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Hemispheric Lateralization of Visual Perception
HEMISPHERIC LATERALIZATION OF VISUAL PERCEPTION C. A. Rubino (Mental Retardation Centre. Toronto, Canada)
Since Broca's (1865) observations and investigations, the two cerebral hemispheres have not been considered functionally symmetrical, although they have been considered morphologically symmetrical. Generally, this has meant that right-handed persons have speech represented in the left hemisphere, and the right hemisphere is subordinate to the left and possibly not involved in speech at all. As a result of these discoveries, efforts were directed toward detecting other functional differences between the cerebral hemispheres. One of these inter-hemispheric differences concerns the apparent lateralization in the right hemisphere of variously defined functions which appear to be visually mediated. These functions have been demonstrated clinically with right hemisphere damaged subjects in the form of impairment on performance type tasks (Piercy and Smyth, 1962; Reitan, 1955), impairment in visual and spatial analysis (McFie, Piercy and Zangwill, 1950), impairment on picture description and interpretation (Ettlinger, 1960), and have been demonstrated experimentally on visual recognition tasks (Kimura, 1963). In refining these investigations of lateral brain damage and differential impairment, the more specific effects of locus of damage have been studied. Minimal effects of unilateral hemispheric lesions in the frontal region are reported while insult to the occipital region is primarily related to visual defects and insult to the parietal region is primarily related to impairment in motor functions (Shure and Halstead, 1958). It appears that the most marked deficits in cognitive processes occur with damage to the temporal areas. It also appears that these deficits differ in nature, depending upon the laterality of the damage.
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Mishkin (1962), referring to Milner's (1954, 1958) and Lansdell's (1961) work on temporal lobe functions, stated that "clearly, the right temporal lobe in man is dominant for visual functions" (p. 105). While he did not discuss the nature of these visual functions, they are generally held to be the perception of "non-verbal" stimuli. At the same time Milner (1962) reported a lack of deficits on "any verbal task" after right temporal damage if that hemisphere is not dominant for speech. She went on to say that the important variable in the detection of deficits with right temporal lobe damage is the visual, "non-verbal" nature of the stimuli presented. Milner concluded that "in man an asymmetry renders one hemisphere dominant for the perception and learning of verbal material, leaving it less important than the other hemisphere for the perception and learning of some non-verbal visual and auditory patterns" (p. 195). Despite these conclusions, there is still much confusion over the role of the right hemisphere and specifically the right temporal area in "non-verbal" and visual functioning. Kimura (1963) investigated the influence of the temporal lobes on visual perception. She administered a series of tachistoscopic and recurring figures tests to epileptics with unilateral temporal lobe lesions, both before and after surgical removal. The tests were designed to explore the hypothesis that the right temeporal lobe aids in rapid visual identification (Milner, 1958). Kimura concluded from her results that, " it thus appears that the two hemispheres do not play equal parts in the perception of all kinds of material" (p. 269). The identification of unfamiliar material was impaired by right temporal damage. The fact that the left temporal group showed impairment in the identification of familiar objects, "suggests that, as material becomes more verbal, its perception depends more on the dominant hemisphere since the final identification of the material is more intimately bound up with the centres for speech" (p. 269). Kimura (1963) suggested a stimulus-familiarity hypothesis as an explanation for deficits found in subjects with unilateral temporal lobe damage. According to this hypothesis the right temporal lobe "participates in the immediate post-exposure trace for all types of tachistoscopic stimuli" (p. 269). Damage to the right lobe does not affect the recognition of familiar stimuli because such recognition "is more dependent on neural connections not available to the unfamiliar stimuli, connections which may, as suggested above, be primarily with the dominant hemisphere" (p. 270). From this it appears that the
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distinction between correct identification of complex visual material in right and left temporal lobe damage is whether or not verbal mediation is used in solution. Investigators are not only concerned with determining differential hemispheric functions based upon the nature of the stimuli but they are also concerned with demonstrating a dominance of one hemisphere over the other for visual perception. As stated earlier, there are no anatomical foundations to account for hemispheric dominance. Although there is an obvious functional difference between hemispheres for speech, it is difficult to account for the implied exclusive functional qualities of the right hemisphere in visual perception. In fact, in man as in monkey, there is a functional interdependence between the striate area and the temporal cortex, greater within than between hemispheres (Mishkin, 1962). This interdependence is symmetrical, however, and cannot be used to account for hemispheric differences. If one hemisphere is truly dominant for visual perception, then it becomes necessary to speculate about this functional difference. Mishkin (1962) has posited an asymmetrical development of transcallosal connections in favour of the pathway from the left striate to the right temporal cortex. This, in combination with strong homolateral pathways already existing, would permit the right temporal cortex to integrate activity from both striate areas. These relationships would lead to the development of right temporal lobe dominance for visual functions. Dorff, Mirsky and Mishkin (1965) conducted an investigation into the perception of letters successively or simultaneously exposed to the left and right visual fields. Basically, the results support the view that "extrageniculostriate mechanisms within both the right and left temporal lobes contribute to the perception of complex visual material and, further, that for each temporal lobe this contribution is greater in the field served by the primary visual system of the same hemisphere" (p. 49). Moreover, the results of the simultaneous task are supportive of Mishkin's postulate of asymmetry of visual functions. That is, while the left temporal group was impaired in the ipsilateral field, the right temporal group was impaired both in ipsilateral and contralateral fields. The left temporal lobe then apparently interacts only with the left striate area while the right temporal lobe interacts with both the
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left and right striate area providing deficits in the ipsilateral and contralateral fields. As Dorff et al. (1965) point out, while this postulate explains the results of the simultaneous task, it does not explain the results of the successive tasks. In this case, lesions in the right temporal and left temporal areas resulted in losses in both ispilateral and contralateral fields. The authors suggest that the scores were artificially depressed due to the symbolic nature of the material. This may be the case since the results of both successive tasks suggest that the right temporal group performed better than the left temporal group when the material was presented in the right visual field. If, as suggested, the left striate area interacts primarily with the left temporal lobe, taking the speech functions of the left temporal lobe into account would explain these results. If the authors had used non-symbolic materials as they suggested, the left temporal patients may have achieved higher recognition scores. However, Mishkin's postulate would suggest an expectation of depressed scores in both visual fields for the right temporal group. Kimura (1966) examined visual field perception of tachistoscopically presented letters, dots and nonsense figures in normal subjects. There were no field differences in the recognition of nonsense figures, but there was differential field recognition for the other material. A left field superiority for the perception of dots appe~red in the same subjects who showed a right field superiority for the perception of letters. These results do not support Mishkin's postulate since it would be expected that the dots would be less accurately perceived in both visual fields. I t appears then that both the right and left hemispheres deal with visual perception. Gazzaniga and Sperry (1967) have illustrated this with tachistoscopic presentations to left and right visual fields in patients with sections of the corpus callosum. They noted that the visual stimuli flashed to the left half were undescribed or reported vocally as just a flash of white light. In this case, the information received from the visual stimulus went to the right hemisphere and was not related to any verbal identification. This was not the case with information presented exclusively to the dominant hemisphere. All of the experiments reported above presented the material to the subject at very fast exposure durations. The exposure duration appears to be an important variable and if the duration is inappropriate it is possible that deficits will go undetected. Several investi-
