9) In the light of the experimental evidence available, PJoO seems to be associated with, or a correlate of, many different kinds of behavioural events, e.g. signal detection, resolution of uncertainty, motor inhibition, orientating response. Such a many-sidedness seems to favour the non-specific interpretation of P,OO; furthermore, these correlates are generally of such a kind that they are, presumably, associated with changes in the state of the organism (arousal decrease), occurring, e.g. with release of attention (detecting a signal, resolution of uncertainty, perhaps also orientating response) or of motor preparation (motor inhibition). (10) PjoO probably correlates with many kinds of non-specific physiological changes (as also CNV possibly does). This question is, unfortunately, rather unexplored, but the convincing demonstration of a close relationship between PsO,, and pupillary dilation response by Friedman et al. (1973) points to the existence of more non-specific correlates for PsoO as the pupillary dilation response is known to correlate with many physiological responses. A fruitful research line in attempting to clarify the issue around P300 (e.g. to which degree such positive phenomena in post-stimulus traces indicate a non-specific change of state) might be to try to separate the specific and non-specific poststimulus events from each other by examining all the other recordable changes following the stimulus and their temporal relationships with PsoO. In the foregoing, much space was devoted to discussing the question of the specific/non-specific nature ofP, or PsoO component. It should be remembered, however, that whatever the correct answer to this question, P3 cannot be a code of selective attention for its long latency; although it is well established that P,,, correlates with the degree of relevance of the stimulus, the use of this late change in research on the neurophysiological basis of selective attention is rather limited for its long latency in comparison to that of the behavioural phenomena involved. For example, the basic question in the field involving the possible existence of a selective filtering mechanism as a neurophysiological basis of selective attention cannot for obvious reasons be approached by studying changes in this component. The evidence referred to in (I)-(10) might give reason for the following kind of hypothesis regarding the ‘realness’ or independence of the middle- and long-latency components of the EP which is called the three-component hypothesis of the EP: there are two negative peaks in succession, those usually called N, and N2 in the literature, in addition to a temporally overlapping longer-duration positive process which might start already at 50-100 msec post-stimulus, peak at 200-500 msec post-stimulus and last up to 1 set but might under some conditions be of rather short duration (terminating already before or at the latency of N2 and appearing as the ‘Pz’ component). Consequently, P, would be nothing but the turning point of the positive-going trace (starting slow positivity) in the negative direction because of the onset of the N1

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component, P2 the most positive point between the two negative components carried into the positive direction by the underlying, now already stronger positive process, and P3 a component composed by (I) the positive-going limb of N2 and the negative-going limb of the slow positive process, or (2) a latter turning point of the slow positive process. The interpretation given to P2 is supported by, e.g. the same direction of changes in the P2 and P3 amplitudes (e.g. Adams and Benson, 1973; Friedman et al., 1973) and difficulties in separating these components especially when N, is lacking (e.g. Squires et al., 1973). The data showing the constancy of the Pz latency versus the variable P3 latency (e.g. Ritter et al., 1972) are by no means in conflict with the present hypothesis as the former latency is strictly locked between the (relatively constant) Nr and Nz latencies. The evidence for the relationship between P, and the other positive components is weaker but not non-existent (see, e.g. Friedman et al., 1973). According to the previously presented hypothesis, under conditions in which the slow positivity is strong, the Nz component should be mainly or entirely located below the baseline ~negativity downwards) {f de posiriu~t~ real/y continues during the occurreme of N2. There is plenty of evidence for that (e.g. Naatanen, 1967, figs. 14-18 and 40-5.5, and Friedman et al., 1973, in which latter study there also existed a relationship between the distances of the N2 and Ps peaks from the baseline, both peaks generally being below it). The N, and N2 components need a stimulus for their evocation, whereas the slow positivity is considered a sudden change in the state of the organism usually, but not necessarily, caused by a stimulus (a momentary release of tonic negativity sustained during wakefulness associated with conditioned, often unconscious anticipation and preparation for the next stimulus”). One prediction would be that the amplitude of the slow positivity would be directly related to the degree of negativity at the stimulus moment. As mentioned in the foregoing, this prediction cannot, however, be tested by measuring the increase in this negativity during the last 1 set period or so (the ‘CNV’) as (1) it does not indicate the total amount of negativity at the stimulus moment; and (2) in fact, it correlates negatively with the initial degree of negativity (Gaillard and NZSuren, 1974). As already mentioned, the preliminary data analyses of a signal-detection experiment (Naatlnen et al., 1974) have shown that the tonic negativity at the S1 moment correlates positively with the amplitude of the slow positivity to S,, thus tentatively corroborating the aforementioned prediction, The extensive topographical distribution of P, fits well into the idea involving P3 as a non-specific change of the state, or as part of it, whereas the “The proposed kind of continuous state of preparedness might often be non-specific in the sense that no particular stimulus is expected, but what is expected is that something will occur soon. The latter kind of expectancy is continuously reinforced.

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considerably less extensive distribution of P, (Vaughan, 1969; Hillyard et al., 1973a) seems to be somewhat problematic to the hypothesis discussed : either there exists an independent P, component (which might summate with the slow positivity) or the slow positivity while gaining in amplitude expands also topographically in time. For the next, a short list of possible artifactual sources of P,oo kind of positivity or its enhancement is given. (1) Stimulus-locked eye artifacts (see, e.g. Rietveld et al., 1966; Waszak and Obrist, 1969; Karlin et al., 1970, fig. 1; Weerts and Lang, 1973) which might more often follow relevant than irrelevant stimuli (because of a motor response associated with the relevant, but not with the irrelevant stimuli, or for some other reason). In a study on the relationship between the CNV and simple RT, Naatanen and Gaillard (1974) observed a relatively sharp positive EOG potential peaking about 300-400 msec post-S, (fig. 4). (2) Long-latency positivity can also be caused by instructed motor movements selectively following the relevant stimuli (as the case may have been in the Corby and Kopell study (1973) in which the relevant stimuli required a fast motor response) or by more involuntary movements (see, e.g. Vaughan, 1969, p. 51, and McAdam and Rubin, 1971). In some experimental situations, the existence of PJoo might perhaps be best explained by its being a correlate of (motor) response inhibition (see, e.g. Waszak and Obrist, 1969, and Karlin et al., 1970r8). (3) Finally, a word of caution should be added with respect to the aspect of time-lockedness of single responses whose degree may systematically vary between experimental conditions and thus produce artificial amplitude differences in averaged records (see, e.g. Hillyard et al., 1971). This point is problematic in all research in which characteristics of single responses have to be inferred from their averages, and especially so with respect to P,,, for its highly variable latency (see, e.g. Ritter et al., 1972). What was said in the foregoing does not mean to claim that no discrete P, component exists in any experimental condition or that that component would not be of psychological and behavioural value (for a review, see Sutton, 1969, and Tueting and Sutton, 1973b). However, many components claimed of this component might to be P, or P,“,, and many reported enhancements in fact signify ‘only’ a non-specific change of the state of the organism (occurring most often after the relevant stimulus) or be of artifactual origin (see the lists given above). The time is not yet ripe to decide with certainty what P3 or Psoo really signifies under the different conditions where such a long-latency

