LEFT HEMISPHERE SUPERIORITY FOR VISUAL SEARCH John M. Polich (Dartmouth College, Hanover, New Hampshire)
INTRODUCTION
Cohen (1973) has reported a study that demonstrated serial processing in the left hemisphere and parallel processing in the right hemisphere. Subjects were tachistoscopically presented with two, three, or four letters to either the left or right visual field of their right eye. Their task was to judge whether all the items in a display were. the same or whether one of them was different. Reaction times for same decisions were found to increase linearly with increases in the number of items for left hemisphere presentations, while right hemisphere response times remained relatively constant, although slower than the left hemisphere's. Different judgments for both hemisphere presentation conditions produced slight increases in reaction time· with increases in the number of stimuli, but no hemisphere processing effect was observed. The increase in response time for left hemisphere projections was interpreted as indicating serial processing of the incoming information. The flat reaction time function for right hemisphere projections was interpreted as indicating parallel processing of visual information. Although the data from other experiments in the same paper were not as clear cut, evidence for a left vs. right hemisphere information processing distinction was generally obtained. In a follow-up study, White and White (1975) used a similar paradigm and had subjects respond same or different to arrays of solid geometric forms or to groups of letters that varied in case. In that experiment, however, generally faster responses for right hemisphere presentations of both types of stimulus materials was found, and no interaction between hemisphere of presentation and number of stimulus items was obtained. The authors specifically noted that no evidence for a serial vs. parallel distinction for the processing capabilities of the cerebral hemisphere was observed. Other applications of the same/different decision paradigm in studies of Cortex (1980) 16, 39-50,
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John M. Polich
hemispheric functioning have produced a similarly confused pattern on results. Considering only studies in which neurologically normal subjects were presented with more than one visual stimulus item simultaneously and required to make a same or different response, stimuli such as letters or words have sometimes been found to produce. faster responses when presented to the left hemisphere (Egeth and Epstein, 1972; Gross, 1972; Hellige, 1975), but have often demonstrated no hemispheric effects (Hardyck, Tzeng and Wang, 1978; Hellige, 1976; Lefton and Haber, 1974). So called" nonverbal stimuli such as geometric forms, complex figures, and lines of various orientations and curvature sometimes produce faster responses when presented to the right hemisphere (Atkinson and Egeth, 1973; Egeth, 1971; Gross, 1972; Hellige, 1975; Longden, Ellis and Iversen, 1976), but have also been found to yield no hemispheric differences or even the opposite effects (Davis and Schmidt, 1973; Dimond and Beaumont, 1972; Hellige, 1976; Klatsky and Atkinson, 1971; Umilta, Bagnara and Simion, 1978). Given the inconsistency of these findings, it is not surprising that the results of Cohen (1973) and White and White (1975) are in disagreement. What is surprising is that these studies appear to be the only investigations extant that directly attack the issue o{ serial vs. parallel processing as an explanation for hemispheric differences by employing a same/different visual search paradigm with multi-element arrays. Most of the reports cited above presented only two stimulus items and generally invoked the verbal vs. nonverbal dichotomy to explain their effects, or used the lack of effects to disclaim laterality differences. The value of comparing several sizes of stimulus arrays with reaction time as the primary dependent measure lies in the potential for discerning distinct patterns of responding which may reflect fundamental information processing capabilities (e.g., Donderi and Zelnicker; 1969; Egeth, Jonides and Wall, 1972; Taylor, 1976; Townsend, 1971, 1974). While Cohen has attempted to employ just this approach, her results were not altogether compelling. However, the possibility of finding distinct information processing differences between the two cerebral hemispheres warrants further exploration with this methodology. The present study was performed in order to help clarify some of the issues rasied in the Cohen (1973) and White and White (1975) papers by replicating and extending Cohen's original approach. Although claiming to improve on Cohen's methodology, White and White employed a design that changed the stimulus materials, greatly reduced the number of trials per condition and subject, and employed binocular viewing as opposed to monocular viewing conditions. Given. the possible effects of stimulus type, the particular sensitivity of hemispheric tasks to number of trials (Hellige, 1976; Ward and Ross, 1977), and the variable influences of monocular vs.
