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Cerebral dominance and lateral differences in perception and memory
Ncuropsychologia, 1973,Vol. 11,pp. 167to il3. PergamonPress.Printedin England.
CEREBRAL DOMINANCE AND LATERAL DIFFERENCES TN PERCEPTION AND MEMORY* HENRY L. DEE?
Neurosensory
and DONALD J. FONTENOT$
Center and Departments of Neurology and Psychology, University of Iowa, Iowa, U.S.A. (Received 23 January 1973)
Abstract-The present investigation was designed to replicate a previous finding that tachistoscopically presented complex forms of low verbal association value would be more accurately recognized when presented to the left visual field than to the right, thus implying dominance of the right hemisphere for the perception of this type of stimulus material. The successful replication of this finding was important because of previous unsuccessful attempts to show such an effect. This finding, in conjunction with the well established right visual field (i.e. left hemisphere) superiority for the perception of verbal material, strongly supports the hypothesis that asymmetry in human perceptual performance reflects hemispheric asymmetry of function rather than peripheral factors. The second question investigated concerned the role of memory in producing perceptual asymmetry. Complex figures were presented for 15-25 msec in either the left or right visual field; after a delay of O-20 set, the subject was required to indicate whether or not a form presented in central vision was the target form. The results indicate that the observed left visual field superiority for these complex forms appears to arise from hemispheric differences in memory rather than from purely perceptual processes.
ASYMMETRY in
human perceptual performance has been a field of active investigation since the early 1950’s. MISHKIN and FORGAYS [I] demonstrated that when English words are presented tachistoscopically to either the left or right visual fields, they are more accurately identified in the right visual field. This finding has been replicated in numerous investigations [2-71. While attempts have been made to explain such results employing peripheral factors, such attempts have met with little success 181. KIMURA [7] suggested that perceptual asymmetries observed under these special conditions reflect the known asymmetry of function of the human brain with respect to the processing of verbal and nonverbal material, i.e. the right field superiority for verbal stimuli might be due to the fact that input from this field is more directly transmitted to the area of the brain (i.e. the left hemisphere) most important for the processing of linguistic material. Similarly, a left field superiority for the identification of nonverbal stimuli might be due to the fact that these stimuli are more directly transmitted to the right hemisphere, which contributes more to the perception of certain nonverbal stimuli [9-121. Consistent with Kimura’s hypothesis, BRYDEN and RAINEY [4] found that both letters and
*This investigation was supported by Research Grant NS-00616 and Program-Project Grant NS-03354 from the National Institute of Neurological Diseases and Stroke. Neurosensory Center Publication No. 268. tThe order of listing of the authors is alphabetical and does not imply unequal responsibility. SPresent Address: Psychology Service, Veterans Administration Hospital, Palo Alto, California 94304, U.S.A. 167
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HENRYL. DEE and DONALD J. FONTENO~
drawings of familiar objects are more accurately identified in the right visual field, while there was no difference between the fields with respect to abstract geometric forms. KIMURA [7], while demonstrating no differences in accuracy between the visual fields with respect to identification of nonsense forms, found that subjects were more accurate in estimating the number of dots (small filled circles) and/or forms presented (in groups) to the left visual field than to the right, and could more accurately identify the locus of a dot in the left visual field than in the right [13]. While these findings provided support for her hypothesis, it must nevertheless be noted that failure to demonstrate left field superiority for presumably “nonverbal” stimuli (forms) represents a serious difficulty for any interpretation of laterality differences in perception which rests upon cerebral dominance or asymmetry of function. FONTENOT [14], in a recent investigation carried out in this laboratory, demonstrated a left field superiority in the recognition of random shapes of high complexity which were low in verbal association value. Accuracy of identification of low complexity shapes did not differ between fields. He also replicated the usual finding of a right field superiority for verbal material (consonant-vowel-consonant trigrams). This is probably the first investigation to control adequately for the verbal association value and complexity of the forms employed as stimuli and may help to explain previous unsuccessful attempts to demonstrate a left field superiority for nonverbal stimuli [7]. In this regard, it should also be noted that Fontenot controlled both visual acuity and lateral phoria in his study by excluding subjects who showed any significant deviation from normal. While the effects of these subject characteristics have not been studied with respect to performance in these experimental situations, and are seldom controlled by investigators in this area, it is interesting to note that the two subjects who were excluded because of lateral phoria showed no difference in accuracy of identification of forms in the lateral visual fields. In the experimental situation described above, i.e. the tachistoscopic presentation of stimuli to either the left or right lateral visual field, the hypothesis that asymmetry in cerebral functioning underlies observed asymmetry in human perceptual performance has thus received considerable support. It is not clear from this explanation, however, whether primacy should be given to perceptual or mnemonic factors. It is notable in this respect that of all the studies conducted on asymmetry in perception, none have been seriously concerned with the possible effects of memory. Especially with regard to the left field superiority for nonverbal material, the experimental paradigms in use fail to distinguish between retention and perception. The typical procedure is to present a form to either the left or right visual field and to then ask the subject to choose the form that he has just seen from an array of several more or less similar forms [7, 141. The choice of a response involves both interference (viewing several similar forms) or the simple passage of time. In the Fontenot study, for example, the subjects chose their responses from a multiple choice array of 32 forms: the length of time it took subjects to look through the forms varied (roughly) from 5 to 40 sec. Thus, either interference or simple decay of the memory trace might produce a decline in the accuracy of recall.