‘*In this particular experiment, the enhanced P3 to the no-response stimulus of a ‘go-no go’ reaction-time task might perhaps be also explained by a faster and, hence, better time-locked resolution of the pre-stimulus negativity-when a response is required by the stimulus the negativity might resolute somewhat later. (For some results of this experiment, see p. 254.)

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500 msec. Fig. 4. An EOG artifact time-locked to Sz in a simple RT experiment. Note the similarity to ‘P3’ or ‘P300’. Each EOG trace represents an average of 16 trials. Positivity upward. (Unpublished data in connection with Nlltlnen and Gaillard, 1974.)

positive component has been recorded. ‘God knows what P, is’ (Hillyard, personal communication, August, 1973). Ex~eri~e~rs with ineun~~~~~~ stir&i. Evidence that EPs do not reflect meaningfulness of the stimuli eliciting them (and, thus, associated kind of selective attention) comes from the second part of the study by Roth et al. (1970). They compared vertex EPs to auditory sense and nonsense monosyllabtes randomly spaced in a list from which the subject had to distinguish nonsense words from meaningful words. The sense words in sequence formed a

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simple story to be detected and remembered by the subject. No EP (amplitude or wave form) differences were found between the two syllable categories. This result is in agreement with that of Feldman and Goldstein (1967) who, however, studied EP latencies rather than amplitudes. Corroborating results were also provided by Shelburne (1972) who, rather than syllables, delivered visually three single letters of the alphabet at 1 set ISIS. The unpredictable third letter determined whether a word or nonsense syllable was formed by the combination of the three letters. The subject’s task was to indicate whether a word or nonsense syllable was presented. The EPs (vertex, right parietal, left parietal) to the third, decisive letter were similar in both cases, but the late positive component (especially that recorded over the vertex) was enhanced as compared to that to the first and second letter. This enhancement was attributed to the decision-making process. Shevrin, Smith and Fritzler (1971) found that larger EPs were elicited by a subliminal meaningful stimulus than by a closely resembling subliminal meaningless stimulus when the stimuli were delivered in a random order. The study was conducted with twin pairs and in one condition it was found (p < 0.05) that the twin with a larger EPamplitude to the meaningful stimulus produced (when a free association procedure was used) a greater amount of veridical verbal associations than the other twin. It was concluded that ‘it would seem reasonable to assume that an act of attention associated with veridical perception and association processes can take place entirely unconsciously and may be reflected in properties of the AER’ (p. 159). There were, however, some physical differences between the two kinds of stimuli: the contrasts in the meaningful stimulus were apparently more effective (regarding the response elicited in the peripheral visual pathways), and this may have caused the EP differences found (see Spehlman, 1965). The authors claim that the correlation between verbal effects and EP components rule out any interpretation entirely based on this difference and emphasize that symbolic meaning must be involved. It is possible, however, that the verbal-association-EP correlation was due to either permanent or temporary intraindividual differences within a twin pair. As to the latter kinds of differences, it might, for example, be that there were systematic intraindividual differences in preparation or EEG arousal at the stimulus moment and that these differences were related to differences in both EP amplitudes and in verbal associations. To sum up this section dealing with EP experiments with meaningful stimuli conducted without the differential-preparation artifact, the central tendency in the results obtained seems to be that meaningfulness showing up not earlier than the stimulus itself produces no or only late EP enhancement (whereas those related studies reviewed on pp. 2444246 have shown that u priori (known in advance) meaningfulness of a stimulus produces many kinds, also rather early, EP changes).

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4. Concluding discussion 4.1. The basic question invohing sefective.@ters Although the basic question or goal, at least of a great deal of psychophysiological work on selective attention, seems to have been put asidein much related EP research-many workers seem only to be interested in knowing whether the so-called ‘attended’ stimuli elicit larger-amplitude EPs than the ‘unattended’ stimuli (see also Vaughan and Ritter, I973)-it still seems to be much the same as it was during the most impressive days of ~ern~ndez-Penn (and his group) almost two decades ago who never lost sight of the goal : is (the psychofogical and behavioural phenomenon qf>selective attention based on selective jiltering or blocking qf sensory impulses ? It appears to this author that this question involving the possible existence of selective filters is a basic one in the field which should be reliably answered before questions involving further distinctions and later post-stimulus processes are introduced. This solution would contribute in a fundamental way to the understanding of attention phenomena and would provide data of crucial importance with regard to choosing between the present ‘psychological’ attention theories (see, e.g. Swets, 1970). If this basic question is kept in mind, experimental planning in EP studies on attention might become more adequate and at least some of the confusion in interpreting research results might be avoided. In the following, an attempt will be made to clarify and elaborate the selective-filtering hypothesis of selective attention (when stimuli with short duration are used). Selective filtering should be understood to involve a passive process on the part of the organism; the differential treatment of the ‘relevant’ and ‘irrelevant’ impulses would be tractable to pre-stimulus events resulting in what could be called a selective sensory state. In such a state, the pathways of the attended modality are facilitated while those of the unattended modalities are inhjbited. Consequently, differences in the size of the EPs to the stimuli of the relevant and irrelevant modalities are a simple reflection of selectively pre-modified impulse-conduction capacity of the afferent sensory pathways. The stimuli of the (in advance) irrelevant modalities ‘collide’ with pre-set gates or filters, whereas the ‘open’ pathways of the attended modality let in any stimulus of that modality. No stimulus-related decision since stimulus onset could affect afferent impulse inflow caused by this stimulus (such an effect would appear as a neurophysioIogjca1 impossibiIity). In the foregoing formulation of the selective-blocking hypothesis, the objects of efferent control have so far involved anatomically and physiologically very clearly distinguishable entities, the pathways of the different sensory modalities, but similar selective pre-stimulus efFects could, in principle, as well also operate within any single sensory modality within the limits of (&I) separable afferent systems (see also NBSinen, 1967, pp. 1X-19). If such a selective e&rent control mechanism does not exist, then the chains of physiological events caused by relevant and irrelevant stimuli could