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gators have used tachistoscopic like presentations to test for asymmetry of hemispheric functioning (e.g. Kimura, 1963; Warrington and James, 1967). However, many have presented the material to be identified at a constant exposure duration (e.g. Kimura, 1963, 1966). Warrington and James (1967) have demonstrated the problem with this procedure by obtaining different results with a constant exposure than obtained with varying exposure durations. It appears then that there would be a certain critical exposure duration at which recognition of different types of material would be maximized. It is possible that, as has been suggested by Glanzer and Clark (1963, 1964), with very fast presentations the degree of verbal mediation taking place is minimized. Since it is difficult to determine what this critical value is, because of the nature of the material, illumination etc., it would be more appropriate to test each subject at a number of varying exposure durations. Perhaps this procedure would eliminate some of the conflicting findings in this area of investigation. On the basis of the information presented above it would seem that an attempt to demonstrate hemispheric asymmetry of visual functioning might be made by employing meaningless material presented in a visual recognition task in which exposure duration was varied. Two examples of such material might be nonsense words and nonsense figures (unfamiliar), both of low association value (relatively nonmeaningful) and both satisfying Benton's (1962) criteria. On the one hand the subject would be required to identify a nonmeaningful familiar item (word) and on the other a nonmeaningful unfamiliar item (figure). The degree of experience within an item may be varied during a pre-testing training session, to test Hebb's (1942) suggestion that brain lesions have more effect upon acquisition than upon what has already been learned. This then was the nature of the following experiment in which nonsense words and nonsense figures were presented to brain-damaged subjects at very brief exposure durations. The subject was required to identify each item immediately upon presentation by selecting the appropriate item from an array of items to be presented and matching non-test items. Prior to testing, half of the test items were presented to the subject in a training session purely for the purpose of making him familiar with them. Therefore, the subject had some experience with half of the test items and no experience with the other half. Each subject was tested on two occasions, once with the nonsense words and once with the nonsense figures.
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MATERIAL AND METHOD
A group of eight nonsense words (WI) each of three letters (consonant-vowel-consonant), was chosen from Noble's (1961) list of CVCs for the Experiment. These words are characterized as nonsense words because of their low meaningfulness. The mean and standard deviation rated association (a') values for the words in WI were 1.38 and .59 respectively. The words were selected within a small range because it has been demonstrated that there is an inverse relationship between association value and recognition thresholds (Kristofferson, 1957). Further, the first letter of each word was different, reducing the possibility of a correct guess based upon recognition of the first letter alone. In addition to the WI words, a further group of eight nonsense words (W2) were also selected from Noble's (1961) list. The words in W2 were also selected for their low association value and were matched individually with the previously selected WI words on the basis of the same first consonant. The mean and standard deviation a' for the words in W2 were 1.39 and .84 respectively. A group of nonsense figures was designed following Graham's (1965, p. 571) procedure and involved the plotting and connecting of 12 points determined from a table of random numbers. A number of such figures were constructed and equated for area. From these, 20 figures were judged by 15 undergraduates as having a minimum of similarity to one another. A further group of 52 undergraduates rated their associations to these figures using the same procedure as Noble (1961) used to develop ratings for CVCs. Two groups of eight of these figures, Fl and F2, were selected for the Experiment. The mean and standard deviation association values (a') for the figures in Fl were 1.87 and .12 and for the figures in F2 were 1.87 and .15. The words and figures were presented to the subject in the same fashion. A Leitz Prodovit Color projector was used to present the test items WI or Fl one at a time by rear projection, on the Stimulus Panel. A second projector (Kodak Carousel , AV900) was used to present the Response Panel, also by rear projection. Both the test (WI and FI) and non-test items (W2 and F2) appeared on the Response Panel. When the subject was being tested for identification of the words, the Response Panel contained the eight words from WI and the eight wordsfrom W2. When the subject was being tested for identification of the figures the Response Panel contained the eight
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figures from Fl and the eight figures from F2. Two Hunter Silenced decade interval timers were used to control the operation of a flag operated rotary solenoid shutter mechanism and therefore the exposure duration. Each test item was projected onto the 2" X 2" Stimulus Panel screen. The Response Panel consisted of two 3" X 6" screens separated by a 2" bar. The bar contained two vertical rows of four buttons. There was also a vertical row of four buttons on either side of the Response Panel. The 16 buttons correspondend to 16, 1 ~" X 1 ~" sections on the Response Panel, eight for each screen. When the Response Panel was illuminated, it contained all eight test (WI or Fl) and eight non-test (W2 or F2) items. The size of the items on both the Stimulus and Response Panels was the same. The figures covered an area approximately 7/8" square and the words an area 5/8" X 2/8". With the distance of the subjects eye being approximately 23" from the Stimulus Panel, the visual angle subtended by the figures was approximately 2 X 2 degrees and by the words approximately 1.4 X .6 degrees. The illumination from both projectors was reduced by Neutral Density filters and was approximately 20 foot candles on both the Stimulus and Response Panels. The subject was seated in an adjustable chair in front of the apparatus. The height of the chair was adjusted so that the Stimulus Panel appeared at eye level. The experimenter sat at the other end of the apparatus and was not in view to the subject. Testing took place in a small examination room under normal lighting conditions. Subjects
Two experimental and one control group of subjects were employed: one group with cerebral damage in the left temporal area; one group with cerebral damage in the right temporal area; one group not impaired by any observable cerebral dysfunction. The experimental subjects were institutionalized epileptics from the Ontario Hospital, Woodstock. The subjects in the non-bra in-damaged group were selected from the attendant staff at the Hospital. The experimental subjects were selected from the population of epileptics on the basis of neurological diagnosis of right or left temporal lobe epileptogenic focus. The discrimination of the cerebral impairment was based upon several EEGs, including recent repeats,
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all of which indicated a well localized and relatively restricted focal abnormality in the brain. In order to be included in one of the experimental groups, it was necessary that the subject display EEG evidence of epileptogenic abnormality predominantly localized in the temporal area, seizure pattern from which inference could be made of lateralized epileptogenic focal disturbance and psychomotor seizures. Each subject's ability to read was tested by having him read the instructions for the task which were typed on a sheet of paper. The subjects in all three groups were right-handed, had normal or corrected vision and exhibited no gross motor impairment in the preferred hand. Each group was composed of 16 subjects, eight males and eight females. In order to equate the groups on some standardized test it was necessary to administer the Picture Completion and Similarities subtests of the WAIS (Wechsler, 1955) to each subject. Since lateralized brain damage has a differential influence upon intelligence tests (e.g., Reitan, 1955), it was necessary to select those subtests which have been shown to be insensitive to brain damage (Dennerll, 1964). If sensitive subtests had been employed then the groups would be equated statistically, though not in fact. That is, if two groups of subjects with lateralized brain damage are equated for scores on a subtest which is sensitive to left hemisphere damage, then on a nonsensitive subtest the left hemisphere group would in fact have a higher score than the right hemisphere group. The age and subtest statistics for each group are shown in Table I. Unfortunately, it was not possible to administer the subtests to the non-brain-damaged group. Information was collected on each experimental subject to provide data on degree of EEG abnormality (moderate or gross). It was not possible to obtain the actual age of onset of damage, however, it was possible to calculate the age of onset of seizures. This latter measure is considered to be a rough index of the age of onset of damage for the purposes of the Experiment. Since the experimental subjects were all selected from an institutional population it seemed important to determine the influence of length of institutionalization upon performance. There was no reason to expect the two dichotomous factors, sex and degree of EEG abnormality, to influence the performance of subjects in these tasks, nevertheless the information was available.
