Sequential and Parallel Operations in Tachistoscopic Recognition* BRIAN BUTLER Queen's University, Kingston, Ontario

ABSTRACT

Three experiments tested a sequential model of letter processing proposed by Merikle, Coltheart, and Lowe (1971). In Experiment 1, processing order was tested using a letter-switching technique adopted from Sperling (1970) and evidence for an ends-first processing strategy was obtained. In Experiments n and in, a visual search task was used to re-examine the selective masking effect (SME), which has been offered as evidence for ends-first processing. The results showed two important limits to the effect. The SME does not occur if the target can be discriminated easily from context items or if the subject can become prepared for a given set of letter features. The results indicate that letter processing cannot be explained by an exclusively parallel or an exclusively sequential model but that both processes are necessary. Two models are considered.

Since i960, much of the research on visual information processing has been concerned with the question of processing order with multi-element displays, especially a comparison of serial and parallel models. A serial model was originally proposed by Heron (1957) in order to account for the superior performance with letters on the left of an array. Later, Sperling (1963, 1967) incorporated a similar operation, called scanning, into a series of process-

ing models. Mewhort (Mewhort, 1966; Mewhort, Merikle & Bryden, 1969) utilized a similar operation to account for performance with higher orders of approximation to English. Subsequent studies, such as those of Smith and Ramunas (1971) and Merikle, Lowe, and Coltheart (1971), have shown that the left side superiority, which was the original basis for scanning, may be due to order of report rather than the initial order of processing. More recently, attention has shifted to parallel processing models, although the scanning model has been used to account for reaction time differences across stimulus position (Lefton, Despite arguments for serial scanning, some aspects of recognition appear to involve parallel operations. Sperling (1970), abandoned a serial model after observing similar effects with letters at either end of a stimulus array. Eriksen and Spencer (1 g6g) also proposed a parallel model after failing to find evidence for a serial operation in a study that compared simultaneous and sequential letter presentation. Parallel models have been proposed most recently by Estes (1972) and Shiffrin (Shiffrin, Gardner, & Allmeyer, 1973; Shiffrin, McKay, & Shaffer, 1976; Schneider & Shiffrin, 1977). The Estes model assumes that stimuli are processed simultaneously through parallel channels but that channels may interact so that performance depends on the number of elements presented. The Shiffrin model, on the other hand, assumes parallel processing with no capacity limits prior to shortterm memory. The Estes and Shiffrin models are unusual in that neither allows for any effect of selective attention prior to shortterm memory. The Shiffrin model, in particular, is based on a series of studies which fail to demonstrate selectivity (e.g. Shiffrin,

* I his research was supported by Grant A-9581 from the National Research Council of Canada to the author. Experiment 1 was presented at the annual meeting of the Canadian Psychological Association (CPA) in Vancouver, 1977. Experiments 11 and in were presented at the annual meeting of CPA in Quebec city, 1975. I should like to thank Steve Eyres and Arnold Campbell for assisting with Experiment 1 and Joan Cristoveanu for Experiments 11 and in. Reprints may be obtained from the author at: Department of Psychology, Queen's University, Kingston, Ontario, Canada, K7L 3N6.

Canad.J. Psychol./Rev. Canad. Psychol., 1978,32 (4)

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McKay, & Shaffer, 1976), although other studies have obtained evidence for it (Butler & Merikle, 1973; Butler, 1974). While most studies have considered left-to-right versus parallel models, a third and more radical model has been derived from a series of studies of the effects of masking stimuli. Merikle and his coworkers (Merikle, Coltheart, & Lowe, 1971; Merikle & Coltheart, 1972; Butler & Merikle, 1973; Merikle, 1974) have found repeatedly that the presentation of a backward masking stimulus has relatively little effect on the report of letters at either end of a letter array, but has considerable effect on the report of centre items. This effect, termed the selective masking effect (SME), appears to occur only if the item to be reported is contained in a mulit-element display (Merikle, Coltheart, & Lowe, 1971) and if the subject must attend to the entire array (Butler & Merikle, 1973; Butler, 1975). Although the SME has involved a mask-no mask comparison, the same effect can be obtained by delaying a backward mask. Merikle and Glick (1976) have shown that delaying a mask initially improves performance for items at the ends of an array, but that longer delays produce a greater increase in performance for items at the centre. Since backward masking should either interrupt the processing operation (Turvey, 1973) or severely degrade the stimulus trace (Eriksen, 1966), Merikle and Glick (1976) have interpreted the results, and the SME, in terms of a limited-capacity, sequential processing operation that identifies and encodes items at the ends of an array prior to items at the centre of the array. To date, this sequential ends-first processing model has been supported only by studies examining masking effects across stimulus position. However, all observations of the masking effects are more consistent with an interpretation in terms of processing order than in terms of specific retinal characteristics or simple masking phenomena (cf. Merikle & Glick, 1976).