METHOD
AND
MATERIAL
Subjects Thirty right-handed undergraduate men between the ages of 18 and 25 were employed in this investigation. They were drawn from the introductory psychology courses at the University of Iowa. All subjects
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underwent visual screening procedures on a Bausch and Lomb Modified Ortho-Rater to insure normal far and near acuity, lateral and vertical phoria at both far and near distance, and stereopsis. Apparatus
A Scientific Prototype Three Channel Tachistoscope (Model GB) was used to present the test stimuli. The background and exposure fields were approximately 3 x 5 inches, subtending a horizontal visual angle of about 10” at the retina @“either side of the fixation point). Exposure duration and brightness were separately adjustable for each field. Stimuli
The test stimuli were drawn in India ink on white 4 x 5 inch cards. Nonverbal stimuli consisted of outline drawings of high complexity (12 point) random shapes taken from VANDERPLAS and GARVIN’S [15] group of forms. Eight stimuli served as target stimuli. Each of these was rated for similarity (by 75 undergraduate volunteers) to thirty other similar shapes. The stimulus judged most similar to each target stimulus was used as the incorrect foil for that item in the identification procedure. Incorrect foils that are highly similar to the target stimuli were necessary, since accuracy in discrimination of forms is partially a function of their similarity. Each of the target stimuli was drawn one inch to the left (on one stimulus card) and one inch to the right (on a second stimulus card), thus appearing 2” to the left or right of fixation. Each shape was approximately + x + inch in size, subtending a solid angle of about 1”. Another set of complex shapes served as interference stimuli (see below). These consisted of 96 additional 16 and 20 point shapes from VANDERPLAS and GARWIN 1151on 2 x 2 inch slides, six stimuli on each slide. These were presented to the subject on a ground glass screenjust one foot to the left of the tachistoscope at the rate of 1 slide every two seconds. A slide projector was used to project the stimuli onto the back of the ground glass screen. Procedure
Before each subject arrived for the experiment, he was randomly assigned to one of two conditions: Interference and Non-Interference. At his arrival, visual screening was carried out. When this was completed, the main experiment began, Test stimuli were presented in either the left or right visual field at very brief durations. After a delay of 0, 5, 10 or 20 set, the subject was shown a card on which was drawn either the test (target) stimulus or the alternative (incorrect) stimulus judged to be the most similar to that of the target stimulus (in the preliminary rating study). He was then asked to say whether the form on the card was the same as the one which had just been presented in the tachistoscope. Half of the time it was and half of the time it was not the correct stimulus. Each of the target stimuli was presented twice in each visual field (at each of four exposure durations). Once it was followed by the correct response card, once it was followed by an incorrect card. The four exposure durations were determined by preliminary investigation. Using the method of descending limits, the mean threshold for recognition (75 per cent correct responses in a same-different judgment) were determined for these stimuli. In the experiment, two duration times above and two below the mean threshold were used. These durations ranged from approximately 10 to 25 msec. The order of presentation of the stimuli within each exposure duration, the visual field in which they were presented, the delay interval, and whether the stimulus was followed by a correct or incorrect response card was determined randomly, with the following constraints: each stimulus appeared twice in each visual field, each was followed once (and only once) in each visual field by the correct response card and each delay interval (0, 5, 10, 20 set) appeared an equal number of times at each duration. The two between groups conditions, Interference and Non-Interference, were defined by the subiect’s activity during the delay or memory interval. In the Non-Interference procedure, the subject looked at the blank (lighted) ground glass screen for 0, 5, 10 or 20 sec. denending unon what the nrotocol called for on that trial. At the end of this period, he was shown the response card and asked if it was different or the same as the stimulus which had just been presented in the tachistoscope. In the Interference procedure, the subject was asked to study forms (the alternative forms described above) presented on the screen (at a rate of 1 slide per 2 set) and to retain them for testing of recall after a certain number of trials. (Note: These groups were instructed as to which condition they were in at the beginning of the experiment.) Because of the procedure outlined above, each condition in the experiment was anticipated to consume approximately two hours for each subject, including rest breaks. It was felt that the Interference vs. NonInterference procedures would best be handled as a between groups variable because of the potential effects of fatigue. Since the zero second interval involved minimal demands upon memory (unlike previous investigations), it would give information regarding differential perception as opposed to memory in the lateral visual fields. If the observed asymmetry in the past has been a function of memory, there should be little difference between the fields at this interval. (This interval might be best characterized as zero plus, since the subject, must move his head from the viewer to look at the response card, which requires roughly half a second.)
HENRY L. DEE and DONALD J. FONTENOT
170
The difference in accuracy of identification would then grow longer as a function of the delay interval. II memory is unimportant in determining differential accuracy of identification in the lateral visual fields, then the maximum difference between the fields will be obtained at the 0 set interval, with this difference either remaining constant or diminishing as a function of the delay interval. The Interference condition was included as one of the variables in the experiment since it has been present in all investigations of form perception to date. The effects upon memory of such interference in an experimental situation such as this is not known. Similarly, whether the presence of such interference interacts with lateral visual field to determine asymmetry is unknown. Thus it seemed important to include an assessment of this factor in our investigation of the possible contribution of memory to asymmetry in perception.