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binocular presentations (d. Gur and Gur, 1977; McKinney, 1967; Porac and Coren, 1976), a more exact replication was deemed necessary to determine whether or not the serial vs. parallel dichotomy for hemispheric functioning holds. MATERIALS AND METHOD
Subjects A total of 12 subjects were employed. All were right handed with no left handed member in their immediate family. All subjects in the monocular viewing condition were right eye dominant. Subjects received course credit for their participation.
Apparatus The stimuli were the capital letters K, T, Y, and Z. All stimuli were presented on a Data Media Elite 1520A cathode ray tube (CRT) computet terminal. The screen of the CRT was covered with a black construction paper mask such that the only apertures were a central cross, and two sets of five rectangular holes arranged in the shape of the five dots on a die. These were located to the right and to the left of the fixation cross and spanned a distance of 3° to 6° of visual angle horizontally and ± 1.5° vertically. When the CRT screen was blank, the subject saw only the outlines of these holes. When the fixation cross appeard,the center cross was illuminated. When a stimulus display was presented, the stimulus items occurred in any two, three, or four of the display holes to the left or right of fixation. Attached to the CRT console was a viewport tube measuring 15 by 15 cm square and 35 cm long. The viewport kept the subject's head orientation and viewing distance constant and provided a means of focusing the subject's gaze on the stimulus presentation area. The keyboard of the terminal was covered with a black plexiglass cover such that only the external buttons controlling the keyboard characters D and K were operational. These characters were chosen as the response keys because their location on the keyboard produced approximately equal spacing between the index fingers of each hand with respect to the center of· the screen. A Polytronic Universal Response Timer (Model 401A) was used to record the subject's response and reaction time to an accuracy of ± .01 msec. The. timer was interfaced between the CRT and a campus timesharing computer and measured the time from the onset of the stimulus display to the subject's button press response (see Potts, 1976). All stimulus presentations and data collection operations were under software control. The apparatus was located in a laboratory room with only the light from the telephone coupler providing minimal illumination during the experimental trials.
Design Subjects were assigned randomly to either a monocular or binocular viewing condition (N = 6 per condition). Following Cohen (1973), subjects in the monocular condition wore an eye patch over the left eye and viewed all stimulus presentations wii:h their right eye. Each subject within a viewing condition received all experimental treatments. The independent variables were visual field (left vs.
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John M. Polich
right), number of stimulus items (two, three, or four), and type of response (all items the same vs. one item different). These three variables combined factorially with all possible presentations positions (with the constraint that in the three and four element conditions, the middle hole had to contain a stimulus item) produced a total of 288 trials that were divided into 12 blocks. Before each set of 6 blocks, subjects received and additional block of 24 practice trials. Data from these practice trials were excluded from all analyses. Each stimulus array was displayed for 102 msec. (the diplay and stimulus control times resulted from the hardware constraints of the system). One half the subjects responded same with the index finger of their right hands and different with the index finger of their left hand. The other half had the opposite arrangement. It should be noted that Cohen (1973) employed a right hand-same and left handdifferent response procedure. White and White (1975) changed this to a dual hand response procedure with both index fingers indicating a same response and both middle fingers indicating a different response. One half of their subjects responded in this fashion and one half had the opposite arrangement. The response hand assignment procedure of the present study was adopted to avoid the criticism of ipsilateral stimulus response bias which Cohen invited, without violating the essentials of the original design procedure.
