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Elec troencephalograph y and Clinical Neurophysiology , 1977, 4 2 : 1 9 8 - - 2 0 4 © Elsevier/North-Holland Scientific Publishers Ltd

THE EFFECTS OF ACTIVE ATTENTION SWITCHING BETWEEN THE EARS ON AVERAGED EVOKED POTENTIALS

TSUNETAKA OKITA and AKIRA OHTANI * Departmen t o f Medical Psychology, Hyogo College o f Medicine, Nishin om iya (Japan)

(Accepted for p u b l i c a t i o n : J u n e 7, 1976)

A large number of studies have investigated the effects of attention on auditory averaged evoked potentials (AEPs) using humans. Karlin (1970), Tecce (1970) and N ~ t ~ n e n (1975)have reviewed these experiments.While most of these studies attempted to show the effects of selective attention on AEPs, several investigators have reported interesting correlations between the passive switching of attention and auditory AEPs. In these studies, a late-positive wave with peak latencies of 210--350 msec (P3 or P300) was observed when infrequent, unpredictable stimuli were presented as compared with conditions using frequent and habituating stimuli. These waves were interpreted as a reflection of subjects' passive shifts in attention or "orienting response" during stimulus presentation (Ritter et al. 1968; Roth 1973; Roth and Kopell 1973; Squires et al. 1975). Psychological studies of human information processing have similarly demonstrated the effects of an active type of switching on perceived dichotic stimuli. In these studies, subjects are required actively and frequently to switch attention from ear to ear during the presentation of auditory stimuli, delivered either simultaneously or alternately to each ear. A series of studies (Axelrod et al. 1968; Axelrod and Guzy 1968; Guzy and Axelrod 1972) reported that subjects perceived a slower presentation rate under conditions using an alternating pattern of stimulus (clicks) presen* The death of Professor Ohtani on April 20, 1976 is reported with regret.

tations as compared to a monotic presentation, although identical stimulus rates were employed in both presentation modes. These data suggest that underestimates of dichotic rates may be due to limitations on subjects' ability to switch attention rapidly from ear to ear. The present experiment was designed to study the relationship between active attention switching and auditory AEPs. Active attention switching was obtained by presenting stimuli in random and alternating patterns versus no switching, as in stimulus presentation to only one ear. In this study random presentation was considered to produce active attention switching since subjects were required to switch attention frequently and were aware that stimuli would be shifting from ear to ear. In addition, this study attempted to assess the differential effects of two types of active switching. An alternating pattern of stimulus presentation provided a situation in which subjects could predict which ear would receive the next stimulus; this was considered to be a condition enhancing active attention switching. On the other hand, a random pattern of stimulus presentation reduced the subjects' ability to predict location of the next stimulus thus allowing for less active attention switching. Finally, this experiment assessed the effects of temporal predictability, where subjects may or may not predict when the next stimulus will be presented. Under conditions of regular stimulus presentation temporal predictability was considered higher thus en-

ATTENTION SWITCHING AND EVOKED POTENTIALS

199 Temporat

hancing active attention switching and under conditions of irregular stimulus presentation temporal predictability was lower resulting in less active attention switching.

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Subjects and task Six adults (4 males, 2 females), used in many previous experiments, served as subject in this study. Each subject was comfortably seated in a reclining chair in a sound-attenuating, shielded room facing a panel m o u n t e d with a small, red light emitting diode (LED}. The LED was used to fixate the subject's gaze in order to reduce ocular artifacts, and also served as a warning light (400 msec) to signal the onset of each trial. A trial consisted of a train of 6, 7, 8 or 9 tone pips (600 c/sec ramp wave, 20 msec duration, and 40 dB above threshold level) delivered through earphones. The train of pips was presented approximately 1.0--2.5 sec following the offset of the warning light. The subjects were instructed to count the number of pips presented to both ears, and report the counted number of pips after each trial. The delivery of tone pips was varied in terms of: (1) four modes of spatial presentation, right ear only (R), left ear only (L), alternating (ALT) and random (RAN}; and (2) two types of temporal presentation, regular interval (REG) and irregular interval (IRREG), as shown in Fig. 1. The first pip on each trial in the ALT mode was randomly delivered to either ear. The regular interval was 1000 msec, and the irregularinterval was an average of 1000 msec ranging from approximately 500 to 1500 msee. Stimuli were presented to all subjects in 6 blocks consisting of 16 trials each. Of the 6 blocks, 3 blocks were REG and the remaining 3 blocks IRREG. These REG or I R R E G blocks were presented via the monotic (L and R~, ALT and RAN modes. In the monotic blocks, the first half trials were R (or L) and the second half trials L (or R). Each block required a b o u t

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Fig. 1. Schematic summary of the stimulus sequence for each condition. (In this and the following figures: R and L, monotic presentations of tone pips to the right and left ears, respectively; ALT, dichotic alternating presentation; RAN, dichotic random presentation; REG, regular interval at 1000 msec; IRREG, irregular interval at an average of 1000 msec ranging approximately 500--1500 msec.) Bars above and below the line indicate presentations to the right and left ears, respectively. In the REG--RAN condition, the bars labeled by "H" and "S" show the two types of the stimulus sequence, whose evoked potentials were separately analyzed as the subdivided conditions of RAN, RAN--HOLD and RAN--SW.