RESULTS The analysis of the results was based on a within subjects analysis of the number of correct identifications in each visual field, each memory interval treated separately, using the method of planned comparisons [16]. The effect of the Interference condition vs the Non-Interference condition was not signihcant (F = 1.38) and the data from the two groups were combined for subsequent analysis. The left visual field was significantly superior to the right in the recognition of the forms. all memory intervals considered together (F = 7.24), and the effect of memory was found to be significant (F =m:3.5). The planned comparisons procedure involved comparing the difference between visual fields at each memory interval. At the 0 set interval, there was a slight right visual field superiority which was not significant (F :- 1.08). The slight superiority of the left visual field over the right at the 5 set interval was also not significant (F < 1.). At 10 set, however, the left visual field was significantly superior to the right (F == 10.30). At the 20 set interval, the left visual field continued to be significantly superior to the right (F = 5.57) although the absolute value of the difference is smaller. Figure 1 summarizes these findings. It graphs the difference in accuracy of recognition (left minus right) between the two visual fields as a function of memory interval.
,’
I’
I’
:-. --._
--._
--._ -.
,‘I
I
L
FIG. 1. Differences in accuracy between the fields as a function of the retention interval.
DISCUSSION The major purposes of the present investigation were twofold. First, it was an attempt to replicate the finding [14] of a left visual field superiority for the recognition of complex forms with low association value. The successful replication of this finding was important because of previous unsuccessful attempts [8] to show such an effect. This finding, in
CEREBRALDOMINANCE
AND LATERAL DlFFERENCESlN PERCEPTION AND MEMORY
171
conjunction with the consistent reports of a right visual field superiority for verbal material, lends strong support to a cerebral dominance explanation of laterality differences in perception. The second purpose of the investigation was to investigate the role of memory in producing apparent perceptual asymmetry. That introducing a retention interval should produce a significant effect in both visual fields is, of course, not surprising since the target stimuli were deliberately chosen to minimize their verbal codeability (thus minimizing the probability that field differences might be influenced by the highly efficient verbal memory processes) and since the incorrect foils were specifically selected because they were high in rated similarity to the target stimuli. It is clear from inspection of the mean number correct at each memory interval (24.4, 22.2, 23.1 and 23.0 for 0, 5, 10 and 20 set retention intervals, respectively) lhat the zero second retention interval was superior to the others. The finding of interest with regard to memory, however, is that the observed left visual field superiority for these complex forms appears to arise from hemispheric differences in memory processes rather than purely perceptual processes. This interpretation seems relatively straightforward, since there was no significant difference between the fields in accuracy of recognition at the shortest intervals, where the performance of the subjects was at its most accurate (roughly 75 per cent) although clearly not so accurate as to mask possible differences between the fields. The significant left field superiority was obtained only at the longer retention intervals (which, incidentally, roughly coincide with the approximate time it takes subjects to search a multiple choice array and select a response foil). The present findings with regard to apparent hemispheric differences in memory performance are consistent with the findings of investigations concerning the memory performance of patients with cerebral lesions. Right hemisphere lesions have been observed to produce impairment in the retention of such nonverbal material as overlapping nonsense figures, recurring geometric designs [17] and unfamiliar faces [8]. Non-brain-damaged depressed patients show selective impairment of verbal learning and retention as a result of having electroconvulsive shock applied selectively to the left side of the cranium and selective impairment in nonverbal retention after electroconvulsive shock to the right side of the cranium [19]. (It is interesting to note that the nonverbal retention task used by HALLIDAY et al. [19] bears a striking resemblance to the dot localization task employed by KIMURA [13] in her demonstration of a left field superiority for localization of a dot.) The performance of the intact subjects in the present investigation provides valuable support for the notion that defects in retention of complex visual material observed in patients with right hemisphere lesions is not an artifact of patient selection in terms of possible group differences in age or size of lesion, visual field abnormalities, etc. In addition, they provide support for MILNER'S [18] suggestion that patients with right temporal lesions showing difficulties in facial recognition are manifesting a mnemonic rather than (or in addition to) a perceptual disorder. It is difficult to delineate the precise parameters and/or processes which produce apparent hemispheric differences in memory performance. The stimuli employed in this study were quite low in rated association value and high in (stimulus defined) complexity. These factors, in combination, produce stimuli low in whatever we mean by “verbalness” and, while it is tempting to let the matter rest at merely pointing out this difference, it does not readily explain the results. That is to say, it does not explain why the right hemisphere should accrue an advantage in the retention of such stimuli. It is, for example, equally correct to point out that such stimuli are low in familiarity and meaningfulness.