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not reflect the degree of stimulus relevance until certain phases of the stimulus processing are reached in which information concerning stimulus relevance is obtained. (Later in this paper, the nature and timing of such decision processes as a function of the experimental situation and the implications on the related EP research will be extensively discussed.) In the related human EP research it might be that selectively attentive states induced in subjects have mostly not been differentiated enough psychologically and physiologically, and that some of the negative findings have resulted from this (e.g. long ISIS and too easy or distracting tasks; see Lindsley, 1969; Hartley, 1970; Wilkinson and Lee, 1972; see pp. 294-295). How, then, to study the selective-filtering hypothesis by using the EP techniques with intact humans ? The following division might be useful in dealing with this problem. There seems to be two different series of physiological processes in a row between the stimulus and its scalp-recorded EP : (a) that between the most peripheral effects of the stimulus (e.g. in audition the cochlear-nerve response with a latency of some 2 msec; Picton et al., 1971) and the first effects in the primary cortex; and (b) the ensuing intracerebral processes, taking a much longer time. (This picture is, of course, further complicated by the extralemniscal pathways whose role in the formation of the total EP complex is not satisfactorily understood; it is, for example, probable that such effects can have a surprisingly short latency (Jewett et al., 1970).) If the primary response of a primary sensory cortex (which some subjects might reliably show under certain conditions) were found to be selectively enhanced this would probably indicate facilitated or increased afferent inflow, provided that the receiving primary area is not in a state of higher excitability or responsiveness than areas in which the irrelevant impulses arrive. (Also such topographically selectively increased excitability, which could be studied by recording, for example, spontaneous pre-stimulus activity over different cortical areas, might form the neurophysiological basis for selective attention.) For this and many other reasons (Horn, 1965; Worden, 1966) it is dangerous even with regard to the primary responses to make conclusions about the afferent sensory inflow on the basis of scalp-recorded EP data. As to the later EP components, and other cortical recording sites than the primary sensory areas, this seems still much more difficult for the possibly increased number of cerebral structures involved and for their functional interactions, and the already overwhelming difficulties are further increased by the possible changes in the excitability of the areas, centres and pathways involved in different phases of the post-stimulus process. Referring to all these and other difficulties in interpreting an attentionrelated selective EP enhancement the author, somewhat exaggerating the point as his habit might be, urges at least some of those people who are not merely interested in knowing whether the ‘relevant’ stimuli evoke larger EPs than the ‘irrelevant’ ones but want to know something about the physiological

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mechanisms of selective attention, to reopen the door of the animal laboratory, (shut most recently in the Introduction to this paper). Although a selectively attentive state might be more difficult to produce, at least to vary, in animals and many kinds of controls might be more problematic to arrange with them, it should seriously be taken into account that-provided that these problems are satisfactorily overcome-in interpreting the results many invincible interpretational problems of human data are not encountered. The methods of animal experimentation referred to in this connection are, of course, stereotactic (also single-unit) recording methods by means of which it should, in principle, be possible to trace post-stimulus organismic processes phase-byphase and thus to determine the points and respective degrees of diversion in the processing of the relevant and irrelevant stimuli. 4.2. DYerent phases in the stimulus processing In an attempt to trace the physiological processes of selective attention as well as of other higher mental functions by using the EP techniques with human subjects, a major obstacle seems to be the lack of firm knowledge with respect to the phase at which the discrimination of the stimulus can take place. This phase seems anyway to be so early that the late positive process (P, or P,,J which during the last few years has drawn most attention, among the host of EP components, of workers in this field, cannot, as explained in the preceding section, be a code of the discrimination process, but a clearly more problematic and perhaps much more important issue relates to how early such a discrimination takes place. This might be an important question even in interpreting the enhancement of such an early EP component as that observed by Hillyard et al. (1973b). Tentatively, if such a discrimination process took place (perhaps in a very elementary form) in a very early post-stimulus phase, a selective effect even on N, (the effect was reported to occur already at a latency of 60-70 msec in most subjects) could be a reaction to a particular outcome of some early step in the discrimination process rather than indicative of pre-set filters which differentially modify afferent activity of sensory pathways (i.e. selective sensory state). It would, of course, be a fundamental error to believe that stimulus discrimination is a discrete event in time; the stepwise, or process, nature of discrimination was already referred to earlier in the present review (e.g. Smith, 1963) and it was proposed that the first step in this process would be that leading to, or associated with, a diffuse experience of ‘something is happening’. Among the products of the next steps of this process in these kinds of experimental situations would perhaps be, for example, some kind of awareness of the modality, of the part of the organism stimulated, of the direction of the stimulus source from the organism, and such more general aspects (these all depending, among others, on the modality and more specific features of the stimulation, certain aspects of the organismic state, and receptor conditions