Procedure The subjects were run individually and instructed verbally by the experimenter. The subject was first shown the stimulus display screen with all of its apertures filled with the letter Z and. the fixation cross illuminated. This display was removed and the subject was shown the blank display area with only the outlines of the stimulus apertures and fixation cross visible. He was then instructed that he would be required to make a same or different judgment about two, three, or four of the items which would be "flashed" to either the left or right display areas. He was informed that there were equal numbers of left and right trials and that he should maximize his chances of being correct by fixating on the center cross when signaled to do so. The subject was also instructed about the nature of the blocked presentation of trials and the rest period. He was informed that the first group of 24 trials for each half session was always a warm-up series. The sequence of events for each trial was as follows: When the subject was ready to begin a block of trials, he pressed the button labeled SAME. A few seconds later the fixation cross was illuminated and an electronic tone sounded. This was the subject's signal to fixate on the cross and to prepare for the stimulus array. Approximately one half second later (493 msec), the fixation cross disappeared and the stimulus array appeared. The subject then responded by depressing either the button marked SAME to designate that all items in the array were the same, or the button marked DIFF to designate that one of the items in the array was different from the rest. If the subject was correct, he heard one tone; if he had made an error, he heard two tones. The next trial occurred automatically seven to ten seconds later. This same sequence continued for the 24 trials in a given block. All nonpractice stimulus presentations which resulted in an incorrect response were reinserted into the presentation sequence at least 10 trials after they occurred. Thus, again following the exact procedure of Cohen (1973), each subject produced a complete set of 288 correct responses.
Left hemisphere superiority for visual search
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A series of four tones informed the subject that the end of the block had been reached and a 60 second rest period was begun. At this time the subject was shown his mean correct reaction time and his percentage of correct responding for the preceding block of trials on another part of the CRT screen. The subject had to remove his head from the viewport in order to observe the overall feedback information. A single tone signaled the end of the rest period (although subjects were permitted to rest as long as they wished between blocks) and the next block was initiated by depressing the SAME button. After the practice trials and six blocks, the subject was required to rest for 15 minutes before starting the second set of trials blocks. After this rest period, the subject responded to another practice set and six more blocks of 24 trials . The entire testing procedure and rest period took approximately one and one half hours. At the end of the experiment, subjects were debriefed as to the nature of the study. RESULTS
The mean reaction time for each combination of the independent variables was computed for each subject. The number of trials producing incorrect responses was counted and computed as a percentage of the total number of responses possible for each combination of the independent variables for each subject. A four factor (Viewing condition X Hemisphere X Number of stimulus items x Response type) analysis of variance was then performed on the rection time data to assess the effects of viewing conditions. Although the monocular viewing condition produced longer overall reaction times compared to the binocular condition (906 vs. 660 msec.), this difference was not significant (F = 2.4, d.f. = 1, 10, P = .15). Viewing condition did not interact statistically with any other variable, thus all analyses will consider both viewing conditions together. Since the type of viewing condition did not strongly affect any of the variables, the mean reaction times have been collapsed over this condition and are presented along with the mean percentage of errors for each hemisphere in Figure 1.
Reaction time data It is apparent from Figure 1 that the same responses for this task did not produce a linear increase in reaction time for the left hemisphere presentations and a flat reaction time curve for the right hemisphere presentations. If anything, the reverse is true. Although the different responses do indicate this effect somewhat, there is no a priori logical reason for separating the two response types since each hemisphere would be expected to process the incoming visual information in the same manner regardless of the outcome of the decision. Hence, these two classes of responses were analyzed together. While a consistent pattern of interactions between hemisphere and
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John M. Polich
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number of stimulus items that might reflect a serial vs. parallel processing distinction was not observed in the present study, one striking feature of the response tiriie data is the clear advantage for left hemisphere presentations of the stimulus items for both types of decisional outcomes. The left hemisphere produced much faster responses than the right hemisphere for all stimulus set sizes (755 vs. 81 msec.) yielding a significant difference between the two presentation conditions, F = 12.1, d.£.. = 1, 10, P < .01. Same responses were significantly faster than different responses (739 vs. 829 msec.), with F == 20.2, d.L = 1, 10, P < .001. The number of stimulus items presented also produced a significant main effect, F = 6.7, d.f. = 2, 20, P < .01. Collapsing over all other variables, the · main difference in reaction times as a function of the nutnbe.r of stimulus items was found with the increase from two to three items with a smaller increase observed going from three to four items (744, 790, 816 msec., respectively). The three variables of hemisphere of presentation response type and number of stimulus items did interact significantly, withF = 11.3, d.f. = 2, 20,