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HENRY L. DEE and DONALD J. FONTEN~I
It is also true that verbal material such as words or trigrams are of necessity processed sequentially, while this is presumably not the way a complex form may be most efficiently processed. Thus the asymmetry in memory performance observed in this study (as well as the asymmetry in performance that has been observed with respect to verbal stimuli) may reflect hemispheric differences in efficiency of processing of familiar versus unfamiliar information and/or differences in the manner in which the hemispheres process or code information. REFERENCES I. MISHKIN, M. and FORGAYS, D. G. Word recognition as a function of retinal locus. J. e.up. P.sycho/. 43, 4348, 1952. 2. HERON, W. Perception as a function of retinal locus and attention. Am. J. PsychoI. 70, 38-48, 1957. 3. TERRACE, H. The effects of retinal locus and attention on the perception of words. J. esp. Psycho/. 58, 382-385, 1959. 4. BRYDEN, M. P. and RAINEY, C. A. Left&right differences in tachistoscopic recognition. J. esp. Psycho/. 66, 568-571, 1963. 5. WYKE, M. and ETTLINGER. G. Eficienc~~ of recognilion in left and right visual fields. Arch. Neural. 5, 659-665, 1961. 6. BRYDEN, M. P. Tachistoscopic recognition, handedness, and cerebral dominance. Newopsychologia 3, l-8, 1965. 7. KIMURA, D. Dual functional asymmetry of the brain in visual perception. Nerwopsycholo~ia 4, 275-285, 1966. 8. WHITE, M. J. Laterality differences in perception: a review. Psycho/. Bull. 72, 378405, 1969. 9. BENTON, A. L. and H~~CAEN, H. Stereoscopic vision in patients with unilateral cerebral disease. Nerrrology 20, 1084-1088, 1970. IO. BENTON, A. L. The ‘minor’ hemisphere. J. His. Med. Allied Sci. 27, 5-14, 1972. I I. MILNER, B. Psychological defects produced by temporal lobe excision. Res. Pub/. ARMND 36, 244-257 1958. 12. H~CAEN, H. Clinical symptomatology in right and left hemispheric lesions. In Interhemispheric Relufiom and Cerebral Dominance, V. MOUNTCASTLE (Editor). Johns Hopkins Press, Baltimore, 1962. 13. KIMURA, D. Spatial localization in left and right visual fields. Call. J. Psycho/. 23, 445-458, 1969. 14. FONTENOT, D. J. Tachistoscopic recognition of verbal and non-verbal stimuli in left and right visual fields. Ph.D. Dissertation, University of Iowa, 1972. 15. VANDERPLAS, J. M. and GARVIN, E. A. The association value of random shapes. J. exp. Psycho/. 57, 147-154,1959. 16. HAYS, W. L. Statistics for Psychologists. Holt, Rinehart and Winston, New York, 1965. 17. KIMURA, D. Right temporal lobe damage. Arch. Neural. 8, 264-271, 1963. 18.MILNER, B. Visual recognition and recall after right temporal-lobe excision in man. Neuropsychologiu 6, 191-209, 1968. 19. HALLIDAY, A., DAVISON, K., BROWNE, M. and KRUGER, L. A comparison of bilateral and unilateral ECT. BY. J. Psychiar. 114, 997-1012, 1968. R&urn&La recherche pr&.ente a et6 rkaliste dans le but de confirmer une constatation anterieure B savoir que les formes complexes de valeur associative verbale basse seraient plus exactement reconnues sous prksentation tachistoscopique, lorsqu’clles Ctaient p&sent&es au champ visuel gauche que lorsqu’elles I’ttaient au champ droit, cette constatation impliquant une dominance de I’hCmisphtre droit dans la perception de ce type de stimulus. It ttait important de rctrouver ces rksultats en raison d’tchecs antCrieurs pour montrer un tel effet. Si on relie ce rCsultat avec le fait bien ktabli de la suptriorite du champ visuel droit (i.e. httmisphkrc gauche) dans la perception du materiel verbal, on a ainsi des arguments importants en faveur de l’hypoth&e que l’asym&rie, dans les performances perceptives humaines, traduit une asymCtrie fonctionnelle h&misph&rique plut6t qu’une asymCtrie des facteurs pCriphCriques. La seconde question envisagke concernait le r61e de la m&moire dans la production de cette asymetrie perceptive. Des figures complexes Ctaient prksentbes pendant 15-20 msec soit g I’hCmisphbre droit, soit B l’hemisphtre gauche; aprks un dClai de O-20 set, on demandait au sujet d’indiquer si une forme prksentte en vision centrale &it ou non la forme cible. D’aprks les rtsultats, il apparait que la suptrioritt du champ gauche pour ces formes complexes provient de diffkrences hCmisphCriques en r&moire plutBt que de processus purement perceptifs.
CEREBRAL
DOMINANCE
AND
LATERAL
DIFFERENCES
IN PERCEPTION
AND MEMORY
Zusammenfassung-Die vorliegende Arbeit sol1 vorausgegangene Untersuchungsbefunde in einem neuen Zusammenhang zeigen. Damals wurde festgestellt, da13 tachistoskopisch dargebotene komplexe Figuren von geringem verbalem Assoziationsgehalt besser und genauer erkannt wurden, wenn man sie in der Ii. Gesichtsfeldhllfte darbot, als in der rechten. Das sprach daftir, daI3 die rechte Hemisphare fur das Erkennen solchen optischen Reizmaterials gegeniiber der linken vorherrscht. Die richtige Deutung dieser Befunde war deswegen wichtig, weil man vorher die Ergebnisse erfolglos zu kllren versuchte. Diese Untersuchungsergebnisse sprachen zusarnmen mit den Befunden iiber die Uberlegenheit des rechten Gesichtsfeldteiles (linke Himhemisphlre) fiir die Wahrnehmung von verbalem Material dafiir, da13 die sich in der Ungleichheit der Wahrnehmungsleistung zeigende Hemisphirenungleichheit Ursache der Erscheinungen war und nicht periphere Faktoren. Die zweite Frame betraf die Bedeutung des Gedachtnisses fur die nesichtsfeldabhangige Ungleichheit der -Wahrnehmungsleistung In einem Versuch wurden komplexe Figuren 15-25 msec lang in die eine oder andere Gesichtsfeldhalfte projiziert, nach Ablauf von O-20 Sekunden mul3ten die Versuchspersonen angeben, ob im zentralen Bereich eine Scheibe gezeigt wurde oder nicht. Die Ergebnisse sprachen dafiir, da8 eine Uberlegenheit der linken Gesichtsfeldhlifte fur das Erinnern an komplexe Formen auf einer hemispharischen Leistungsdifferenz fur Gedlchtnisfunktion beruht und nicht mit dem Wahrnehmungsprozeg allein zu erkllren ist .