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TABLE I
Group Means and Standard Deviations for Age and W AIS Subtest Scores Group
Sex
N
RT
M
8
LT
NBD
Age 36.4 8.25
F
8
34.1 11.61
M
8
34.9 9.18
F
8
35.1 9.28
M
8
34.1 8.39
F
=
8
35.4 8.31
l
Legend. RT right temporal group; LT damaged group.
Sub tests Similarities P. Completion
35.3 10.13
9.6 1.12
9.3 0.92
35.0 9.23
9.3 1.09
9.4 0.97
34.8 8.77
= left
temporal group; NBD
= non
brain-
Procedure Training. The subject was told to attend carefully to the items which would appear on the Stimulus Panel. These items were four words from W1 during one training situation and four figures from F1 during another training situation. Each item was presented for three seconds per Exposure. The number of Exposures (two for words and eight for figures) per stimulus item was determined by pilot study as those which would produce 100% recognition. The interval between Exposures was three seconds. The subject was not required to make any overt response. The training items were selected randomly for each subject from the original eight test items. This was done in such a fashion that each of the eight test items was selected the same number of times. The order of presentation of the training items was randomized in blocks of four trials with the items appearing in a different order within each block. The procedure of selecting a different four items
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for each subject was necessary in order to control for the effects of "intrinsic" familiarity. Testing. At a given exposure duration all eight test items (W1 or P1) were shown one after another in random order. The subject was required to make a response after each item was presented. At the next longer exposure duration all the items were again shown in a different random order. This procedure continued, including those items aready identified, for a minimum of 15 exposure durations or until all items had been correctly identified to criteria. The order of presentation was the same for each subject. The first exposure duration was 25 milliseconds. Each subsequent duration increased by 20 milliseconds. The test and non-test items (W1 and W2 or P1 and F2) were projected onto the Response Panel after each test item had been presented on the Stimulus Panel. The subject was required to respond after each item was presented by pushing the button beside the item previously exposed on the Stimulus Panel. The order of the items on the Response Panel was random and changed every trial for eight trials then repeated itself. That is, the Response Panel changed with every item exposed on the Stimulus Panel. The Response Panel was projected with the offset of the Stimulus Panel and remained on until the subject made his response. The procedure for each item then became: Stimulus Panel on Stimulus Panel off - Response Panel on - subject responds Response Pa~el off. The justification for this procedure was twofold: if the Response Panel remained constant and on throughout the testing period, the degree of experience achieved in the training session may change over trials; with this procedure, each subject had to go through the same searching procedure. This procedure of forced response was identical in both testing situations. The subject who was seated at the apparatus was again told to attend carefully to the items which would appear" at a very fast speed" on the Stimulus Panel. The subject was then given a trial demonstration of the procedure. The maximum testing time for each testing situation was approximately 60 minutes. The order in which the subjects were tested with the words and with the figures was counterbalanced within groups.
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RESULTS
The recognition of a word or figure was defined as the exposure duration at which the last of three consecutive correct identifications occurred. The scores used in the statistical analyses were the Total Recognition Score for Familiar items, the Total Recognition Score for Unfamiliar items and the Combined Recognition Score (Familiar + Unfamiliar). The Familiar items are the items on which the subjects received training prior to testing. When the testing was completed each subject had obtained four Total Recognition Scores; one for Familiar Figures, one for Unfamiliar Figures, one for Familiar Words and one for Unfamiliar Words. The group scores are illustrated in Table II. TABLE II
Group Means and Standard Deviations for Recognition Scores Group
Figures Combined
Familiar
Words Unfamiliar
Combined
Familiar
Unfamiliar
LT
47.3 12.94
22.3
6.10
25.0 7.22
74.4 22.16
37.1 11.93
37.3 11.26
RT
86.4 27.39
41.5 14.10
44.9 14.13
44.3 16.26
20.4 7.36
23.9 10.02
NBD
38.9 7.25
17.6 3.24
21.3 4.49
27.6 2.Bl
12.9 1.05
14.7 2.02
Total Recognition Scores
In order to measure any differences statistically and also to determine the effects of other factors, an analysis of variance was performed on the Total Recognition Scores. A three (Groups-RT, LT, NED) by 16 (Subjects) factorial design with repeated measurements on two factors (Material-Figures, Words; Treatment-Familiar, Unfamiliar) was used to analyze the data (Winer, 1962). Significant main effects of Material and Groups were found, however, they were precluded because of the significant interaction of Material by Groups. This interaction is illustrated in Figure 1 where the Combined (Familiar + Unfamiliar) Recognition Scores for
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each Treatment and each Group are plotted. It is obvious from these results that the predicted hemispheric asymmetry based upon the nature of the test material was supported. Multiple t-tests of the Combined Recognition Scores indicated that the non-brain-damaged group was superior to the right temporal group which in turn was superior to the left temporal group in the recognition of the Words. However, the non-brain-damaged group did not perform at a significantly better level than the left temporal group although the left temporal group was superior to the right temporal group in the recognition of the Figures. It appears obvious that had the size of the groups been larger, the difference between the non-brain-damaged group and the left temporal group would have been significant. The 100
80 G>
~
o o
CJ)
s:: 60
o
..-!
.p
·M
s::
bD
o o
G> a:::
40
x NED o LT
20
• RT
o Figures Fig. 1 -
Combined (Familiar
Words
+
Unfamiliar) Group Recognition Scores.