Left hemisphere superiority for visual search
45
p. < .001. This effect apparently was due to the difference~ between the left and right hemisphere presentations as a function of the two response conditions. No other interactions between any combination of these veriables approached significance. Error data
A similar four factor analysis of variance was performed on the percentage of errors obtained under each stimulus presentation condition. Monocular viewing demonstrated significantly more errors than binocular viewing (32.2% vs. 21.8%), with F = 12.1, d.£. = 1, 10, P < .01. However, viewing condition did not interact with any other variable for the error data. As illustrated in the lower portion of Figure 1, stimulus projections to the left hemisphere produced fewer overall errors of responding than stimulus projections to the right hemisphere (26.4% vs. 30.6%), although this effect was only marginally significant, F = 4.6, d.£. = 1, 10, P = .058. No effect of response type was found for the error data, but the number of errors did increase significantly with the number of stimulus items (26.0%, 27.8% 31.7%), F = 5.3, d.f. = 2,20, P <- .05. The interaction between hemisphere of presentation, response type, and number of stimulus items was also significant for the error data, F = 15.0, d.£. = 2, 20, P < .01. However, this interaction did not appear to reflect any reasonable pattern of effects, either when all of the subjects' data were averaged together or when viewed individually. Thus, Figure 1 illustrates the overall main finding from the error data, generally fewer errors for left hemisphere presentations than for right hemisphere presentations. No other interactions between any combination of variables were significant.
DISCUSSION
Several points about these results need to be emphasized. As noted above, a distinction between the left and right cerebral hemispheres based upon a serial vs. parallel processing dichotomy is not supported by these data. The overall pattern of interactions between the left and right hemisphere presentations and whether an array of stimulus items required a same or different decision was not consistent. This finding alone makes a definitive statement about hemispheric processing differences in terms of a strong serial or parallel distinction untenable. However, evidence was obtained that the left hemisphere rather than the right can more rapidly process visual arrays of variable number of stimulus items within the context of a same/different judgment task.
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John M. Polich
The type of processing required to produce this effect can best be viewed if the reaction time data presented in Figure 1 are collapsed over the response type variable. One then observes two monotonically increasing parallel lines, with the left hemisphere demonstrating substantially shorter reaction times than the right. The parallel relationship of the two curves suggests that either both hemispheres performed the task in essentially the same fashion or that projections to the right hemisphere were transferred to the .left for processing at a fairly constant rate. While there is no ready means for distinguishing between these two alternatives from the present data, the fact that both curves are very similar and increase systematically as a function of the number of stimulus items processed, does suggest how this visual search task was accomplished. The most straightforward assumption is that after the initial perceptual processing at the time of input, the array items are scanned item-by-item. If all items match, a same decision is made; if a stimulus item is found to be unlike the others, a different decision is made. Thus, as the number of stimulus items is increased, array processing time also increases as more time is required to complete the search. This processing view of hemispheric functioning implies that subjects employed some form of serial analysis for both same and different stimulus displays and that the left hemisphere performed the task better than the right for both types of stimulus presentations. One difficulty with this interpretation is that such an item-by-item analysis of a visual display predicts faster .different decisions relative to same decisions, since the subject would encounter the aberrant item of a different display before the entire array was processed. Nonlaterality reaction time studies concerned with this problem have indicated that other factors related to properties of the stimulus array itself can account for this finding. Krueger (1978) has recently proposed that same responses are generally made faster than different responses in this task (e.g., Bamber, 1969; Egeth and Blecker, 1971; Krueger, 1970; Nickerson, 1967) because "perceptual noise" produced by the aberrant item in the display will slow processing of the stim~lus. Since a same stimulus display will contain no aberrant item that might interfere with perceptual processing, it can produce a rapid response. A different stimulus array, on the other hand, is processed more slowly because the subject engages in a rechecking of the stimulus input after detecting an aberrant item in order to insure that no error is made in the response. This additional processing takes real time and is reflected in overall longer reaction times to the different stimulus arrays - just the results of the present study. The interaction obtained between hemisphere of input and type of response over the various stimulus display sizes is somewhat more perplexing, although many studies employing same/different tasks and hemispheric