173
CEREBRAL DOMINANCE AND LATERAL DIFFERENCES TN PERCEPTION AND MEMORY* HENRY L. DEE?
Neurosensory
and DONALD J. FONTENOT$
Center and Departments of Neurology and Psychology, University of Iowa, Iowa, U.S.A. (Received 23 January 1973)
Abstract-The present investigation was designed to replicate a previous finding that tachistoscopically presented complex forms of low verbal association value would be more accurately recognized when presented to the left visual field than to the right, thus implying dominance of the right hemisphere for the perception of this type of stimulus material. The successful replication of this finding was important because of previous unsuccessful attempts to show such an effect. This finding, in conjunction with the well established right visual field (i.e. left hemisphere) superiority for the perception of verbal material, strongly supports the hypothesis that asymmetry in human perceptual performance reflects hemispheric asymmetry of function rather than peripheral factors. The second question investigated concerned the role of memory in producing perceptual asymmetry. Complex figures were presented for 15-25 msec in either the left or right visual field; after a delay of O-20 set, the subject was required to indicate whether or not a form presented in central vision was the target form. The results indicate that the observed left visual field superiority for these complex forms appears to arise from hemispheric differences in memory rather than from purely perceptual processes.
ASYMMETRY in
human perceptual performance has been a field of active investigation since the early 1950’s. MISHKIN and FORGAYS [I] demonstrated that when English words are presented tachistoscopically to either the left or right visual fields, they are more accurately identified in the right visual field. This finding has been replicated in numerous investigations [2-71. While attempts have been made to explain such results employing peripheral factors, such attempts have met with little success 181. KIMURA [7] suggested that perceptual asymmetries observed under these special conditions reflect the known asymmetry of function of the human brain with respect to the processing of verbal and nonverbal material, i.e. the right field superiority for verbal stimuli might be due to the fact that input from this field is more directly transmitted to the area of the brain (i.e. the left hemisphere) most important for the processing of linguistic material. Similarly, a left field superiority for the identification of nonverbal stimuli might be due to the fact that these stimuli are more directly transmitted to the right hemisphere, which contributes more to the perception of certain nonverbal stimuli [9-121. Consistent with Kimura’s hypothesis, BRYDEN and RAINEY [4] found that both letters and
*This investigation was supported by Research Grant NS-00616 and Program-Project Grant NS-03354 from the National Institute of Neurological Diseases and Stroke. Neurosensory Center Publication No. 268. tThe order of listing of the authors is alphabetical and does not imply unequal responsibility. SPresent Address: Psychology Service, Veterans Administration Hospital, Palo Alto, California 94304, U.S.A. 167
168
HENRYL. DEE and DONALD J. FONTENO~
drawings of familiar objects are more accurately identified in the right visual field, while there was no difference between the fields with respect to abstract geometric forms. KIMURA [7], while demonstrating no differences in accuracy between the visual fields with respect to identification of nonsense forms, found that subjects were more accurate in estimating the number of dots (small filled circles) and/or forms presented (in groups) to the left visual field than to the right, and could more accurately identify the locus of a dot in the left visual field than in the right [13]. While these findings provided support for her hypothesis, it must nevertheless be noted that failure to demonstrate left field superiority for presumably “nonverbal” stimuli (forms) represents a serious difficulty for any interpretation of laterality differences in perception which rests upon cerebral dominance or asymmetry of function. FONTENOT [14], in a recent investigation carried out in this laboratory, demonstrated a left field superiority in the recognition of random shapes of high complexity which were low in verbal association value. Accuracy of identification of low complexity shapes did not differ between fields. He also replicated the usual finding of a right field superiority for verbal material (consonant-vowel-consonant trigrams). This is probably the first investigation to control adequately for the verbal association value and complexity of the forms employed as stimuli and may help to explain previous unsuccessful attempts to demonstrate a left field superiority for nonverbal stimuli [7]. In this regard, it should also be noted that Fontenot controlled both visual acuity and lateral phoria in his study by excluding subjects who showed any significant deviation from normal. While the effects of these subject characteristics have not been studied with respect to performance in these experimental situations, and are seldom controlled by investigators in this area, it is interesting to note that the two subjects who were excluded because of lateral phoria showed no difference in accuracy of identification of forms in the lateral visual fields. In the experimental situation described above, i.e. the tachistoscopic presentation of stimuli to either the left or right lateral visual field, the hypothesis that asymmetry in cerebral functioning underlies observed asymmetry in human perceptual performance has thus received considerable support. It is not clear from this explanation, however, whether primacy should be given to perceptual or mnemonic factors. It is notable in this respect that of all the studies conducted on asymmetry in perception, none have been seriously concerned with the possible effects of memory. Especially with regard to the left field superiority for nonverbal material, the experimental paradigms in use fail to distinguish between retention and perception. The typical procedure is to present a form to either the left or right visual field and to then ask the subject to choose the form that he has just seen from an array of several more or less similar forms [7, 141. The choice of a response involves both interference (viewing several similar forms) or the simple passage of time. In the Fontenot study, for example, the subjects chose their responses from a multiple choice array of 32 forms: the length of time it took subjects to look through the forms varied (roughly) from 5 to 40 sec. Thus, either interference or simple decay of the memory trace might produce a decline in the accuracy of recall.