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difference between the Combined Recognition Score for Words and the Combined Recognition Score for Figures was significant for all three groups. All of these differences were statistically significant at the .01 level of probability. The only other factor to prove significant in the analysis of variance was the main effect of Treatments (Familiar, Unfamiliar). It had been predicted that a significant interaction of Treatment by Groups by Material would be found; however, this was not the case. This result indicates that the only difference between the Familiar and the Unfamiliar items occurred across Groups and across Material. That is, the Recognition Score for the Familiar items was always superior to that of the Unfamiliar items. Correlations
Pearson Product Moment correlational analyses of age, length of institutionalization and age of onset of seizures, with Combined Word Recognition Score and Combined Figure Recognition Score indicated no significant relationships for any of the groups. Point-Biserial correlations of Combined Word Recognition Score and Combined Figure Recognition Score for each group, with sex and degree of EEG abnormality (Moderate versus Gross) also proved not to be significant. The fact that none of these correlations were significant is not surprising considering the small size of the groups. However, there were significant positive correlations between the Combined Word and Combined Figure Recognition Scores for both the right and left temporal groups. They were respectively .79 and .71 (p < .01, r = .623, df = 14). The correlation between these scores for the non-brain-damaged group was not significant (r = .08). These results are not surprising considering the large variations in Recognition Scores within each of the experimental groups and the small variations within the non-brain-damaged group.
DISCUSSION
The results of this investigation clearly indicate the existence of hemispheric asymmetry of visual functions at least with respect to the
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material employed in this Experiment. That is, the recognition of the familiar nonmeaningful items (words) was impaired by left temporal lobe damage while the recognition of the unfamiliar, nonmeaningful items (figures) was impaired by right temporal lobe damage. There was a clear difference in the recognition of words and figures, the direction being dependent upon the laterality of the damage. None of the left temporal damaged subjects displayed any indication of receptive aphasia as shown by their ability to comprehend and follow the instructions. As for expression, although a few subjects displayed a mild degree of expressive aphasia, its influence was well controlled by not requiring the subject to respond verbally. Simple confrontation tests which grossly measure the peripheral visual fields detected visual field deficits in three left temporal subjects and five right temporal subjects. A look at the scores of these subjects showed that while their Total Recognition Scores were generally higher than those of subjects without field defects, they were not sufficient either in number or magnitude to account for the large group differences. Although central fields were not tested, Warrington (1962) has shown that they also do not account for the group differences. In addition to the asymmetry described above, the experimental groups performed at a lower level than the non-brain-damaged group. There are two possible explanations for this result: one assuming a general depression in performance as a result of brain damage; another is the influence of anti-convulsant drugs (mysoline, dilantin, phenobarbital) which all of the brain-damaged subjects were receiving at the time of testing. It is not possible to determine here if these results are due to one or both of these factors. Because of Hebb's (1949) suggestion that brain-damage impairs the learning of new material, it was predicted that there would be a differential influence of pre-testing exposure to half of the stimulus items. That is, the items with which the subject was given some experience (training) would be recognized sooner than the nontrained items. It was expected that this would be the case for the nonbrain-damaged group for both words and figures but only true for the left temporal group with the figures and the right temporal group with the words. That is, experience with the figures and with the words would not enhance the performance of the right temporal group and the left temporal group respectively. In fact there was a difference in Recognition Scores between the Familiar and Unfamiliar items with the former being recognized sooner
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than the latter. However, this result was independent of the type of Material and of Groups contrary to that predicted. This result probably does not reflect on the validity of Hebb's hypothesis. It probably indicates that the training session was not adequately designed to tap these differences. One problem which appeared after the fact was the assumption that the time of individual exposures summated. Dainoff and Haber (1967) have demonstrated that in a recognition task one long look at the stimulus was always superior to two or more shorter looks, summing to the same total presentation time. The results were relatively independent of the influence of such factors as age, sex and length of institutionalization as indicated by the correlations obtained. However, because of the small number of subjects in each group these findings were expected. The only correlation factor which approached significance was age on onset of seizures for the subjects in the right temporal group. This factor is considered to be a rough representation of the age of onset of damage and as such indicated an inverse relationship with Combined Recognition Scores of this group for both words and figures. Hebb (1942) has suggested that age of onset of damage is an important variable which deserves investigation particularly with respect to speech development. Specifically, since at early ages the central nervous system has a good deal of plasticity, damage to the speech area may not produce as drastic effects as damage at a later age when various functional qualities become more anatomically restricted. Although the correlations obtained here for the left temporal group are positive, they are very small. However, the correlations for the right temporal group which are sufficient to suggest some relationships, are in the wrong direction. Hebb's (1942» hypothesis would suggest a positive correlation between Recognition Scores and age of onset but negative correlations were obtained. That is, as the age of onset of seizures increased performance on the tasks in this Experiment improved. An unsupported suggestion for these results would necessitate a different relationship between age of onset of damage and performance for each hemisphere. These results tend to support Hebb's suggestions and the author's conviction that such factors as age of onset of damage and duration of damage should be investigated fully. The strikingly large inter-subject variations for the right and left temporal groups and small variations· within the non-brain-damaged group account for the significant correlations between word
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0/
visual perception
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and figure Recognition Scores for the former group and lack of significance for the latter group. The variations within the experimental groups probably reflects two things, the degree of damage and the degree of dominance. There is no reason to assume that the degree of damage or dominance is equal for each subject. Luria ( 1966) examined this inequality and concluded that "correspondingly, with lesions of the dominant hemisphere, speech (as well as its related functions) is disturbed to a different degree in different subjects and may re-develop unequally being restored comparatively satisfactorily in some and hardly at all in others. These findings cannot entirely be explained by the severity of the lesion (the size of the focus, the presence of complicating factors, etc.). It is evident that the degree of dominance of one hemisphere in relation to lateralized processes such as speech varies considerably from case to case and this factor introduces a considerable element of diversity into the local pathology of higher cortical function. This may also account for the fact that circumscribed lesions in the same location may produce symptoms of unequal severity in different individuals " (p. 89). These conclusions are as relevant for the right hemisphere as for the left hemisphere. The most important results of course are those which indicate a deficit in the recognition of words by subjects with left temporal lobe damage and in the recognition of figures by subjects with right temporal lobe damage. Results similar to the ones here have generally been explained on the basis of verbal mediation (e.g. Milner, 1958). Weinstein (1962) referred to "non-verbal" achievements depending more on the right than left hemisphere. McFie and Zangwill (1960), Boller and De Renzi (1967) and others have also described this asymmetry in relation to "non-verbal" functions. Kimura (1963) found the perception of unfamiliar material to be impaired by right temporal damage and suggested that, "as material becomes more verbal, its perception depends more on the dominant hemisphere, since the final identification of the material is more intimately bound up with the centers for speech" (p. 269). The only apparent model to account for hemispheric asymmetry is based upon the assumption that there are differences in functions between the hemispheres which are independent of the sensory modality involved. The unfortunate aspect of this model is that it implies a neurological system which must act as a filter mechanism to inhibit left temporal activity in the perception of "non-verbal"
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material and right temporal activity in the perception of "verbal" material. In summary, it appears that although both hemispheres partiClpate in visual recognition tasks, differences in function can be demonstrated with highly specific tasks. Demonstrating these differences has required testing subjects with well localized and discrete cerebral lesions. However, when the important variables have been adequately defined, it should be possible to detect these differences in hemispheric functions by employing non-brain-damaged subjects.