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and orientation at the moment of stimulation) before the finer steps of the process (see, e.g. Smith, 1963, and Lehtiii, 1970). In the Hillyard et al. (1973b) study, the discrimination between a relevant and an irrelevant stimulus could presumably take place very soon after the stimulus onset, as only such an elementary phase of the process as that involving the laterality (left or right side) of the stimulus source (or the ear stimulated) was necessary. It might be possible to reach this post-stimulus stage well before 60-70 msec from the stimulus onset (the latency of the effect reported). If this were the case this might explain the selective enhancement observed: for examp!e, it might be that some of the cerebral tissue whose activity is recorded via the vertex electrode, reacted fast enough to the first ‘experience’ (or to its physiological correlate) of, or related to, task relevance (in this case to the ‘left tone’ or ‘right tone’, depending on which side is relevant). Such a fast reaction might perhaps be rapidly increased excitability or impulse activity related to preparation for, or performance of, the difficult pitchdiscrimination task involved and this (rather than differences in afferent inflow between relevant and irrelevant standard stimuli) might be reflected by the enhanced N, component. Moreover, it is also possible that Ni enhancement in a more direct way reflects the positive outcome of the decision between a relevant and an irrelevant stimulus. Thus, it is not unequivocal that the selective enhancement reported reflects pre-decision processes, such as impulse inhibition in the afferent pathway of the unattended ear or selectively increased excitability somewhere along the ‘relevant’ afferent system, notwithstanding the early latency of the effect. Again, we have to emphasize the necessity of experimental tracing of still earlier effects in testing the selective-filtering hypothesis of selective attention. The authors themselves seem to favour a filter explanation: ‘The early latency of the attention effects upon N, suggests that the underlying attentional process is a tonically maintained set favoring one ear over the other rather than an active discrimination and recognition of each individual stimulus’ (Hillyard et al. 1973b, p. 179). A similar explanation can also be applied to many other related studies. The earliest enhancement in the well-controlled investigation of Harter and Salmon (1972) occurred in a negative component of the occipital EP peaking as late as between 220 and 250 msec from stimulus onset. (This observation reached only the 0.05 significance level.) The visual modality and the relatively dim stimuli used account for part of this long latency, but most of the delay in comparison to the Hiltyard et al. (1973b) study might be attributed to the same or partially same afferent sensory channels along which the impulses from the relevant and irrelevant stimuli proceeded upwards. (Stimuli were projected on the same or partially same retinal area.) For the effect with a latency of approximately 130 msec of the Eason et al. (1969) study, many alternative explanations have already been proposed

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in the preceding section. In their task, relevant and irrelevant stimuli were delivered to different retinal areas, relevant to the right and irrelevant to the left visual field of the subject or vice versa. This apparently greatly expedited discrimination between relevant and irrelevant stimuli in comparison to the Harter and Salmon (I 972) study. The rank order of the latencies of the effect observed in these three investigations seems clearly to reflect the time needed to discriminate between relevant and irrelevant stimuli in these respective studies. The implication of this discussion on the latency of different post-stimulus decision-making or discrimination processes to the EP research on selective attention is clear: special care should be taken in designing the task-relevant feature of the stimuli of the task. If the main question to be asked is, at least implicitly, whether efferent selective modulation of afferent inflow is operating during selective attention or not, the moments of occurrence of certain phases of stimulus processing should be of special importance: this is because the question with every claimed demonstration of a selective effect would naturally be whether the enhancement is related to pre- or post-decision processes. (Only data involving pre-decision processes bear direct relevance to the question of the selective efferent control.) This point often seems to have been partially or completely ignored in the phase of data interpretation and, what is more unfortunate, in experimental planning, namely, in determining the way in which certain stimuli are ‘task-relevant’ and in which some ‘task-relevant’ stimuli are signals. By varying (a) different properties of these stimuli especially with respect to each other, i.e. the dimensions of their similarities and differences and interstimulus distances along these dimensions (for a more systematic presentation of this point, see later); (b) instructions (e.g. emphasis on speed versus accuracy); probably also(c) pay-off and punishing systems; and (d) permanent and temporal characteristics of subjects, it apparently is possible to a great extent to determine which kinds of discrimination processes will be involved, how profound or risky they will be, at which latencies and in which order they will take place (see, e.g. Lehtio, 1970) and so on. Thus, the experimenter can greatly affect the nature and timing of the possible two main types of decisions presumably occurring serially (see, e.g. Hillyard et al., 1973b; Wilkinson and Spence, 1973) in the kind of experimental work reviewed : (I) is the stimulus ‘task-relevant’ or not; and (IT) is the ‘task-relevant’ stimulus a signal or not. In principle, both these types of decisions can involve many kinds of dimensions in which stimuli may vary. The most parsimonious and practical subclassification for these dimensions might be as follows19: “Since this was written, the author has become aware of a somewhat similar distinction by Weinberger (1971) in connection with his important analysis with stimulus processing during attention.

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(1) the temporal and probability aspects qf‘ the stimulus (regularity versus irregularity and duration of the ISI, temporal relationships with other events; hence, predictability versus unpredictability from the subject’s point of view); (2) the spatial aspects of the stimulus of which there are two kinds: distal and proximal in the sense of Brunswik (1956) or, which division seems to be largely the same, allocentric and autocentric (Worden, 1966); to concretize this subdivision, a sound can be localized to the left or right (outer-world) side by the subject or to the left or right ear; (3) the sensory modality of the stimulus; and (4) thephysicalproperties qf’the stimulus for which there is a host of dimensions of which some are specific to a certain sensory modality. In experimental planning, all these aspects should be adequately taken into account as these, in interaction with the instructions, the pay-off and punishing systems, and the permanent and temporal characteristics of the subject (b)-(d) presumably determine (pp. 2922293), what the subject actually will do in the experimental situation, i.e. to what we are searching for a physiological substrate. These factors are assumed to determine what strategy the subject will adopt, usually, hopefully, to maximize his success in the experimental task, sometimes only to earn his $1.50 with the least possible effort. (It would indeed be, as Sutton (1969) maintains, very important to reduce what he calls ‘subject option’; see also Vaughan and Ritter, 1973). Hence, these factors are proposed to determine in which phase of stimulus processing the critical decisions (I and II) are made and, consequently, which processes are pre- and which post-decision processes (relative to each kind of decision). To take two examples: in the Hillyard et al. (1973b) study, decision I could apparently be reached at a very early phase because of the subjectively very clear difference in the spatial aspect between the relevant and irrelevant standard stimuli (whereas the temporal and modality-aspects could not be used for decision 1 for the lacking objective basis). Additionally, if a selective filtering mechanism was reducing the afferent inflow of the unattended ear, then it is reasonable to believe that the discrimination between the relevant and irrelevant stimuli was still easier and earlier for the additional difference in subjective intensity. If decision I was negative, the discrimination process probably terminated (for self-terminating models; see, Lehtiii, 1970) and the trial was over from the subject’s point of view; if it was positive, decision I1 was made on the basis of Hz value (pitch) of the tone as there was no other objective basis for this decision. The dual-decision view herein introduced and the interpretation given by Hillyard et al. (1973b) to their results agree with respect to the nature of the discrimination process between the ‘relevant standard’ stimulus and the signal, but disagree with respect to the easier and apparently earlier discrimination between the relevant and irrelevant standard stimuli. For Hillyard et al., this apparently is accomplished by selective filters or, to use the terminology