METHOD
AND
MATERIAL
Subjects Thirty right-handed undergraduate men between the ages of 18 and 25 were employed in this investigation. They were drawn from the introductory psychology courses at the University of Iowa. All subjects
CEREBRAL DOMINANCE AND LATERAL DIFFERENCESIN PERCEPTION AND MEMORY
169
underwent visual screening procedures on a Bausch and Lomb Modified Ortho-Rater to insure normal far and near acuity, lateral and vertical phoria at both far and near distance, and stereopsis. Apparatus
A Scientific Prototype Three Channel Tachistoscope (Model GB) was used to present the test stimuli. The background and exposure fields were approximately 3 x 5 inches, subtending a horizontal visual angle of about 10” at the retina @“either side of the fixation point). Exposure duration and brightness were separately adjustable for each field. Stimuli
The test stimuli were drawn in India ink on white 4 x 5 inch cards. Nonverbal stimuli consisted of outline drawings of high complexity (12 point) random shapes taken from VANDERPLAS and GARVIN’S [15] group of forms. Eight stimuli served as target stimuli. Each of these was rated for similarity (by 75 undergraduate volunteers) to thirty other similar shapes. The stimulus judged most similar to each target stimulus was used as the incorrect foil for that item in the identification procedure. Incorrect foils that are highly similar to the target stimuli were necessary, since accuracy in discrimination of forms is partially a function of their similarity. Each of the target stimuli was drawn one inch to the left (on one stimulus card) and one inch to the right (on a second stimulus card), thus appearing 2” to the left or right of fixation. Each shape was approximately + x + inch in size, subtending a solid angle of about 1”. Another set of complex shapes served as interference stimuli (see below). These consisted of 96 additional 16 and 20 point shapes from VANDERPLAS and GARWIN 1151on 2 x 2 inch slides, six stimuli on each slide. These were presented to the subject on a ground glass screenjust one foot to the left of the tachistoscope at the rate of 1 slide every two seconds. A slide projector was used to project the stimuli onto the back of the ground glass screen. Procedure
Before each subject arrived for the experiment, he was randomly assigned to one of two conditions: Interference and Non-Interference. At his arrival, visual screening was carried out. When this was completed, the main experiment began, Test stimuli were presented in either the left or right visual field at very brief durations. After a delay of 0, 5, 10 or 20 set, the subject was shown a card on which was drawn either the test (target) stimulus or the alternative (incorrect) stimulus judged to be the most similar to that of the target stimulus (in the preliminary rating study). He was then asked to say whether the form on the card was the same as the one which had just been presented in the tachistoscope. Half of the time it was and half of the time it was not the correct stimulus. Each of the target stimuli was presented twice in each visual field (at each of four exposure durations). Once it was followed by the correct response card, once it was followed by an incorrect card. The four exposure durations were determined by preliminary investigation. Using the method of descending limits, the mean threshold for recognition (75 per cent correct responses in a same-different judgment) were determined for these stimuli. In the experiment, two duration times above and two below the mean threshold were used. These durations ranged from approximately 10 to 25 msec. The order of presentation of the stimuli within each exposure duration, the visual field in which they were presented, the delay interval, and whether the stimulus was followed by a correct or incorrect response card was determined randomly, with the following constraints: each stimulus appeared twice in each visual field, each was followed once (and only once) in each visual field by the correct response card and each delay interval (0, 5, 10, 20 set) appeared an equal number of times at each duration. The two between groups conditions, Interference and Non-Interference, were defined by the subiect’s activity during the delay or memory interval. In the Non-Interference procedure, the subject looked at the blank (lighted) ground glass screen for 0, 5, 10 or 20 sec. denending unon what the nrotocol called for on that trial. At the end of this period, he was shown the response card and asked if it was different or the same as the stimulus which had just been presented in the tachistoscope. In the Interference procedure, the subject was asked to study forms (the alternative forms described above) presented on the screen (at a rate of 1 slide per 2 set) and to retain them for testing of recall after a certain number of trials. (Note: These groups were instructed as to which condition they were in at the beginning of the experiment.) Because of the procedure outlined above, each condition in the experiment was anticipated to consume approximately two hours for each subject, including rest breaks. It was felt that the Interference vs. NonInterference procedures would best be handled as a between groups variable because of the potential effects of fatigue. Since the zero second interval involved minimal demands upon memory (unlike previous investigations), it would give information regarding differential perception as opposed to memory in the lateral visual fields. If the observed asymmetry in the past has been a function of memory, there should be little difference between the fields at this interval. (This interval might be best characterized as zero plus, since the subject, must move his head from the viewer to look at the response card, which requires roughly half a second.)
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The difference in accuracy of identification would then grow longer as a function of the delay interval. II memory is unimportant in determining differential accuracy of identification in the lateral visual fields, then the maximum difference between the fields will be obtained at the 0 set interval, with this difference either remaining constant or diminishing as a function of the delay interval. The Interference condition was included as one of the variables in the experiment since it has been present in all investigations of form perception to date. The effects upon memory of such interference in an experimental situation such as this is not known. Similarly, whether the presence of such interference interacts with lateral visual field to determine asymmetry is unknown. Thus it seemed important to include an assessment of this factor in our investigation of the possible contribution of memory to asymmetry in perception.