SUMMARY
The existence of hemispheric asymmetry of function with special reference to the function of the right temporal lobe was investigated. Two types of material (nonsense words and nonsense figures) were selected on the basis of low association (a') value, to be presented to three groups of subjects, non-brain-damaged, left temporal ana right temporal. The subjects were tested in a tachistoscopic like task in which the material to be identified was presented at varying exposure durations. Identifications were made by selecting the appropriate item from a visual array of test and non-test items. Prior to testing the subject was given some experience with half of the test items. Each subject was tested on two occasions, once with the words and once with the figures. The left temporal subjects displayed a deficit in the iuentification of the words and the right temporal subjects displayed a deficit in the identification of the figures. Both experimental groups were inferior to the non-brain-damaged group. Prior experience with some of the test material enhanced performance but contrary to that predictec. this effect was not related to the type of material and the groups. These results indicated the probable existence of a hemispheric asymmetry of function independent of the sensory modality involved. This asymmetry was discussed in light of suggestions that the right temporal lobe is responsible for non-verbal functioning and the left temporal lobe for verbal functioning.
Acknowledgment. The research presented here was carried out in partial fulfillment of the Ph. D. degree at York University, Toronto. The author is grateful to the members of his Supervising Committee, Drs. 1. Howard and P. Herzberg, and in particular to the Chairman of the Committ~e,_ Dr. D. Randall.
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REFERENCES BENTON, A. L. (1962) Clinical symptomatology in right and left hemisphere lesions, in Interhemispheric Relations and Cerebral Dominance, ed. by V. B. Mountcastle, Johns Hopkins Press, Baltimore. BOLLER, F., and DE RENZI, E. (1967) Relationship between visual memory defects and hemispheric locus of lesion, "Neurology," 17, 1052-1058. BROCA, P. (1865) Sur ta faculte du langage articute, "Bull. Societe d'Anthropo!'," 6, 493-494. DAINOFF, M., and HABER, R. (1967) How much help do repeated presentations give to recognition processes, "Perception and Psychophysics," 2, 136-137. DENNERLL, R. (1964) Cognitive deficits and lateral brain dysfunction in temporal lobe epilepsy, "Epilepsia," 5, 177-191. DORFF, J., MIRSKY, A., and MISHKIN, B. (1965) Effects of unilateral temporal lobe removals in man on tachistoscopic recognition in the left and right visual fields, "Neuropsychologia," 3, 39-5l. ETTLINGER, G. (1960) The description and interpretation of pictures in cases of brain lesions, "J. Ment. Sci.," 106, 1337-1346. GAZZANIGA, M., and SPERRY, R. (1967) Language after section of the cerebral commissures, "Brain," 90, 131-148. GLANZER, B., and CLARK, W. (1963) The verbal-loop hypothesis: binary numbers, "J. Verb. Learn. Behav.," 2, 301-309. - , - (1964) The verbal loop hypothesis: conventional figures, "Amer. J. Psycho!.," 77, 621-626. GRAHAM, C. H. (1965) Visual form perception, in Vision and Visual Perception, ed. by C. H. Graham, Wiley, New York. HEBB, D. O. (1942) The effects of early and late brain injury upon test scores and the nature of normal adult intelligence, "Proc. Amer. Phi!. Soc.," 85, 275-292. (1949) The Organization of Behavior, Wiley, New York. KIMURA, D. (1963) Right temporal lobe damage, "Arch. Neuro!.," 8, 264-271. (1966) Dual functional asymmetry of the brain in visual perception, "Neuropsychologia," 4, 275-285. KRISTOFFERSON, A. (1957) Word recognition, meaningfulness, and familiarity, "Percept. Mot. Skills," 7, 219-220. LANSDELL, H. c. (1961) Two selective deficits found to be lateralized in temporal neurosurgery patients, Paper read at 32nd Annual Meeting of Eastern Psychological Association, Philadelphia. LURIA, A. (1966) Higher Cortical Functions in Man , Basic Books, New York. McFIE, ]., and ZANGWILL, O. L. (1960) Visual-constructive disabilities associated with lesions of the left cerebral hemisphere, "Brain," 83, 243-259. MILNER, B. (1954) Intellectual functions of the temporal lobes, "Psycho!. Bull.," 51, 42-62. (1958) Psychological defects produced by temporal lobe excision, "Res. Pub. Assoc. Res. Nerv. Ment. Dis.," 36, 244-257. (1962) Laterality effects in audition, in Interhemispheric Relations and Cerebral Dominance, ed. by V. B. Mountcastle, Johns Hopkins Press, Baltimore. MISHKIN, M. (1962) A possible link between interhemispheric integration in monkeys and cerebral dominance in man, in Interhemispheric Relations and Cerebral Dominance, ed. by V. B. Mountcastle, Johns Hopkins Press, Baltimore. NOBLE, C. E. (1961) Measurements of association value (a'), rated associations (a'), and scaled meaningfulness (m') for 2100 CVC combinations of the English alphabet, "Psycho!' Rep.," 8, 487-52l. PIERCY, M., and SMYTH, B. (1962) Right hemisphere dominance for certain non-verbal intellectual skills, "Brain," 85, 775-790. REITAN, R. (1955) Certain differential effects of left and right cerebral lesions in human adults, "J. Compo Physio!. Psycho!.," 48, 474-477. SHURE, G., and HALSTEAD, W. (1958) Cerebral localization of intellectual processes, "Psycho!. Monogr.," 72, 1-40.
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WARRINGTON, E. (1962) The completion of visual forms across hemianopic field defects, 'T Neurol. Neurosurg. Psychiat.," 25, 208-217. - , and JAMES, M. (1967) Tachistoscopic number estimation in patients with unilateral cerebral lesions, "J. Neurol. Neurosurg. Psychiat.," 30, 468-474. WECHSLER, D. (1955) Wechsler Adult Intelligence Scale, Psychological Corporation, New York. WEINSTEIN, S. (1962) Difference in effects of brain wounds implicating right or left hemisphere: differential effects on certain intellectual and complex perceptual functions, in Interhemispheric Relations and Cerebral Dominance, ed. by V. B. Mountcastle, Johns Hopkins Press, Baltimore. WINER, B. (1962) Statistical Principles in Experimental Design, McGraw-HilL Ne m York. Dr. C. A. Rubino, Mental Retardation Centre, 2 Surrey Place, Toronto 5, Ontario, Canada.