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of this section, selective sensory state, whereas according to the present view, the latter discrimination is also accomplished by an active discrimination and recognition of each individual stimulus. In the Wilkinson and Ashby (1974) experiment, the ISI was constant at 1 set and the relevant and irrelevant stimuli were delivered either in the alternating or randomized order. There was a clear Hz difference between the two kinds of stimuli but the signals differed from the relevant and irrelevant standard stimuli in duration. The possibility was suggested (p. 260) that under the unpredictable conditions the subject might have utilized only the stimulus duration in his search for the signal stimuli (this might have been subjectively easier or more successful) and thus skipped decision I which the authors were studying. This possibility receives some support from their result that the amplitudes of the EPs to both relevant and irrelevant (standard) stimuli under the unpredictable condition were equal and approximately equalled those of EPs to relevant stimuli in the predictable setting. (The authors themselves doubted the real irrelevance of the so-called irrelevant stimuli in the unpredictable setting on the grounds of the necessity to discriminate between the relevant and irrelevant stimuli by paying attention to the physical characteristics of all the stimuli.) Under the predictable runs of the same experiment, decision I was apparently completely made on the basis of the temporal aspect” and upon arrival, or already in anticipation, of the ‘relevant’ stimulus the subject solely concentrated on decision II, keenly attending to and estimating the stimulus duration. It would not be completely certain, however, that a possible objective basis for a significant degree of subject-option had been eliminated in this study if the signals had been ‘traditional’, i.e. in this case deviating by a small frequency difference from the ‘relevant’ standard stimuli so that all the three kinds of stimuli would have varied along the same dimension. (This remark apparently concerns all those studies which have utilized the unidimensional stimulus variation under the unpredictable situations.) This is because of the possibility that decision I might perhaps be skipped by the subject also under such experimental conditions (without the experimenter knowing it). This means that what the subject might do is to compare the template, or what have you, directly with the other kinds of stimuli, with the ‘relevant’ and ‘irrelevant’ standard stimuli. In such a case it is only natural that no at least relatively early EP differences could be discovered between the ‘relevant’ and ‘irrelevant’ ‘“The temporal basis might be a preferred basis for such decisions; it can be utilized already before the delivery of the stimulus, being, thus, the first possible basis offering itself in time for the subject for decision; furthermore, the utilization of this basis is advantageous in terms of energy expenditure and it allows short rest periods for the subject during the strenuous experimental task. (There is presumably some post-stimulus control mechanism to ascertain that the stimulus expected to occur at a certain point of time meets the pre-set criteria.)

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standard stimuli as they will be similarly processed until the (relatively late) stage of the comparison with the template. It might be justified to describe such an experimental situation so that there are, in fact, no ‘relevant’ and ‘irrelevant’ standard stimuli but, instead, there are two other kinds of stimuli besides the signal, one more and one less resembling the latter. (it is, of course, evident and well established that a bigger difference is detected earlier than a smaller difference.) It appears that which strategy or manner of processing is going to be realized by the subject greatly depends on the inter-stimulus diferences along the dimension in which the stimuli differ from each other. Thus, apparently, the risk of eliminating decision I is increased by (a) increasing the difference between the ‘relevant’ standard stimulus and the signal, i.e. by making the task easier, and (b) decreasing the difference between the ‘relevant’ and ‘irrelevant’ standard stimuli. A systematic parametric study in which these two presumably critical differences and their proportions are varied might probably reveal the limit values for the occurrence of decision I proposed. Earlier in this review (pp. 250-251), it was maintained that under the unpredictable setting, no stimuli, even those called irrelevant, can really be subjectively immediately irrelevant (see also Wilkinson and Ashby, 1974), but have to be processed until the negative decision of the phase which we now can identify as decision I is reached. (In case the irrelevant stimuli use different afferent sensory systems than the relevant ones and the sensory impulses caused by the former stimuli are effectively blocked by efferent selective control mechanisms, no decision I will be made, of course, as the task is taken care of by pre-set filters). As this negative decision cannot be made on the temporal basisZo it must be made on the basis of spatial, modality or specific physical aspects of the stimulus, all of which require some stimulus processing. It is, for example, clear that, as mentioned in the foregoing, the greater the physical difference (e.g. in Hz) between the ‘relevant’ and ‘irrelevant’ stimuli, the faster decision I is reached. (This can be inferred, for example, from the results of choice-reaction experiments.) And, as maintained in the foregoing, to allow clearly separate afferentj sensory channels for these two kinds of stimuli as Hillyard et al. (1973b) did, would probably considerably shorten the latency of decision 1 compared to that the sole basis for this decision would have been the other difference between the two kinds of stimuli, that involving the frequency (although a difference of considerable size). It is possible that for this reason decision I took place much later in the Wilkinson and Lee study (1972), which used binaural stimulus delivery, than in the former study, and that this difference would be related to the later effect observed in the latter investigation. (The nature and explanation of these respective effects might still be quite different; see the preceding section.) Hence, these results conflict neither with Hillyard et al.‘s (1973b) interpretation nor with the dual-decision view presented herein.