RESULTS The analysis of the results was based on a within subjects analysis of the number of correct identifications in each visual field, each memory interval treated separately, using the method of planned comparisons [16]. The effect of the Interference condition vs the Non-Interference condition was not signihcant (F = 1.38) and the data from the two groups were combined for subsequent analysis. The left visual field was significantly superior to the right in the recognition of the forms. all memory intervals considered together (F = 7.24), and the effect of memory was found to be significant (F =m:3.5). The planned comparisons procedure involved comparing the difference between visual fields at each memory interval. At the 0 set interval, there was a slight right visual field superiority which was not significant (F :- 1.08). The slight superiority of the left visual field over the right at the 5 set interval was also not significant (F < 1.). At 10 set, however, the left visual field was significantly superior to the right (F == 10.30). At the 20 set interval, the left visual field continued to be significantly superior to the right (F = 5.57) although the absolute value of the difference is smaller. Figure 1 summarizes these findings. It graphs the difference in accuracy of recognition (left minus right) between the two visual fields as a function of memory interval.
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FIG. 1. Differences in accuracy between the fields as a function of the retention interval.
DISCUSSION The major purposes of the present investigation were twofold. First, it was an attempt to replicate the finding [14] of a left visual field superiority for the recognition of complex forms with low association value. The successful replication of this finding was important because of previous unsuccessful attempts [8] to show such an effect. This finding, in
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conjunction with the consistent reports of a right visual field superiority for verbal material, lends strong support to a cerebral dominance explanation of laterality differences in perception. The second purpose of the investigation was to investigate the role of memory in producing apparent perceptual asymmetry. That introducing a retention interval should produce a significant effect in both visual fields is, of course, not surprising since the target stimuli were deliberately chosen to minimize their verbal codeability (thus minimizing the probability that field differences might be influenced by the highly efficient verbal memory processes) and since the incorrect foils were specifically selected because they were high in rated similarity to the target stimuli. It is clear from inspection of the mean number correct at each memory interval (24.4, 22.2, 23.1 and 23.0 for 0, 5, 10 and 20 set retention intervals, respectively) lhat the zero second retention interval was superior to the others. The finding of interest with regard to memory, however, is that the observed left visual field superiority for these complex forms appears to arise from hemispheric differences in memory processes rather than purely perceptual processes. This interpretation seems relatively straightforward, since there was no significant difference between the fields in accuracy of recognition at the shortest intervals, where the performance of the subjects was at its most accurate (roughly 75 per cent) although clearly not so accurate as to mask possible differences between the fields. The significant left field superiority was obtained only at the longer retention intervals (which, incidentally, roughly coincide with the approximate time it takes subjects to search a multiple choice array and select a response foil). The present findings with regard to apparent hemispheric differences in memory performance are consistent with the findings of investigations concerning the memory performance of patients with cerebral lesions. Right hemisphere lesions have been observed to produce impairment in the retention of such nonverbal material as overlapping nonsense figures, recurring geometric designs [17] and unfamiliar faces [8]. Non-brain-damaged depressed patients show selective impairment of verbal learning and retention as a result of having electroconvulsive shock applied selectively to the left side of the cranium and selective impairment in nonverbal retention after electroconvulsive shock to the right side of the cranium [19]. (It is interesting to note that the nonverbal retention task used by HALLIDAY et al. [19] bears a striking resemblance to the dot localization task employed by KIMURA [13] in her demonstration of a left field superiority for localization of a dot.) The performance of the intact subjects in the present investigation provides valuable support for the notion that defects in retention of complex visual material observed in patients with right hemisphere lesions is not an artifact of patient selection in terms of possible group differences in age or size of lesion, visual field abnormalities, etc. In addition, they provide support for MILNER'S [18] suggestion that patients with right temporal lesions showing difficulties in facial recognition are manifesting a mnemonic rather than (or in addition to) a perceptual disorder. It is difficult to delineate the precise parameters and/or processes which produce apparent hemispheric differences in memory performance. The stimuli employed in this study were quite low in rated association value and high in (stimulus defined) complexity. These factors, in combination, produce stimuli low in whatever we mean by “verbalness” and, while it is tempting to let the matter rest at merely pointing out this difference, it does not readily explain the results. That is to say, it does not explain why the right hemisphere should accrue an advantage in the retention of such stimuli. It is, for example, equally correct to point out that such stimuli are low in familiarity and meaningfulness.