Since Broca's (1865) observations and investigations, the two cerebral hemispheres have not been considered functionally symmetrical, although they have been considered morphologically symmetrical. Generally, this has meant that right-handed persons have speech represented in the left hemisphere, and the right hemisphere is subordinate to the left and possibly not involved in speech at all. As a result of these discoveries, efforts were directed toward detecting other functional differences between the cerebral hemispheres. One of these inter-hemispheric differences concerns the apparent lateralization in the right hemisphere of variously defined functions which appear to be visually mediated. These functions have been demonstrated clinically with right hemisphere damaged subjects in the form of impairment on performance type tasks (Piercy and Smyth, 1962; Reitan, 1955), impairment in visual and spatial analysis (McFie, Piercy and Zangwill, 1950), impairment on picture description and interpretation (Ettlinger, 1960), and have been demonstrated experimentally on visual recognition tasks (Kimura, 1963). In refining these investigations of lateral brain damage and differential impairment, the more specific effects of locus of damage have been studied. Minimal effects of unilateral hemispheric lesions in the frontal region are reported while insult to the occipital region is primarily related to visual defects and insult to the parietal region is primarily related to impairment in motor functions (Shure and Halstead, 1958). It appears that the most marked deficits in cognitive processes occur with damage to the temporal areas. It also appears that these deficits differ in nature, depending upon the laterality of the damage.
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103
Mishkin (1962), referring to Milner's (1954, 1958) and Lansdell's (1961) work on temporal lobe functions, stated that "clearly, the right temporal lobe in man is dominant for visual functions" (p. 105). While he did not discuss the nature of these visual functions, they are generally held to be the perception of "non-verbal" stimuli. At the same time Milner (1962) reported a lack of deficits on "any verbal task" after right temporal damage if that hemisphere is not dominant for speech. She went on to say that the important variable in the detection of deficits with right temporal lobe damage is the visual, "non-verbal" nature of the stimuli presented. Milner concluded that "in man an asymmetry renders one hemisphere dominant for the perception and learning of verbal material, leaving it less important than the other hemisphere for the perception and learning of some non-verbal visual and auditory patterns" (p. 195). Despite these conclusions, there is still much confusion over the role of the right hemisphere and specifically the right temporal area in "non-verbal" and visual functioning. Kimura (1963) investigated the influence of the temporal lobes on visual perception. She administered a series of tachistoscopic and recurring figures tests to epileptics with unilateral temporal lobe lesions, both before and after surgical removal. The tests were designed to explore the hypothesis that the right temeporal lobe aids in rapid visual identification (Milner, 1958). Kimura concluded from her results that, " it thus appears that the two hemispheres do not play equal parts in the perception of all kinds of material" (p. 269). The identification of unfamiliar material was impaired by right temporal damage. The fact that the left temporal group showed impairment in the identification of familiar objects, "suggests that, as material becomes more verbal, its perception depends more on the dominant hemisphere since the final identification of the material is more intimately bound up with the centres for speech" (p. 269). Kimura (1963) suggested a stimulus-familiarity hypothesis as an explanation for deficits found in subjects with unilateral temporal lobe damage. According to this hypothesis the right temporal lobe "participates in the immediate post-exposure trace for all types of tachistoscopic stimuli" (p. 269). Damage to the right lobe does not affect the recognition of familiar stimuli because such recognition "is more dependent on neural connections not available to the unfamiliar stimuli, connections which may, as suggested above, be primarily with the dominant hemisphere" (p. 270). From this it appears that the
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distinction between correct identification of complex visual material in right and left temporal lobe damage is whether or not verbal mediation is used in solution. Investigators are not only concerned with determining differential hemispheric functions based upon the nature of the stimuli but they are also concerned with demonstrating a dominance of one hemisphere over the other for visual perception. As stated earlier, there are no anatomical foundations to account for hemispheric dominance. Although there is an obvious functional difference between hemispheres for speech, it is difficult to account for the implied exclusive functional qualities of the right hemisphere in visual perception. In fact, in man as in monkey, there is a functional interdependence between the striate area and the temporal cortex, greater within than between hemispheres (Mishkin, 1962). This interdependence is symmetrical, however, and cannot be used to account for hemispheric differences. If one hemisphere is truly dominant for visual perception, then it becomes necessary to speculate about this functional difference. Mishkin (1962) has posited an asymmetrical development of transcallosal connections in favour of the pathway from the left striate to the right temporal cortex. This, in combination with strong homolateral pathways already existing, would permit the right temporal cortex to integrate activity from both striate areas. These relationships would lead to the development of right temporal lobe dominance for visual functions. Dorff, Mirsky and Mishkin (1965) conducted an investigation into the perception of letters successively or simultaneously exposed to the left and right visual fields. Basically, the results support the view that "extrageniculostriate mechanisms within both the right and left temporal lobes contribute to the perception of complex visual material and, further, that for each temporal lobe this contribution is greater in the field served by the primary visual system of the same hemisphere" (p. 49). Moreover, the results of the simultaneous task are supportive of Mishkin's postulate of asymmetry of visual functions. That is, while the left temporal group was impaired in the ipsilateral field, the right temporal group was impaired both in ipsilateral and contralateral fields. The left temporal lobe then apparently interacts only with the left striate area while the right temporal lobe interacts with both the
Hemispheric lateralization of visual perception
105
left and right striate area providing deficits in the ipsilateral and contralateral fields. As Dorff et al. (1965) point out, while this postulate explains the results of the simultaneous task, it does not explain the results of the successive tasks. In this case, lesions in the right temporal and left temporal areas resulted in losses in both ispilateral and contralateral fields. The authors suggest that the scores were artificially depressed due to the symbolic nature of the material. This may be the case since the results of both successive tasks suggest that the right temporal group performed better than the left temporal group when the material was presented in the right visual field. If, as suggested, the left striate area interacts primarily with the left temporal lobe, taking the speech functions of the left temporal lobe into account would explain these results. If the authors had used non-symbolic materials as they suggested, the left temporal patients may have achieved higher recognition scores. However, Mishkin's postulate would suggest an expectation of depressed scores in both visual fields for the right temporal group. Kimura (1966) examined visual field perception of tachistoscopically presented letters, dots and nonsense figures in normal subjects. There were no field differences in the recognition of nonsense figures, but there was differential field recognition for the other material. A left field superiority for the perception of dots appe~red in the same subjects who showed a right field superiority for the perception of letters. These results do not support Mishkin's postulate since it would be expected that the dots would be less accurately perceived in both visual fields. I t appears then that both the right and left hemispheres deal with visual perception. Gazzaniga and Sperry (1967) have illustrated this with tachistoscopic presentations to left and right visual fields in patients with sections of the corpus callosum. They noted that the visual stimuli flashed to the left half were undescribed or reported vocally as just a flash of white light. In this case, the information received from the visual stimulus went to the right hemisphere and was not related to any verbal identification. This was not the case with information presented exclusively to the dominant hemisphere. All of the experiments reported above presented the material to the subject at very fast exposure durations. The exposure duration appears to be an important variable and if the duration is inappropriate it is possible that deficits will go undetected. Several investi-
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gators have used tachistoscopic like presentations to test for asymmetry of hemispheric functioning (e.g. Kimura, 1963; Warrington and James, 1967). However, many have presented the material to be identified at a constant exposure duration (e.g. Kimura, 1963, 1966). Warrington and James (1967) have demonstrated the problem with this procedure by obtaining different results with a constant exposure than obtained with varying exposure durations. It appears then that there would be a certain critical exposure duration at which recognition of different types of material would be maximized. It is possible that, as has been suggested by Glanzer and Clark (1963, 1964), with very fast presentations the degree of verbal mediation taking place is minimized. Since it is difficult to determine what this critical value is, because of the nature of the material, illumination etc., it would be more appropriate to test each subject at a number of varying exposure durations. Perhaps this procedure would eliminate some of the conflicting findings in this area of investigation. On the basis of the information presented above it would seem that an attempt to demonstrate hemispheric asymmetry of visual functioning might be made by employing meaningless material presented in a visual recognition task in which exposure duration was varied. Two examples of such material might be nonsense words and nonsense figures (unfamiliar), both of low association value (relatively nonmeaningful) and both satisfying Benton's (1962) criteria. On the one hand the subject would be required to identify a nonmeaningful familiar item (word) and on the other a nonmeaningful unfamiliar item (figure). The degree of experience within an item may be varied during a pre-testing training session, to test Hebb's (1942) suggestion that brain lesions have more effect upon acquisition than upon what has already been learned. This then was the nature of the following experiment in which nonsense words and nonsense figures were presented to brain-damaged subjects at very brief exposure durations. The subject was required to identify each item immediately upon presentation by selecting the appropriate item from an array of items to be presented and matching non-test items. Prior to testing, half of the test items were presented to the subject in a training session purely for the purpose of making him familiar with them. Therefore, the subject had some experience with half of the test items and no experience with the other half. Each subject was tested on two occasions, once with the nonsense words and once with the nonsense figures.