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In Corby and Kopell’s (1973) experiment, the subject was instructed to make a motor response as fast as he was capable of, either to a flashed vertical arrow, to a reversal (a change in the orientation of an oblique arrow from one side to the other) or to both (see the preceding section). Here we do not know with certainty whether the subject really reacted on the basis of the orientation of the stimuli or merely on the basis of their spatial aspects. This is because it is possible that, for example, in the situation in which the reversals were relevant stimuli, the subject found it to be the easiest strategy to fixate to one of the two corners of the visual field which the oblique arrow present at that moment did not pierce and to respond immediately when a fovea1 stimulus (part of the oblique arrow) appeared. This means that it might have been possible for the subject to change the task, apparently meant to be a choice-reaction task, to a simple-reaction task. This might partially explain the relatively short latency of the effect reported and the lack of difference in the latency of the late positive component between the predictable and unpredictable conditions (see pp. 260-261). The literature reviewed in this paper would provide topics in abundance for these kinds of considerations of which only a few have been dealt with here for illustrative purposes. The essential is that for reaching the goal of the experimental task the subjects can often find or use many ways different from those that the experimenter intends to study, either on the basis of what would resemble conscious reasoning (different ‘strategies’) or less deliberately (certain situations tend to elicit certain types of cognitive and other processes (sorry for the experimenter if he guesses wrong in this point) which may even vary from individual to individual or from time to time). In the previously presented examples we have mainly limited ourselves to considering the effects of the mere physical stimulus situation and the basic type of the task on the subject’s overt and covert responses with the emphasis on the nature and latency of the decision concerning the task-relevance of a stimulus (decision I), i.e. on the possible point of diversion in the stimulus processing after which point the ongoing stimulus-triggered physiological activity (or activity changes) might be treated, and responded to, differentially, according to the experienced (or brain detected; not necessarily experienced) relevance, by the organism. In case of irrelevance, the process might terminate (see Lehtio, 1970); otherwise it may go on to phases of still finer discrimination providing the basis for making decision II. In the following, a brief, more general examination of the above-listed factors (a)-(d) (p. 287) proposed to determine the nature and latency of different phases of discrimination in the stimulus processing is given with respect to a common situation (phasic or continuous) in the research work reviewed. This situation involves subjective certainty that the next stimulus is a signal to be responded to by a fast motor response, such as a key press (e.g. the situation of a simple RT task).

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Figure 5 illustrates a basic situation in which the signal stimulus is the only stimulus and is delivered at regular ISIS of 3 sec. In such a situation the subject apparently already responds at the very early phase of the stimulus processing involving the experience of ‘something is happening’; his triggering level or decision criterium is very low. It is a common observation in such reaction situations that any stimulus, e.g. a sound outside from the laboratory, occurring at about the expected time of the signal triggers the response; such a stimulus does not have even to be of the same modality as the signal (see the discussion on Naatanen’s (1967) first expt. ; p. 250). It might indeed be that by the expected moment of the signal the subject has abandoned all the other, more specific criteria for decision-making: ‘something’, somewhat discrete from the background stimulation and neural noise is enough for the decision of ‘signal’. The lowest dotted curve in fig. 5 represents the assumed lowering of the decision criterium as a function of the time since the preceding signal (or increasing expectancy or decreasing time-uncertainty; see Klemmer, 1956). If the signal were presented occasionally unexpectedly early in such a situation, the response would be greatly delayed (Mowrer, 1940), which apparently is due to both a higher decision criterium and lower preparedness of the response mechanisms (perhaps also to the increased central integration time; see Lansing et al., 1959). Presumably, the decision criterium would not yet be nonspecific to the same degree as that at the expected moment of the stimulus delivery. This was an example of a possible way how the temporal aspect of the stimuli of the task could have an influence on both the latency and nature of the decision (decision criterium). As to the effect of the instruction (b) (pp. 287) in this situation, if speed is emphasized at the expense of accuracy (premature responses are not at all

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discouraged), the ‘something is happening’ level might be lowered increasing the number of false alarms and premature responses (see Naatanen, 1971). If, on the other hand, correctness of performance is emphasized in the instruction, this level would probably be elevated. These kinds of effects, presumably, and the general level of performance are affected by the pay-off and punishing systems (c) (p. 287). And as to the permanent and temporal characteristics of the subject (d), these naturally have many kinds of influences on each aspect of decision-making in an experimental situation. If instead of the simple reaction, the task involves a disjunctive one, e.g. the auditory stimuli are to be responded to but the shock stimuli are not, the decision criterium cannot be any more a non-specific ‘something’. Now the subject has to wait until he is sure (or relatively sure) that the stimulus is auditory with the consequence that the latency of the decision is increased. The assumed form of the change of the latency of this decision as a function of the time elapsed since the preceding auditory stimulus (if the auditory stimuli are delivered at regular intervals of 3 set and the shocks at random intervals) is shown by the middle curve in fig. 5 (response latencies at intervals deviating from the constant can be measured by occasional deliveries of the signal at those deviating intervals; see Mowrer, 1940; a difficult problem would be faced in trying to isolate the contribution of the possibly increased decision time to the total increase of the response latency). Probably in this kind of situation the relative frequencies of the two kinds of stimuli also have such an influence on the criterium that the fewer shocks are given the lower the decision criterium of ‘auditory stimulus’. If we now modify the task so that the auditory stimuli are still the relevant ones and the shocks irrelevant but only occasional, slightly louder clicks should be detected by the subject, we are in the midst of a most typical situation of the EP studies on selective attention. Decision I is presumably reached at a very early phase as probably almost all the possible bases to decide between a click and a shock are available, whereas the latency of decision II is in any case much longer and mainly depends on the difficulty of the discrimination within the auditory modality (see fig. 5). (As mentioned above, the relative frequencies of stimuli might also have an influence on the stimulus processing-a possibility not sufficiently taken into account in experimental planning.) The latency of this decision as well as of all those described earlier should also be dependent on the degree of certainty at which the subject wants to operate which is a function of the instructions, the pay-off and punishment system of the experiment and the characteristics of the subject, In the foregoing, a two-stage hypothesis of decision processes usually involved in the kinds of experimental tasks used in EP studies on selective attention was outlined. This hypothesis proposes the existence of two sequential decisions (under most conditions) when the outcome of the first decision is not ‘irrelevant’ (and only the first one if the stimulus is regarded as irrelevant).