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It is also true that verbal material such as words or trigrams are of necessity processed sequentially, while this is presumably not the way a complex form may be most efficiently processed. Thus the asymmetry in memory performance observed in this study (as well as the asymmetry in performance that has been observed with respect to verbal stimuli) may reflect hemispheric differences in efficiency of processing of familiar versus unfamiliar information and/or differences in the manner in which the hemispheres process or code information. REFERENCES I. MISHKIN, M. and FORGAYS, D. G. Word recognition as a function of retinal locus. J. e.up. P.sycho/. 43, 4348, 1952. 2. HERON, W. Perception as a function of retinal locus and attention. Am. J. PsychoI. 70, 38-48, 1957. 3. TERRACE, H. The effects of retinal locus and attention on the perception of words. J. esp. Psycho/. 58, 382-385, 1959. 4. BRYDEN, M. P. and RAINEY, C. A. Left&right differences in tachistoscopic recognition. J. esp. Psycho/. 66, 568-571, 1963. 5. WYKE, M. and ETTLINGER. G. Eficienc~~ of recognilion in left and right visual fields. Arch. Neural. 5, 659-665, 1961. 6. BRYDEN, M. P. Tachistoscopic recognition, handedness, and cerebral dominance. Newopsychologia 3, l-8, 1965. 7. KIMURA, D. Dual functional asymmetry of the brain in visual perception. Nerwopsycholo~ia 4, 275-285, 1966. 8. WHITE, M. J. Laterality differences in perception: a review. Psycho/. Bull. 72, 378405, 1969. 9. BENTON, A. L. and H~~CAEN, H. Stereoscopic vision in patients with unilateral cerebral disease. Nerrrology 20, 1084-1088, 1970. IO. BENTON, A. L. The ‘minor’ hemisphere. J. His. Med. Allied Sci. 27, 5-14, 1972. I I. MILNER, B. Psychological defects produced by temporal lobe excision. Res. Pub/. ARMND 36, 244-257 1958. 12. H~CAEN, H. Clinical symptomatology in right and left hemispheric lesions. In Interhemispheric Relufiom and Cerebral Dominance, V. MOUNTCASTLE (Editor). Johns Hopkins Press, Baltimore, 1962. 13. KIMURA, D. Spatial localization in left and right visual fields. Call. J. Psycho/. 23, 445-458, 1969. 14. FONTENOT, D. J. Tachistoscopic recognition of verbal and non-verbal stimuli in left and right visual fields. Ph.D. Dissertation, University of Iowa, 1972. 15. VANDERPLAS, J. M. and GARVIN, E. A. The association value of random shapes. J. exp. Psycho/. 57, 147-154,1959. 16. HAYS, W. L. Statistics for Psychologists. Holt, Rinehart and Winston, New York, 1965. 17. KIMURA, D. Right temporal lobe damage. Arch. Neural. 8, 264-271, 1963. 18.MILNER, B. Visual recognition and recall after right temporal-lobe excision in man. Neuropsychologiu 6, 191-209, 1968. 19. HALLIDAY, A., DAVISON, K., BROWNE, M. and KRUGER, L. A comparison of bilateral and unilateral ECT. BY. J. Psychiar. 114, 997-1012, 1968. R&urn&La recherche pr&.ente a et6 rkaliste dans le but de confirmer une constatation anterieure B savoir que les formes complexes de valeur associative verbale basse seraient plus exactement reconnues sous prksentation tachistoscopique, lorsqu’clles Ctaient p&sent&es au champ visuel gauche que lorsqu’elles I’ttaient au champ droit, cette constatation impliquant une dominance de I’hCmisphtre droit dans la perception de ce type de stimulus. It ttait important de rctrouver ces rksultats en raison d’tchecs antCrieurs pour montrer un tel effet. Si on relie ce rCsultat avec le fait bien ktabli de la suptriorite du champ visuel droit (i.e. httmisphkrc gauche) dans la perception du materiel verbal, on a ainsi des arguments importants en faveur de l’hypoth&e que l’asym&rie, dans les performances perceptives humaines, traduit une asymCtrie fonctionnelle h&misph&rique plut6t qu’une asymCtrie des facteurs pCriphCriques. La seconde question envisagke concernait le r61e de la m&moire dans la production de cette asymetrie perceptive. Des figures complexes Ctaient prksentbes pendant 15-20 msec soit g I’hCmisphbre droit, soit B l’hemisphtre gauche; aprks un dClai de O-20 set, on demandait au sujet d’indiquer si une forme prksentte en vision centrale &it ou non la forme cible. D’aprks les rtsultats, il apparait que la suptrioritt du champ gauche pour ces formes complexes provient de diffkrences hCmisphCriques en r&moire plutBt que de processus purement perceptifs.
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Zusammenfassung-Die vorliegende Arbeit sol1 vorausgegangene Untersuchungsbefunde in einem neuen Zusammenhang zeigen. Damals wurde festgestellt, da13 tachistoskopisch dargebotene komplexe Figuren von geringem verbalem Assoziationsgehalt besser und genauer erkannt wurden, wenn man sie in der Ii. Gesichtsfeldhllfte darbot, als in der rechten. Das sprach daftir, daI3 die rechte Hemisphare fur das Erkennen solchen optischen Reizmaterials gegeniiber der linken vorherrscht. Die richtige Deutung dieser Befunde war deswegen wichtig, weil man vorher die Ergebnisse erfolglos zu kllren versuchte. Diese Untersuchungsergebnisse sprachen zusarnmen mit den Befunden iiber die Uberlegenheit des rechten Gesichtsfeldteiles (linke Himhemisphlre) fiir die Wahrnehmung von verbalem Material dafiir, da13 die sich in der Ungleichheit der Wahrnehmungsleistung zeigende Hemisphirenungleichheit Ursache der Erscheinungen war und nicht periphere Faktoren. Die zweite Frame betraf die Bedeutung des Gedachtnisses fur die nesichtsfeldabhangige Ungleichheit der -Wahrnehmungsleistung In einem Versuch wurden komplexe Figuren 15-25 msec lang in die eine oder andere Gesichtsfeldhalfte projiziert, nach Ablauf von O-20 Sekunden mul3ten die Versuchspersonen angeben, ob im zentralen Bereich eine Scheibe gezeigt wurde oder nicht. Die Ergebnisse sprachen dafiir, da8 eine Uberlegenheit der linken Gesichtsfeldhlifte fur das Erinnern an komplexe Formen auf einer hemispharischen Leistungsdifferenz fur Gedlchtnisfunktion beruht und nicht mit dem Wahrnehmungsprozeg allein zu erkllren ist .
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