Left hemisphere superiority for visual search
47
presentations have reported similar findings (e.g., Bradshaw, Gates and Patterson, 1976; Cohen, 1973; Egeth and Epstein, 1972; HeIlige, 1976; Lefton and Haber, 1974; Patterson and Bradshaw, 1975; White and White, 1975). While a clear explanation of these effects is not currently possible, there is evidence that structural properties of the stimulus display can affect processing times for this task and may affect the outcomes of laterality differences. Such factors as the symmetry of the array (Egeth, Brownell and Geoffrion, 1976; Fox, 1975), its overall Gestalt quality (Garner and Sutliff, 1974; Millspaugh, 1978), and the number of individual elements in the display (Connor, 1972; Donderi and Zelnicker, 1969) all have been shown to influence the processing times for this task. In the present study, the display structure changed from trial-to-trial and thereby varied several structural components of the stimulus. Thus, some of the variability of these findings as well as those of other studies may be the result of such structural influences on the processing of same and different stimulus displays. Despite this difficulty, however, a generally strong left hemisphere advantage was observed for both same and different stimulus displays and indicates a definite advantage for left hemisphere processing in visual search. A more important consideration concerning the overall left hemisphere effect for the present task is the fact that letters were used as stimulus items. It could be argued that subjects were merely using their left hemisphere processing capabilities to perform the same/different decision (Gazzaniga and Sperry, 1967; Milner, 1971; Levy and Treavarthen, 1976). Several recent studies, however, suggest that visual hemispheric differences are largely determined by the nature of the processing required for the specific task, rather than by the type of stimulus employed. For example, under appropriate processing conditions, a right hemisphere advantage can be found for verbal stimulus items such as letters or words (Bryden and Allard, 1976; Gibson, Dimond and Gazzaniga, 1972; Polich, 1978), or a left hemisphere advantage obtained for nonverbal stimulus items such as geometric figures or random forms (Hellige and Cox, 1976; Hellige, 1978; Umilta et a1., 1978). In the present study, the left hemisphere effects were quite large (on the order of 60 msec.) and robust relative to previous same/different studies reporting such effects (Egeth and Epstein, 1972; Gross, 1972; Hellige, 1975), or demonstrating no hemispheric differences (Hardyck, Tzeng and Wang, 1978; Hellige, 1976; Lefton and Haber, 1974). Given this sensitivity of visual laterality differences to the type of processing task performed and the inconsistency of previous results based on the assumption of a simple verbal vs. nonverbal stimulus dichotomy, ascribing the effects obtained in the present study to hemispheric specialization for verbal stimuli finds little real support. Rather, since the processing demands necessary to
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John M. Policb
make a same/different decision (when the stimulus items are presented simultaneously) appear to require very rapid serial analysis, it is reasonable to conclude that the left hemisphere can engage in this type of processing much more efficiently than the right. Two notes of caution must be advanced before accepting the results of the present study as conclusive evidence for left hemisphere superiority for visual search. The first concerns the very high error rates observed for both types of viewing conditions. For both monocular and binocular presentation modes, the trials producing an erroneous response were repeated by the subject so that all reaction times employed in the analysis were from correct responses. Averaging over viewing conditions, 28% of all trials were replaced. This finding makes strong conclusions about the relative processing differences between the hemispheres based on reaction time somewhat suspect. Although the left hemisphere error rates were less than right hemisphere error rates, and no evidence of a speed for accuracy tradeoff was apparent, lower overall error proportions (gained, perhaps, with a slightly longer exposure duration) would enhance statements about hemispheric processing differences. The second cautionary note concerns the number of trials employed for each combination of stimulus presentation variables. All subjects in the present study produced a comparatively large number of individual trials. However, a few recent studies have implied that changes in laterality phenomena can occur as the subject becomes more practiced (Hellige, 1976; Ward and Ross, 1977). Future research with this paradigm should consider employing a large number of trials in order to assure measurement of steady-state processing rather than potentially transient phenomena. SUMMARY
A replication of Cohen (1973) was conducted in an attempt to substantiate the claim for serial processing in the left hemisphere and parallel processing in the right hemisphere for visual information. Subjects made same or different judgments to hemispherically projected visual arrays of two, three, or four letters and reaction times were measured. No support for a serial vs. parallel hemispheric processing distinction was obtained, but strong support for overall left hemisphere superiority for visual search was found with both the reaction time and error percentage data.· The implications for an information processing strategy for this task as well as various methodological considerations are discussed. Acknowledgements. I would like to thank George Wolford for his help during the course of this research. Requests for reprints should be sent to the author who is now at the Cognitive Psychophysiology Laboratory, Department of Psychology, University of Illinois, Champaign, Illinois 61820.