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As an active discrimination process based on different kinds of features of the stimulus is proposed, the sensory impulses from all the stimuli are understood to reach a sufficiently high, presumably cortical, level of the central nervous system. Essentially, this hypothesis represents an elaborated version of NH&nen’s 1967 view: ‘the non-specific reactiveness of the organism and the physical properties of the stimulus, but not its relevance of meaningfulness seem . . . to be the main determinants of evoked potentials amplitudes, and even of those of vertex potentials, often regarded as reliable indicators of the relevance and importance of the stimuli’ (p. 174). In this connection it is interesting to note that Wilkinson and Lee (1972), referring to NI - P, of the El’, write that ‘conceivably this basic component of the stimulus trace, which unlike PsoO is present when stimuli go ignored and unresponded to (Wilkinson and Morlock, 1967), may be associated in some way with brief retention of the raw sensory input in a preliminary precategorical store (Sperling, 1963) before any selection is made for transmission to more permanent stores of more limited capacity’ (pp. 416-417). The basic view of both Wilkinson and Lee (1972) and of Naatanen (1967) clearly is that a veridical (in the sense of being unbiased by attentional influences) representation of the proximal stimulation (see Brunswik, 1956) takes place as high as at the cortical level and attentional effects might modulate it only thereafter. The crucial test between the previously presented hypothesis and that assuming the existence of selective filters would involve the latency of the first (true) effect on the EP reflecting stimulus significance. If the latter hypothesis were correct, this latency should be very short (maximally some 20-40 msec depending on the time the afferent volley needs for reaching the primary sensory cortex). The other hypothesis could not explain such very early effects but might be able to explain the earliest (reasonably reliable) effect so far found with an onset latency of some 60-70 msec (Hillyard et al., 1973b) extensively discussed in the foregoing. According to Barlow (1964) the perception time for visual stimuli is approximately 15 msec and even simple motor RTs can under favourable conditions be as short as 100 msec. In the author’s laboratory the shortest mean (auditory) RTs measured have been of the order of 90 msec. To separate between these two possibilities, it would be necessary to be able to reliably record very early stimulus-related activity (for works giving promise in this connection, see, e.g. Jewett et al., 1970; Velasco et al., 1973; Picton et al., 1974), for example, in a setting like that used by Hillyard et al. (1973b, second experiment). If still no earlier effect than that with a latency of some 60-70 msec is observed, a selective-filter interpretation for such an effect cannot be valid. Finally, it is important to note that not every demonstration of the lacking sufficiently early effect means that the seiective-filtering hypothesis is false: there are presumably many kinds of attention and it might be that only certain

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forms of it are based on selective filtering of sensory impulses. The following kinds of conditions are likely to increase the probability that the filtering mechanism will be set into operation (if it exists). (1) The relevant and irrelevant stimuli utilize d$erent aflerent systems (as discussed extensively in the foregoing). Very similar stimuli could not be differentially treated by a filtering system but their discrimination can occur only after their detailed (more central) internal representation, i.e. in a later post-stimulus stage (response-set; Broadbent, 1970). (2) The subject has a clear, ‘vivid’ conception qf at least the relevant, perhaps also of the irrelevant, stimulus. (Otherwise he might not be able to ‘tune’ the filtering system.) Therefore, the onset of the filtering system probably does not occur in the very beginning of an experimental session or run, but the subject has to have an opportunity to familiarize himself thoroughly with the stimuli used. (3) The expectancy, i.e. the momentary subjective probability of the immediate delivery, of the relevant stimulus is high (which is usually the case with short ISIS of relevant stimuli or just before the occurrence of a relevant stimulus with regular ISIS of relevant stimuli). This assumption reflects the present conception of the selective sensory state as a transient state maintained with high energy expenditure. If this were the case, a CNV or tonic negativity should coincide with the active period of the filtering system (whether stimuli are delivered or not). Paragraph (3) might be the reason for difficulties to be really selectively attentive under unpredictable conditions (see Lindsley, 1969, and Lindsley and Wicke, 1974) except for, perhaps, with very short ISIS of relevant stimuli (Wilkinson and Lee, 1972; Hillyard et al., 1973b) and for some of the EP differences between the predictable and unpredictable conditions. Presumably, the filtering mechanism could be in operation mainly only under the regular conditions. (4) The subject works under time stress so that there is a need,for a very early stimulus analysis (the ‘cocktail-party phenomenon’). In such a case, the stimulus is not processed, for example, on the basis of its effects on the shortterm memory, which kind of analysis manner might be utilized with long ISIS such as those used in most of the EP studies on selective attention. 4.3. Present and proposed.future research trends: an overview and evaluation Generally, when looking backward a common problem in the EP research conducted on the neurophysiological substrate of selective attention seems to have been lack of knowledge of what the subject really has done in the experimental situation, what feature he has attended and responded to, what he has ignored in reality, and what his strategy has been in the experimental situation. Giving instructions does not necessarily mean that they are literally followed, and the ‘relevant’ or ‘attended’ and ‘irrelevant’ or ‘unattended’ stimuli as defined by the experimenter might even acquire completely different significancies in the underestimated mind of the subject who really does not look very

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clever when anxiously looking at the wires and other devices of the lab, apprehensive of the risk of electric shock, and demonstrating a complete lack of the cultivating knowledge of psychophysiology. It might even be said that when speaking about ‘relevant’ and ‘irrelevant’ stimuli we in fact often assume what we should investigate. We have been looking for electrophysiological correlates often for something about which we have had a quite too vague conception (see also Moray, 1969). What is needed is more information about the covert actions of the subject, obtained independently of knowing the specific contents of the instructions given to the subject and the external features of the experimental task. This is, of course, a persistent problem in the whole of psychological research, but the research work reviewed herein somewhat surprises with its little concern on this point. For example, in only a few research reports is anything mentioned about the subject’s descriptions of his experiences, strategies or feelings during the experimental run. If such reports are encouraged and carefully analysed with full awareness of the unavoidable biases of such data, it might be possible to infer at least whether the subject was following the instructions or not and to reveal some of the alternative strategies. This would lead to reduced subject-option (Sutton, 1969, see also Tecce, 1970, p. 3.51). (In any case, special attention should be paid to clarity and simplicity of instructions and that they are easy to follow and make sense to the subject; see Sutton, 1969.) Another related feature of the research work reviewed is the common fixation to one basic paradigm and within it to certain single arbitrary values of even the most important variables. If there had been more flexibility and variation in this respect, a more detailed ‘map’ of the relations between, on one hand, the experimental situations and, on the other hand, the subject’s overt behaviour (task-performance and other behaviour) and physiological data would have been possible to obtain. This would, presumably, form a much safer basis for the inferences concerning covert ‘psychological’ processes. Before looking for physiological correlates it should be known what phenomenon the correlate to should be sought. To sum up some central tendencies in the recent research work on the EP correlates of selective attention, they could be listed as follows. (1) The shift of emphasis from animal to human work. (2) The increasing tendency to study EPs not as separate phenomena but in interaction with, and in context of, other brain potentials such as slow potentials (mainly CNV and readiness potentials) and the background EEG. In .. . 1967, the necessity of this change was emphasized by Naatanen as follows: ‘It may be asked whether too much valuable information is sacrificed by excluding all cycles slower than 1 cps for the sake of having a steady baseline in the EEG records. This exclusion of the slow electrophysiological phenomena seems to be an arbitrary restricting operation, which in many cases prevents the factual brain events from being reflected in the EEG records as they take