Hemispheric lateralization of visual perception
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MATERIAL AND METHOD
A group of eight nonsense words (WI) each of three letters (consonant-vowel-consonant), was chosen from Noble's (1961) list of CVCs for the Experiment. These words are characterized as nonsense words because of their low meaningfulness. The mean and standard deviation rated association (a') values for the words in WI were 1.38 and .59 respectively. The words were selected within a small range because it has been demonstrated that there is an inverse relationship between association value and recognition thresholds (Kristofferson, 1957). Further, the first letter of each word was different, reducing the possibility of a correct guess based upon recognition of the first letter alone. In addition to the WI words, a further group of eight nonsense words (W2) were also selected from Noble's (1961) list. The words in W2 were also selected for their low association value and were matched individually with the previously selected WI words on the basis of the same first consonant. The mean and standard deviation a' for the words in W2 were 1.39 and .84 respectively. A group of nonsense figures was designed following Graham's (1965, p. 571) procedure and involved the plotting and connecting of 12 points determined from a table of random numbers. A number of such figures were constructed and equated for area. From these, 20 figures were judged by 15 undergraduates as having a minimum of similarity to one another. A further group of 52 undergraduates rated their associations to these figures using the same procedure as Noble (1961) used to develop ratings for CVCs. Two groups of eight of these figures, Fl and F2, were selected for the Experiment. The mean and standard deviation association values (a') for the figures in Fl were 1.87 and .12 and for the figures in F2 were 1.87 and .15. The words and figures were presented to the subject in the same fashion. A Leitz Prodovit Color projector was used to present the test items WI or Fl one at a time by rear projection, on the Stimulus Panel. A second projector (Kodak Carousel , AV900) was used to present the Response Panel, also by rear projection. Both the test (WI and FI) and non-test items (W2 and F2) appeared on the Response Panel. When the subject was being tested for identification of the words, the Response Panel contained the eight words from WI and the eight wordsfrom W2. When the subject was being tested for identification of the figures the Response Panel contained the eight
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C. A. Rubino
figures from Fl and the eight figures from F2. Two Hunter Silenced decade interval timers were used to control the operation of a flag operated rotary solenoid shutter mechanism and therefore the exposure duration. Each test item was projected onto the 2" X 2" Stimulus Panel screen. The Response Panel consisted of two 3" X 6" screens separated by a 2" bar. The bar contained two vertical rows of four buttons. There was also a vertical row of four buttons on either side of the Response Panel. The 16 buttons correspondend to 16, 1 ~" X 1 ~" sections on the Response Panel, eight for each screen. When the Response Panel was illuminated, it contained all eight test (WI or Fl) and eight non-test (W2 or F2) items. The size of the items on both the Stimulus and Response Panels was the same. The figures covered an area approximately 7/8" square and the words an area 5/8" X 2/8". With the distance of the subjects eye being approximately 23" from the Stimulus Panel, the visual angle subtended by the figures was approximately 2 X 2 degrees and by the words approximately 1.4 X .6 degrees. The illumination from both projectors was reduced by Neutral Density filters and was approximately 20 foot candles on both the Stimulus and Response Panels. The subject was seated in an adjustable chair in front of the apparatus. The height of the chair was adjusted so that the Stimulus Panel appeared at eye level. The experimenter sat at the other end of the apparatus and was not in view to the subject. Testing took place in a small examination room under normal lighting conditions. Subjects
Two experimental and one control group of subjects were employed: one group with cerebral damage in the left temporal area; one group with cerebral damage in the right temporal area; one group not impaired by any observable cerebral dysfunction. The experimental subjects were institutionalized epileptics from the Ontario Hospital, Woodstock. The subjects in the non-bra in-damaged group were selected from the attendant staff at the Hospital. The experimental subjects were selected from the population of epileptics on the basis of neurological diagnosis of right or left temporal lobe epileptogenic focus. The discrimination of the cerebral impairment was based upon several EEGs, including recent repeats,
Hemispheric lateralization of visual perception
109
all of which indicated a well localized and relatively restricted focal abnormality in the brain. In order to be included in one of the experimental groups, it was necessary that the subject display EEG evidence of epileptogenic abnormality predominantly localized in the temporal area, seizure pattern from which inference could be made of lateralized epileptogenic focal disturbance and psychomotor seizures. Each subject's ability to read was tested by having him read the instructions for the task which were typed on a sheet of paper. The subjects in all three groups were right-handed, had normal or corrected vision and exhibited no gross motor impairment in the preferred hand. Each group was composed of 16 subjects, eight males and eight females. In order to equate the groups on some standardized test it was necessary to administer the Picture Completion and Similarities subtests of the WAIS (Wechsler, 1955) to each subject. Since lateralized brain damage has a differential influence upon intelligence tests (e.g., Reitan, 1955), it was necessary to select those subtests which have been shown to be insensitive to brain damage (Dennerll, 1964). If sensitive subtests had been employed then the groups would be equated statistically, though not in fact. That is, if two groups of subjects with lateralized brain damage are equated for scores on a subtest which is sensitive to left hemisphere damage, then on a nonsensitive subtest the left hemisphere group would in fact have a higher score than the right hemisphere group. The age and subtest statistics for each group are shown in Table I. Unfortunately, it was not possible to administer the subtests to the non-brain-damaged group. Information was collected on each experimental subject to provide data on degree of EEG abnormality (moderate or gross). It was not possible to obtain the actual age of onset of damage, however, it was possible to calculate the age of onset of seizures. This latter measure is considered to be a rough index of the age of onset of damage for the purposes of the Experiment. Since the experimental subjects were all selected from an institutional population it seemed important to determine the influence of length of institutionalization upon performance. There was no reason to expect the two dichotomous factors, sex and degree of EEG abnormality, to influence the performance of subjects in these tasks, nevertheless the information was available.