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EXPERIMENT I

In 1970, Sperling reported an experiment that attempted to discriminate between a parallel and a serial model. Although the data were considered only in the light of these two models, the results could be relevant to the ends-first model as well. In that study, subjects were shown five letters briefly, but, after one-half of the exposure duration, one of the letters was switched to a second letter; for example, M T K L X changed to M T K L Z. Each display was followed by a visual noise mask. Sperling (1970) examined the probability of reporting the first and second of the switched letters when the change occurred at either the left or right end of a five-letter array. Since it was found that performance for these two positions was virtually identical, Sperling rejected the notion of a left-toright scan in favour of a parallel model. Unfortunately, Sperling did not report performance when the letter change occurred at the centre positions in the array, and such a comparison is essential in order to discriminate between a parallel and a sequential ends-first process. In order to obtain the missing data, the present experiment repeated Sperling's procedure withfiveand eight letter arrays. The procedure used in the present experiment is shown in Figure 1. A PDp/8e computer was used to drive a rapid phosphor CRT display monitor. The display presented a five or eight letter array for 100 msec immediately followed by a visual noise mask. On each trial, one of the presented letters was switched to a second letter after 50 msec. On each trial, this switch occurred at only one of the five or eight positions occupied by letters. Subjects were not warned about the switch, and, apparently, did not notice it. Subjects were asked to report as many letters as possible on each trial. While none of the processing models can predict the effect of stimulus position for

B. Butler

EXPERIMENTAL 5 ITEMS

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the switched letters, the three general classes of models must make different predictions for the relative effects of stimulus position for each of the two letters in the switched position. A left-to-right scanning model must predict that the first of the switched letters will be more accurately reported from the left of the array while the second letter will be more accurately reported on the right. A parallel processing model, on the other hand, must predict identical stimulus position effects for each of the switched letters. Finally, the sequential, ends-first model should predict that the first of the switched letters will be most accurately reported at the ends of each array while the second letter will be reported best from the centre positions. While the mask should disrupt processing, either through integration or interruption, any processing that does occur in the centre should be relatively more beneficial for the second letter than the first, thus yielding an inverted U-shaped position function for the second letter. It should be noted that this position function for the second letter is contrary to the usual observation that performance is poorer for the centre positions when a mask follows the display (e.g. Merikle, Coltheart, & Lowe, 1971). In order to ensure that any effects observed are due to processing strategy, the present study did incorporate a third condition to control for retinal position and masking. As shown in Figure 1, the control

Sequential and parallel operations

condition required subjects to report a single letter presented in a context of masking stimuli. While a single letter was shown for 20% of the trials, 80% of the trials involved a switch in the letter after one-half of the exposure duration. Since this control condition involved the same masking parameters as the experimental test conditions, any effects of retinal locus or masking should show up in both conditions. The control condition is especially interesting because there are reasons to expect parallel processing in this case. Estes (1972) obtained evidence for parallel processing, using a similar set of stimuli for a non-confusible background with letter search. Triesman (1977) would predict parallel processing since the target letter shares no features with the adjacent stimuli, This control condition, however, is less than ideal for two reasons. First, it was presented after the subject had completed all experimental test trials and, second, stimuli were shown for 50 msec only. These measures had to be adopted since a pilot study had demonstrated that performance was error-free at 100 msec and subjects often noticed the letter switch in this condition, The control condition, therefore, was always presented after the test series in order to ensure that subjects would not suspect a letter switch during the test series. While this produces a confound with practice, the limited practice in this case should not alter the subject's processing strategy.