5 min for completion. Orders of these blocks were randomized across the subjects, and the subjects were instructed concerning the mode of spatial presentation.

Recording Evoked potentials were recorded from the vertex (Cz) electrode referred to the left earlobe with the right earlobe as a ground, and amplified using a Nihon Koden RB-5 preamplifier (time constant 2.0 sec, high frequency cut-off at 100 c/sec). The vertical electro-oculogram (EOG) was also recorded to monitor eye movements and blinks and amplified with a Nihon Koden RDU-5 high gain DC pre-amplifier. Scalp potentials and EOG were stored along with trigger pulses on an FM tape recorder (TEAC R-260) for off-line analysis.

200

T. O K I T A , A. O H T A N I

Data processing

tion to the same ear (this type of order is indicated by H for hold in Fig. 1); and (2) those cases where two successive pips were given to the different ears, requiring subjects to switch attention (S for switch in Fig. 1). Finally, N1--P: peak-to-peak amplitude and N1 and P: peak latencies from the onset of pips were measured in each subject's AEP. Scalp potentials obtained on trials which were contaminated by ocular or other artifacts were omitted from the analyses. Also, the averages of the EOGs, which were separately computed, ensured that the AEPs obtained were free of ocular artifacts.

The first 32 scalp potentials for each condition were summed on a 500 msec analysis time using a Nihon Koden ATAC-250 (sampling rate: 256 samples in 500 msec). For this summation, however, the potential from the first pip of each train was omitted, since the amplitude of this first potential was generally larger than those following as previously reported by Fruhstorfer et al. (1970) and Fruhstorfer (1971). These data were analyzed as a 4 × 2 factorial within-subject design (4 levels of spatial presentation mode versus 2 levels of temporal regularity) using the average of 32 scalp potentials as the unit of analysis. In addition, data for the RAN mode were subdivided and separately analyzed for: (1) those cases where two or more pips were presented in a row to the same ear, thus permitting subjects to hold atten-

Results

Figs. 2 and 3 present the AEPs of 2 representative subjects for the 8 conditions and the 4 subdivided conditions of the RAN mode.

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The prominent evoked components under all conditions and across all subjects were a negative deflection (N1, mean latency = 93 msec) and following positive deflection (P2, mean latency = 177 msec). However, other components (PI, N2 and P3) noted in earlier studies were inconsistent between subjects, and did not systematically vary with conditions.

tion and the two monotic modes ( P < 0.05). In addition, higher amplitudes were found for the ALT mode as compared to the L, but not R mode (P < 0.05). There were no significant

22

[• Amplitude effects The mean NI--P2 amplitudes (see Fig. 4) across all 6 subjects for the different conditions were analyzed using a 4 × 2 × 6 (spatial presentation, temporal regularity, subject) analysis of variance design. A significant effect due to spatial presentation ( F ( 3 , 1 5 ) = 9.82, P < 0.005) was obtained. Subsequent t tests were performed to analyze the difference between these spatial presentation modes. Significantly higher amplitudes were obtained for the RAN mode as compared to ALT presenta-

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202

T. OKITA, A. OHTANI

amplitude effects due to temporal regularity and no significant interactions. A 2 × 2 × 6 (HOLD vs. SW, temporal regularity, subject) analysis of variance was c o m p u t e d for the subdivided RAN data. A significant effect due to RAN--SW versus RAN-- HOLD was obtained (F(1, 5 ) = 2 5 . 2 3 , P < 0.005) with the higher amplitudes produced in the RAN--SW condition. There were no other significant main effects or interactions.