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place in experimental subjects. Perhaps the LLexpectancy waves”, such as demonstrated by Walter (1964) and Walter et al. (1964) with the d.c.-recording technique, and other slow potentials, should be given a chance to show up in all electroencephalographic records; otherwise, valuable information is lost. So far these slower electrophysiological events have been a special research area partially separated from evoked potential and other EEG studies’ (p. 155). (3) Emphasis on the adequacy of the temporal structure of the task (randomization and, hence, unpredictability of different stimuli) to separate the effects of specific and non-specific physiologica processes associated with attentive states on EPs (note, however, the risk of decreased selectiveness and intensiveness of such states as a possible result of increased event and temporal uncertainty; see Lindsley, 1969, and Lindsley and Wicke, 1974). (4) Increased control of ocular potentials by EOG records which, at least partialIy, can be ascribed to an increased interest in slower potentials in the selective-attention context (and to longer time-constants of recording systems used). (On the other hand, a general increase in carefulness and adequacy of peripheral sensory control is not observed; there were early studies with highly commendable arrangements for such a control, e.g. that of Chapman and Bragdon, 1964, and there are plenty of recent ones with clearly unsatisfactory peripheral sensory control.) (5) As to the EP wave form itself, ten years ago the (then)’ late’ amplitude N1 - P, was the main object of interest and measurement (people appeared also to be searching for really early, primary, components, with minor success, however); nowadays, N1 - Pz is referred to as an early or middle-latency amplitude and PsoO a late component which latter is seen to hold most psychological promise {whereas the former is understood to be, at least to a much greater extent, a function of non-psychological factors, such as physical stimulus energy). (6) Earlier only one measure of the EP was often taken (this being most commonIy the largest ‘peak-to-peak’ amplitude N, - P2) with the most notable exception perhaps in the approach of Ciganek (see, e.g. 1964); nowadays more measures are generally taken, which trend cannot perhaps be totally attributed to the widening of the area of interest to involve also the late post-stimulus and slow pre-stimulus potentials. (7) Nowadays, conclusions are supported by statistical evidence rather than by only showing selected graphs. Rejection of large portions of data and the variable, perhaps biased, criteria for rejection may still be a major problem. (8) Increasing interest in the topographical aspects of the different EP components. (9) After certain disadvantages of averaging processes became apparent, a (very welcome) increase of interest towards single responses can be observed (e.g. in work by Donchin, pioneering in many respects).

Unfortunately, much progress can be observed neither in task design (except for that mentioned in (3)) nor in understanding the psychological and behavioural aspects of the experimental task (as extensively discussed in the foregoing). Finally, some recommended trends for the future related work are indicated. (1) More emphasis on topographical aspects which would be helpful, for example, for identifying various intracranial generators and separating different spatially and temporally overlapping potentials or components as well as identifying the same potentials or components in different experimental situations (see, e.g. Vaughan, 1969). (2) In the EP work, the other brain potentials (CNV, readiness potentials, ‘background EEG’) should more generally be considered possible sources of information of central importance with respect to the interpretation of the EP data in many kinds of paradigms and should be adequately measured and related to the latter data. (3) More interest should also be directed to other physiological variables in the selective-attention context such as the GSR (ohman and Lader, 1972), the pupillary dilation response (Friedman et al., 1973) or the heart rate which is especially sensitive to attentional factors (Lacey, 1967). A fruitful research line in attempting to clarify the issue around PIOO(e.g. to which degree such positive phenomena in post-stimulus traces indicate a non-specific change of state) might be to try to separate the specific and non-specific post-stimulus events from each other by examining all the other recordable changes following the stimulus and their temporal relationships with P,,,. There is rather strong experimental indication that P,,, is a part of a larger response entity, which possibility can be studied by more representative recording of physiological changes in different experimental situations. (4) More emphasis should be put on the task construction and variation and on the subject’s overt and covert behaviour during the task. There are many critical variables in these experiments which have often had only a single arbitrary value in a condition and, thus, have not received experimental attention in proportion to their importance. Among such variables are: ISI, task difficulty, task-relevant features of the stimulation, differences between different stimulus categories of the task, relative frequencies of the different stimuli, objective basis for anticipation of when a stimulus and which stimulus is delivered. (Special attention should be paid to the stimulus randomization in settings in which it is desirable to produce a state of maximal event uncertainty in the subject.) We really should read more about psychological attention research. (5) The promises

of the single-trial approach are still largely unexplored. (6) A revival of interest in animal studies of selective attention is encouraged for the great interpretational difficulties encountered with even methodologically flawless human data.

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(7) Presently, the measurement of the data still seems generally to be too stereotyped and superficial, being mainly confined to peak-to-peak amplitudes; for example, baseline-to-peak measures are largely neglected-to stress the importance of this point: if Hillyard et al. (1973b) had not used a latter kind of measure, the important result involving N1 component obtained might have remained undetected. Also measures other than those based on amplitudes, e.g. duration measures (for example, as an indicator of the degree of time-lockedness of components of single responses) and those involving wave forms, might prove to be useful. In light of the possibilities to test the selectivefiltering hypothesis by means of scalp-recorded EP data, special emphasis should be paid to the very early stimulus-locked activity. (8) More emphasis should be put on the adequacy of peripheral sensory control. (9) The EP components are usually measured as they appear in the graphs without due consideration of the possibility that different components might be temporally overlapping or redundant. (One such possibility called Z%Pthreecomponent hypothesis qfthe EP was outlined in the preceding section.) A most urgent task would be to isolate the really independent components of the EP from each other. (I 0) The relationship between the phenomena of attention and habituation should be exhaustively explored under the same experimental setting (see ijhman and Lader, 1972).

Acknowledgements This work was supported by The Research Council for the Humanities of The Finnish Academy. The author is greatly indebted to several of his colleagues for their advice and constructive criticism and is especially grateful to Miss S. JBrvelH, Miss M.-L. Moltsi and Mr. R. Ellonen, M.A., for typing the manuscript, to Miss S. Mantysalo, M.A., for help in references and to Mr. A. Merisalo, M.A., for drawing or reproducing the figures.

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