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METHOD

Subjects

Twelve subjects were hired from a student job service at Queen's University and were paid $3.00 each for participating. Procedure

Following a brief practice session, subjects were given 130 trials with a row of letters shown for 100 msec on each trial. Subjects were asked to report immediately as many letters as possible from each display. For each trial, eitherfiveor eight letters were presented. The condition for each trial was determined at random with the constraint that eight letter displays be presented on 80 trials and five letter displays on 50 trials. The letters displayed on each trial were randomly selected by a PDp/8e minicomputer. On each trial, either four or seven letters remained visible for 100 msec, but one letter was switched to a second letter after 50 msec. The position for the letter switched was selected at random for each trial, with the restriction that each position in thefiveand eight letter rows was to be tested ten times. The second letter appearing in the switched position was also selected at random with the restriction that it could not be identical with the first letter in that position. Following the 100 msec display of letters, a visual mask was shown for 1000 msec and appeared in the same location as the letter rows. The mask consisted of an illuminated series offiveby seven dot matrices, with each matrix covering a position previously occupied by a letter. Between trials, a small dot was presented in a position corresponding to the centre of the array, and subjects were asked to maintain fixation on that point while the letters were presented. After the 130 test trials, subjects were given 80 trials during which one letter was presented at one of eight possible positions on the screen. The eight possible target positions corresponded to the positions used with the eight letter displays and the seven remaining positions on each trial were occupied by masking stimuli. The single letter displays were shown for 25 msec; then a second letter was presented in the same position for another 25 msec. The exposure duration had to be decreased for this condition since a pilot study indicated that a total exposure duration of 100 msec would allow perfect performance for both the first and second letters. The letters presented on each trial and the position tested were selected at random with the restriction that each position was tested ten times. In addition, on 20% of the trials the first and second letters were identical, i.e., the letter

244

did not change. The latter restriction was introduced in an attempt to maintain the subjects' belief that one item was being shown on each trial. Again a mask followed the display and was shown for 1000 msec. Apparatus

All stimuli were presented as bright letters on a dark background, using a Tektronic display monitor (model 604), with a P4 phosphor, coupled to a PDp/8e computer. Letters were formed by illuminating the appropriate dots in a five by seven dot matrix. As viewed by the subject, each matrix subtended approximated 20' by 28' of visual angle. A row of eight letters subtended about 3°45' by 28' while a five letter row was about 2°2i' by 281. For each trial, the stimuli presented briefly to the subject were recorded on a typewriter terminal. The verbal response of the subject was recorded on the terminal by the experimenter. RESULTS

Each subject's data was scored to obtain separate scores for each stimulus position in the two test conditions and the control condition. Scores were obtained for (a) probability of recalling the first of the switched letters only, (b) probability of recalling the second of the switched letters only, (c) probability of recalling both of the switched letters, and (d) the number of other letters recalled from the display. Since the experimental design does not form a factorial combination of position and array length, separate analyses were conducted for the five letter arrays, eight letter arrays, and the control condition. For each condition, analysis of variance with trend analysis was used to compare the (a) and (b) scores across position. Stimulus Position Effects The mean probabilities for reporting the first, second, or both of the switched letters are shown for each stimulus position in Figure 2. The pattern of results clearly supports the sequential ends-first processing model, rather than either the serial or parallel models. With both five and eight letter arrays, subjects were more accurate at reporting the first, rather than the second,

B. Butler

5



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FIGURE 2 The mean probability of recalling the first, second, or both of the switched letters for each stimulus position with thefiveand eight letter arrays.

of the switched letters, F(i, n ) = 14.76,/»< .01 and F(i, 11) = 7.84,/? < .02, but, more importantly, the stimulus position effects are quite different for each of the switched letters. The letter presented first in the switched position is most likely to be reported correctly if the switch occurs at either end of the display. Performance for the second of the switched letters appears to show an opposite effect; the second of the switched letters is most accurately reported if the switch occurs in the centre of the array. For five letter arrays, an analysis of variance showed that 67% of the variance due to the interaction of the first and second letter report by stimulus position can be attributed to a reversal in the quadratic component of stimulus position; F(i, 11) = 12.41, p < .005; with eight letter arrays a reversal of the quadratic component accounts for 76% of the variance due to the letters X position interaction; F(i, 11) = 21.38,/* < .001. A multiple range test (Duncan, 1955) was used to compare the report of each of the switched letters at the ends and at the middle. With the five letter arrays, the range test indicated a significant

Sequential and parallel operations

superiority for the report of the first letter in the first position, (7(10, gg) = 1.617,/? < .05, and the fifth position, ^(6, 99) = 1.588, p < .05, but no significant difference between switched letters for the third position. Similar results were found for the first position, O

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FIGURE 6 The mean detection score, corrected for guessing, for each stimulus position under each combination of context similarity and masking, with fixed-target procedure, in Experiment m.