Latency effects The mean peak latencies of N~ and P2 components for all subjects are presented in Fig. 5. Separate analyses of variance as c o m p u t e d for the amplitudes were performed for the N~ and P2 latencies. For P: data there were no observed effects due to either spatial or temporal presentation and no significant interactions. Analysis of the subdivided RAN data indicated that a significantly prolonged P2 latency was obtained for the RAN--SW condition as compared with RAN--HOLD (F(1, 5 ) = 8 . 3 8 ,

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P < 0.05). There were no other significant main effects or interaction. A significant temporal regularity effect on N1 latency was obtained for both the spatial × temporal × subject analysis and for the subdivided RAN data (F(1, 5) = 12.59, P < 0.025; F(1, 5) = 9.69, P < 0.05, respectively), In both cases a prolonged N~ latency was obtained for the I R R E G presentation as compared with the REG presentation. The average difference between the REG and I R R E G intervals was very small, of approximately 5 msec duration. There were no other significant main effects or interactions for either analyses. Discussion

The data from this experiment indicate an enhancing effect o n N l - - P 2 amplitude as a function of dichotic presentation modes which required subjects to switch attention from ear to ear as compared to presentation modes which required no switching (one ear only). Our findings also indicate that unpredictable attention switching as in the RAN mode further enhanced N~--P2 amplitude in comparison with predictable attention switching as in the ALT mode. Similarly, P2 latency was also affected by this switching phenomenon as indicated by the prolongation of latency found in RAN--SW as compared to the RAN--HOLD conditions. Thus, these effects of active attention switching were found on the earlier N~--P~ components in contrast to passive switching effects on the P3 c o m p o n e n t as in the previous studies on the orienting response (Ritter et al. 1968; Fruhstorfer and BergstrSm 1969; Roth 1973; Roth and Kopell 1973; Squires et al. 1975). On the other hand, N1 latency was increased with an I R R E G temporal presentation, compared to REG presentation. The enhancing effects of this apparent attention switching phenomenon on N1--P2 amplitude and P2 latency together with the enhancing effect of an unpredictable temporal presentation (IRREG) on N~ latency may be related to stage models of human information processing. The finding that NI and P2 latencies were

ATTENTION SWITCHING AND EVOKED POTENTIALS affected by different factors may indicate that these components reflect different processing stages. The P~ latency was prolonged under conditions requiring subjects to switch attention between ears; this switching effect, however, was not obtained for N~ latency. N~ latency was only prolonged under conditions of irregular temporal stimulus presentation. One interpretation of these data is that the prolonged latencies are the result of subjects' reduced ability to predict stimulus occurrence under conditions involving irregular temporal and dichotic stimulus presentation. Thus, N~ may reflect an earlier processing stage, where arrhythmic presentation may increase subjects' processing time by reducing stimulus predictability. Under conditions using rhythmic stimulus input, subjects are better able to predict stimulus timing and thus exhibit shorter processing periods. On the other hand, P2 may reflect a later processing stage, unaffected by temporal variations, but sensitive to spatial factors such as dichotic versus monotic presentation. As in those studies showing that interaural switching of attention requires a certain amount of time (Axelrod et al. 1968; Axelrod and Guzy 1968; Guzy and Axelrod 1972), the delays in P~ latency observed in the present study could be considered as the result of the additive processing time due to attention switching. The delayed P2 latency effect could be found only in the RAN--SW condition, a condition which tended to maximize the unpredictability of stimulus location and thus the attention switching effect. The amplitude data from this study may also be interpreted in relation to the processing stage concerned with attention switching. That is, the increased N~--P2 amplitudes were observed primarily in those conditions requiring attention switching such as RAN, ALT, RAN--SW. Further, in the two dichotic conditions (RAN versus ALT), both requiring attention switching, amplitudes under the RAN condition were greater than for the ALT condition. In effect, this amplitude difference may be due to a situation where attention

203 switching is time-locked to the onset of the pip under the RAN condition since the subjects' ability to predict stimulus location is reduced. On the other hand, in the ALT situation subjects are able to predict stimulus location and thus their attention switching may occur prior to onset of the pip. Although the data presented in this study lend support to the hypothesis that observed latency and amplitude changes in AEP are a function of stages in human information processing, we must be careful to consider the possible effects due to stimulus variables. Butler et al. (1969) has suggested that N~--P2 amplitude reduction of the auditory AEP is produced by repetitively eliciting "the same pattern of neural activity". If we accept Butler et al.'s suggestion, our enhanced N~--P2 amplitudes in dichotic modes may be derived from less repetitive activations of the same neural pattern than in our monotic modes. In the same manner, the difference between the RAN and ALT modes could be also interpreted on the basis of the average repetition rate which is slower in the RAN mode. However, these criticisms do not appear to detract from the relatively strong evidence of a switching effect as shown by the prolonged P2 latency. A clearer assessment of these switching effects may be obtained by controlling for stimulus variables and by recording behavioral data on attention switching.

Summary The relationship between averaged vertex evoked potentials and active switching of attention between the ears was studies in 6 trained subjects. A train of tone pips from 6 to 9 on each trial was presented in four modes, of which two modes were monotic presentations to either ear and the other two were dichotic, alternating and random presentations. These pips were delivered at either regular or irregular intervals which averaged 1000 msec. N~--P2 amplitudes were found to be larger when attention switching was required between the ears

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in the dichotic presentations as compared with monotic presentations which allowed subjects to restrict attention to one ear. The random presentation m o d e produced a further enhancement of N1--P2 amplitude as compared to the alternating mode. Within the random mode, attention switching elicited delayed P2 latency relative to a condition n o t requiring subjects to switch attention. Irregular stimulus presentation delayed N1 latency regardless of the attention switching requirement. These results were related to the successive stages of human information processing.