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(1976) Changes in same-different laterality patterns as a function of practice and stimulus quality, Percep. Psychophysics, 20, 267-273. (1978) Visual laterality patterns for pre- versus mixed-list presentation, J. Exp. Psycho!.: Human Percep. Perfor., 4, 121-13l. _, and Cox, P. (1976) Effects of concurrent verbal memory on recognition of stimuli from the left and right visual fields, J. Exp. Psycho!.: Human Percep. Perfor., 2, 21O-22l. KLATSKY, R.L., and ATKINSON, R.C. (1971) Specialization of the cerebral hemispheres in scanntng for information in short-term memory, Percep. Psychophysics, 10, 335-338. KRUEGER, L.E. (1970) Effect of bracketing lines on speed of "same"-"different" judgment of two adjacent letters, J. Exp. Psycho!', 84, 324-330. (1978) A theory of perceptual matching, Psych. Rev., 85, 278-304. LEFTON, L., and HABER, R. (1974) Information extraction from different retinal locations, J. Exp. Psycho!., 102, 975-980. LEVY, J., and TREVARTHEN, C. (1976) Metaeontrol of hemispheric function in human split-brain patients, J. Exp. Psycho!.: Human Percep. Perfor., 2, 299-312. LONGDEN, K., ELLIS, c., and IVERSEN, S. (1976) Hemispheric differences in the discrImination of curvature, Neuropsychologia, 14, 195-202.
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Mc KINNEY, J. (1967) Handedness, eyedness and perceptual stability of the left and right visual fields, Neuropsychologia, 5, 339-344. MILLSPAUGH, J. (1978) Effects of array organization on same-different judgments, Percep. Psychophysics, 23, 27-35. MILNER, B. (1971) Interhemispheric differences in the localization of psychological process ini man, Brit. Med. Bull., 27, 272-277. NICKERSON, R. (1967) "Same"-"different" response times with multi-attribute stimulus differences, Percept. Mot. Skills, 24, 543-554. PATTERSON, K., and BRADSHAW, J. (1975) Differential hemispheric mediation of nonverbal visual stimuli, J. Exp. Psychol.: Human Percep. Perfor., 3, 246-252. PORAC, c., and COREN, S. (1976) The dominant eye, Psychol. Bull., 83, 880-897. POLICH, J. (1978) Hemispheric differences in stimulus identification, Percep. Psychophysics, 24, 49-57. POTTS, G. (1976) Use of a campus-wide timesharing system to run reaction time experiments, Behav. Res. Meth. Instr., 8, 179-181. TAYLOR, D. (1976) Processing of repeated letters in search and matching tasks, Percep. Psychophysics, 19, 63-68. TOWNSEND, J. (1971) A note in the identifiability of parallel and serial processes, Percep. Psychophysics, 10, 161-163. (1974) Issues and models concerning the processing of a finite number of inputs, in Human Information Processing: Tutorials in Performance and Cognition, ed. B. H. Kantowitz, Lawrence Erlbaum Associates, Hillsdale. UMILTA, c., BAGNARA, S., and SIMION, F. (1978) Laterality effects for simple and complex geometrical figures and nonsense patterns, Neuropsychologia, 16, 43-49. WARD, T., and Ross, L. (1977) Laterality differences and practice effects under central and backward masking conditions, Memory Cogn., 5, 221-226. WmTE, M ., and WHITE, K. (1975) Parallel-serial processing and hemispheric function, Neuropsychologia, 13, 377-381.
John Polich, Cognitive Psychophysiology Laboratory, Department of Psychology, University of Illinois, Champaign , Illinois 61820.