(Estes & Taylor, 1966; Estes & Wessel, 1966; Bjork & Estes, 1971; Estes, 1972) have been visual search tasks with fixedtargets. The results of the present experiment suggest that this procedure allows subjects to become set for a target and adopt a parallel processing strategy. Certainly a comparison of the similar context conditions in Figures 5 and 6 shows that a variable-target procedure yields a SME while a fixed-target procedure does not. If this is correct, then the Estes model may provide a good description of the parallel processing component, but completely ignores sequential processing because of the nature of the tasks considered. In essence, Estes's 253

model may be a limited version of a combined parallel-sequential model. CONCLUSIONS

None of the three discrete models presented in the introduction can provide a complete account of the order of processing with a multiple letter display. Experiment i was designed to discriminate between these models and the results do provide good evidence for a sequential endsfirst processing model. The control condition, in Experiment i, does show that the evidence for sequential processing is not due to spatio-temporal characteristics of the retinae, but the results with the control condition suggest parallel rather than sequential discrimination between the forms presented. The possibility of parallel processing was explored more fully in Experiments II and in by examining the effects of masking across stimulus position in a visual search task. The results of Experiment n showed a typical SME when the target shared features with other letters in the display, but failed to show a SME when the target was dissimilar to the context items. The results of Experiment in, when compared with Experiment n, further demonstrate that the SME may disappear completely when a fixed-target procedure is used. These results suggest that the processing strategy used by the subject may vary depending on the constraints of the task. Since the SME is best explained in terms of sequential processing (cf. Merikle & Glick, 1976), subjects appear to adopt a parallel processing strategy under some circumstances. The most parsimonious explanation of the present results can be offered either by a dual-process model or a model that utilizes an operation that is capable of mimicking the complementary operation. One possibility is the model suggested by Butler (1975) which incorporates a limited capacity parallel operation coupled with a focal directable process that can handle only a limited number of items. The paral254

lel operation could be the dominant process when the target is readily discriminable or when the subject is set for a given combination of features, while the sequential operation is dominant when the task is more demanding of capacity. Such a model could also explain some cases in which sequential operations have not been observed (e.g., Eriksen & Spencer, 1969; Shiffrin & Gardner, 1972; Shiffrin, Gardner, & Allmeyer, 1 973)> D u t would also be quite compatible with demonstrations of selective attention and the known limits to selection (Butler, 1974; Dallas & Merikle, 1976; Eriksen & Hoffman, 1973; Underwood, 1976). A model combining parallel and sequential characteristics may make sense in the light of similar proposals for other perceptual phenomena. Cantor and Thomas (1977) have proposed a dual-process model to account for size illusions with briefly presented geometric stimuli; Matthews (1978) has made a similar suggestion for the discrimination of faces. Weisstein, Ozog, and Szoc (1975) and Breitmeyer and Ganz (1976) have proposed dual-process models to account for metacontrast effects in vision, although it does seem unlikely that their holistic stages, i.e. transient responses from the retina, could account for any aspects of letter search. The present dualprocess model seems most similar to Broadbent's (1977) proposal that word recognition involves an initial global analysis followed by a more active focal process. A second possibility exists and, to some extent, contradicts the distinction that has been drawn between the different processing models. The present results can be explained in terms of a single operation, providing that operation is capable of mimicking the features of the complementary process. A parallel operation could duplicate a sequential process if the stimulus trace produces a set of parallel input channels which can be weighted to vary the rate at which feature information is processed. If all channels were weighted equally, this would yield evidence for parallel processing; unequal weighting would produce B. Butler

results that could not be distinguished from those predicted by a sequential model. Such a model could explain any observation that is compatible with a dual-process model, provided that channel weighting could be adjusted during the operation of the system. Thus, information could be obtained at the same rate from all positions initially, but the rate of processing for some channels could be increased if it seems more probable that the target is located on those channels, or if the stimuli on those channels are easy to recognize. Such a model must include definite capacity limitations so that the rate of processing for each channel depends on the number of channels in operation, i.e., the number of stimuli, and the weighting, i.e., attention, assigned to the channel. If a switch in the initial weightings required either time or capacity, this could account for masking effects that persist with conditions that permit selective attention. Unlike a dual-process model, a weighted parallel channels model could produce a large number of different weightings for each stimulus channel and would seem to be ideally suited for reading or word recognition, since channels could be differentially weighted to match the discriminant letters in words. It should be noted that the present studies are concerned primarily with the initial letter recognition process and do not centre on subsequent processes such as articulatory recoding and rehearsal in short-term memory. These latter processes should be important in the report of a series of items, but should play a minimal role in letter search. The proposal, for either a dual-process or weighted channels model, does centre on the initial order of recognition; later processes could involve other modes of operation, such as serial left-toright operation (e.g., Scheerer, 1973). RESUME