Rdsumd Effets du changement actif d'attention d'une oreille d l'autre sur les potentiels dvoquds moyens La relation entre les potentiels ~voqu~s moyens au vertex et le changement actif d'attention d'une oreille ~ l'autre a ~t6 ~tudi~ chez 6 sujets bien entrain~s. Un train de sons trSs brefs comportant 6 ~ 9 sons ~ chaque essai est pr~sent~ suivant quatre modalit~s, deux d'entre elles ~tant des presentations monaurales ~ chacune des deux oreilles et les deux autres ~tant des presentations binaurales alternantes et al~atoires. Ces pips sont d~livr~es des intervalles soit rdguliers, soit irrdguliers qui sont en m o y e n n e de 1000 msec. Les amplitudes de N1 et P2 s'av6rent plus grandes lors qu'il est demandd de changer l'attention d'une oreille ~ l'autre en presentation binaurale par rapport ~ la presentation monaurale qui permet aux sujets de restreindre leur attention une oreille. La prdsentation al~atoire produit une augmentation ultdrieure de l'amplitude de NI et P2 par comparaison ~ la modalit6 alternante. A l'int6rieur de la modalit6 aldatoire, !e changement d'attention provoque un retard de latence de P2 par rapport ~ une condition qui ne demande pas aux sujets de changer leur attention. Les pr6sentations de stimulus irrdguli6res retardent la latence de N1 ind6pendamment du fait qu'il est ou non demand6 de changer l'attention. Ces r6sultats sont mis en

T. OKITA, A. OHTANI

relation avec les stades successifs du traitement de l'information chez l'homme. The authors wish to thank Dr. Gilfred Tanabe for his many helpful suggestions in the revision of this manuscript and Mr. Keishi Koyama for his assistance in running the experiment.

References Axelrod, S. and Guzy, L.T. Underestimation of dichotic click rates: results using methods of absolute estimation and constant stimuli. Psychonom. Sci., 1968, 12: 133--134. Axelrod, S., Guzy, L.T. and Diamond, I.T. Perceived rate of monotic and dichotically alternating clicks. J. acoust. Soc. Amer., 1968, 43 : 51--55. Butler, R.A., Spreng, M. and Keidel, W.D. Stimulus repetition rate factors which influence the auditory evoked potential in man. Psychophysiology, 1969, 5: 665--672. Fruhstorfer, H. Habituation and dishabituation of the human vertex response. Electroenceph. clin. Neurophysiol., 1971, 30: 306--312. Fruhstorfer, H. and Bergstr6m, R.M. Human vigilance and auditory evoked responses. Electroenceph. clin. Neurophysiol., 1969, 27 : 346--355. Fruhstorfer, H., Soveri, P. and J//rvilehto, T. Shortterm habituation of the auditory evoked response in man. Electroenceph. clin. Neurophysiol., 1970, 28: 153--161. Guzy, L.T. and Axelrod, S. Interaural attention shifting as response. J. exp. Psychol., 1972, 95: 290-294. Karlin, L. Cognition, preparation, and sensory-evoked potentials. Psychol. Bull., 1970, 73 : 122--136. N~/~/ti/nen,R. Selective attention and evoked potentials in humans -- a critical review. Biol. Psycho]., 1975, 2: 237--307. Ritter, W., Vaughan Jr., H.G. and Costa, L.D. Orienting and habituation to auditory stimuli: a study of short-term changes in average evoked responses. Electroenceph. clin. Neurophysiol., 1968, 25: 550--556. Roth, W.R. Auditory evoked responses to unpredictable stimuli. Psychophysiology, 1973, 10: 125-138. Roth, W.T. and Kopell, B.S. P 3 0 0 - - a n orienting reaction in the human auditory evoked response. Percept. Motor Skills, 1973, 36: 219--225. Squires, N.K., Squires, K.C. and Hillyard, S.A. Two varieties of long-latency positive waves evoked by unpredictable auditory stimuli in man. Electroenceph, clin. Neurophysiol., 1975, 38: 387--401. Teece, J.J. Attention and evoked potentials in man. In D.I. Mostofsky (Ed.), Attention: contemporary theory and analysis. Appleton--Century--Crofts, New York, 1970: 331--365.

The effects of active attention switching between the ears on averaged evoked potentials.

198 Elec troencephalograph y and Clinical Neurophysiology , 1977, 4 2 : 1 9 8 - - 2 0 4 © Elsevier/North-Holland Scientific Publishers Ltd THE EFFEC...
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