Trois experiences sur le modele sequentiel propose par Merikle, Coltheart, et Lowe (1971) sur le traitement visuel d'un materiel constitue de lettres. L'experience 1, qui utilise une technique

Sequential and parallel operations

de changement de lettres, adoptee par Sperling (1970), pour etudier le traitement de ce materiel, demontre l'existence d'une strategic de traitement donnant la priorite aux lettres extremes. Les experiences 11 et m utilisent une tache de recherche visuelle pour re-examiner l'effet de masquage selectif deja propose pour expliquer le traitement prioritaire des extremites. Les resultats revelent deux limites importantes a cet effet. L'effet ne se produit pas si la cible peut facilement se distinguer des items contextuels ou si le sujet peut se preparer a un ensemble donne de caracteristiques des lettres. Ces resultats indiquent que le traitement des lettres ne peut pas s'expliquer par un modele exclusivement sequentiel ou exclusivement parallele, mais que ces deux processus sont necessaires. Deux modeles sont consideres.

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and transient channels for theories of visual pattern masking, saccadic suppression, and information processing. Psychol. Rev., 1976,83, 1-36 BROADBENT, D.E. T h e hidden preattentive processes. Am, Psychol., 1977,3a, log— 118 BUTLER, B. T h e limits of selective attention in tachistoscopic recognition. Canad.J. Psychol., 1974, 38, '99-2'3 BUTLER, B. Selective attention and target search with brief visual displays. Quart. J. exp. Psychol., 1975, VJ, 467-477 BUTLER, B., & MERIKLE, P.M. Selective maskingand pro-

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selective effects of a patterned masking stimulus. Canad.J. Psychol., 1971, 25, 264—279 MEWHORT, D.J.K. Sequential redundancy and letter spacing as determinants of tachistoscopic recognition. Canad.J. Psychol. 1966, 20, 435—444 MEWHORT, D.J.K., MERIKLE, P.M., & BRYDEN, M.P. Oil the

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NICE, D.S., & HARCUM, E.R. Evidence from mutual masking for serial processing of tachistoscopic letter patterns.Percept. motorSkilLs, 1976,42,991-1003 SCHEERER, E. A further test of the scanning hypothesis in tachistoscopic recognition. Canad.J. Psychol., •973. * 7 . 9 5 - 1 0 2 SCHNEIDER, w., & SHIFFRIN, R.M. Controlled and au-

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to forty-nine spatial positions at one.J. exp. Psychol.: Human Perc. & Per/., 1976, a, 14-22. SMITH, M.C, 8c RAMUNAS, s. Elimination of visual field effects by use of a single report technique: Evidence for order-of-report artifact.^, exp. Psychol., 1971,87, 23—28 SPERLING, G. A model for visual memory tasks. Human factors, 1963,5, 19-31 SPERLING, G. Successive approximations to a model for short-term memory. Ada Psychology 1967, 27, 285-292 SPERLING, G. Short-term memory, long-term memory, and scanning in the processing of visual information. In F.A. YOUNG and D.B. LINDSLEY (Eds.), Early experience and visual information processing in perceptu and reading disorders. Washington, D.C: National Academy of Sciences, 1970 SPERLING, G., BUDIANSKY.J., SP1VAK, J.G., & JOHNSON,

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Psychol., 1970, 21, 339-366 TRIESMAN, A. Focused attention in the perception and retrieval of multidimensional stimuli. Percept. Psychophys., 1977, 22, 1 — 11 TURVEY, M.T. On peripheral and central processes in vision: Inferences from an information-processing analysis of masking with patterned stimuli. Psychol. Rev., 1973,80, 1-52. UNDERWOOD, G. Semantic interference from unattended printed words. Brit.J. Psychol., 1976,67, 324-338 WEISSTEIN, N., O7.OG, G., & szoc, R. A comparison and elaboration of two models of meta-contrast. Psychol. Rev., 1975, 82,325-343 (First received 31 October 1977) (Dateaccepted 7 September 1978)

B. Butler

Sequential and parallel operations in tachistoscopic recognition.

Sequential and Parallel Operations in Tachistoscopic Recognition* BRIAN BUTLER Queen's University, Kingston, Ontario ABSTRACT Three experiments test...
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