C. A. Rubino
110
TABLE I
Group Means and Standard Deviations for Age and W AIS Subtest Scores Group
Sex
N
RT
M
8
LT
NBD
Age 36.4 8.25
F
8
34.1 11.61
M
8
34.9 9.18
F
8
35.1 9.28
M
8
34.1 8.39
F
=
8
35.4 8.31
l
Legend. RT right temporal group; LT damaged group.
Sub tests Similarities P. Completion
35.3 10.13
9.6 1.12
9.3 0.92
35.0 9.23
9.3 1.09
9.4 0.97
34.8 8.77
= left
temporal group; NBD
= non
brain-
Procedure Training. The subject was told to attend carefully to the items which would appear on the Stimulus Panel. These items were four words from W1 during one training situation and four figures from F1 during another training situation. Each item was presented for three seconds per Exposure. The number of Exposures (two for words and eight for figures) per stimulus item was determined by pilot study as those which would produce 100% recognition. The interval between Exposures was three seconds. The subject was not required to make any overt response. The training items were selected randomly for each subject from the original eight test items. This was done in such a fashion that each of the eight test items was selected the same number of times. The order of presentation of the training items was randomized in blocks of four trials with the items appearing in a different order within each block. The procedure of selecting a different four items
Hemispheric lateralization of visual perception
111
for each subject was necessary in order to control for the effects of "intrinsic" familiarity. Testing. At a given exposure duration all eight test items (W1 or P1) were shown one after another in random order. The subject was required to make a response after each item was presented. At the next longer exposure duration all the items were again shown in a different random order. This procedure continued, including those items aready identified, for a minimum of 15 exposure durations or until all items had been correctly identified to criteria. The order of presentation was the same for each subject. The first exposure duration was 25 milliseconds. Each subsequent duration increased by 20 milliseconds. The test and non-test items (W1 and W2 or P1 and F2) were projected onto the Response Panel after each test item had been presented on the Stimulus Panel. The subject was required to respond after each item was presented by pushing the button beside the item previously exposed on the Stimulus Panel. The order of the items on the Response Panel was random and changed every trial for eight trials then repeated itself. That is, the Response Panel changed with every item exposed on the Stimulus Panel. The Response Panel was projected with the offset of the Stimulus Panel and remained on until the subject made his response. The procedure for each item then became: Stimulus Panel on Stimulus Panel off - Response Panel on - subject responds Response Pa~el off. The justification for this procedure was twofold: if the Response Panel remained constant and on throughout the testing period, the degree of experience achieved in the training session may change over trials; with this procedure, each subject had to go through the same searching procedure. This procedure of forced response was identical in both testing situations. The subject who was seated at the apparatus was again told to attend carefully to the items which would appear" at a very fast speed" on the Stimulus Panel. The subject was then given a trial demonstration of the procedure. The maximum testing time for each testing situation was approximately 60 minutes. The order in which the subjects were tested with the words and with the figures was counterbalanced within groups.
C. A. Rubino
112
RESULTS
The recognition of a word or figure was defined as the exposure duration at which the last of three consecutive correct identifications occurred. The scores used in the statistical analyses were the Total Recognition Score for Familiar items, the Total Recognition Score for Unfamiliar items and the Combined Recognition Score (Familiar + Unfamiliar). The Familiar items are the items on which the subjects received training prior to testing. When the testing was completed each subject had obtained four Total Recognition Scores; one for Familiar Figures, one for Unfamiliar Figures, one for Familiar Words and one for Unfamiliar Words. The group scores are illustrated in Table II. TABLE II
Group Means and Standard Deviations for Recognition Scores Group
Figures Combined
Familiar
Words Unfamiliar
Combined
Familiar
Unfamiliar
LT
47.3 12.94
22.3
6.10
25.0 7.22
74.4 22.16
37.1 11.93
37.3 11.26
RT
86.4 27.39
41.5 14.10
44.9 14.13
44.3 16.26
20.4 7.36
23.9 10.02
NBD
38.9 7.25
17.6 3.24
21.3 4.49
27.6 2.Bl
12.9 1.05
14.7 2.02
Total Recognition Scores
In order to measure any differences statistically and also to determine the effects of other factors, an analysis of variance was performed on the Total Recognition Scores. A three (Groups-RT, LT, NED) by 16 (Subjects) factorial design with repeated measurements on two factors (Material-Figures, Words; Treatment-Familiar, Unfamiliar) was used to analyze the data (Winer, 1962). Significant main effects of Material and Groups were found, however, they were precluded because of the significant interaction of Material by Groups. This interaction is illustrated in Figure 1 where the Combined (Familiar + Unfamiliar) Recognition Scores for
Hemispheric lateralization of visual perception
113
each Treatment and each Group are plotted. It is obvious from these results that the predicted hemispheric asymmetry based upon the nature of the test material was supported. Multiple t-tests of the Combined Recognition Scores indicated that the non-brain-damaged group was superior to the right temporal group which in turn was superior to the left temporal group in the recognition of the Words. However, the non-brain-damaged group did not perform at a significantly better level than the left temporal group although the left temporal group was superior to the right temporal group in the recognition of the Figures. It appears obvious that had the size of the groups been larger, the difference between the non-brain-damaged group and the left temporal group would have been significant. The 100
80 G>
~
o o
CJ)
s:: 60
o
..-!
.p
·M
s::
bD
o o
G> a:::
40
x NED o LT
20
• RT
o Figures Fig. 1 -
Combined (Familiar
Words
+
Unfamiliar) Group Recognition Scores.
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C. A. Rubino
difference between the Combined Recognition Score for Words and the Combined Recognition Score for Figures was significant for all three groups. All of these differences were statistically significant at the .01 level of probability. The only other factor to prove significant in the analysis of variance was the main effect of Treatments (Familiar, Unfamiliar). It had been predicted that a significant interaction of Treatment by Groups by Material would be found; however, this was not the case. This result indicates that the only difference between the Familiar and the Unfamiliar items occurred across Groups and across Material. That is, the Recognition Score for the Familiar items was always superior to that of the Unfamiliar items. Correlations
Pearson Product Moment correlational analyses of age, length of institutionalization and age of onset of seizures, with Combined Word Recognition Score and Combined Figure Recognition Score indicated no significant relationships for any of the groups. Point-Biserial correlations of Combined Word Recognition Score and Combined Figure Recognition Score for each group, with sex and degree of EEG abnormality (Moderate versus Gross) also proved not to be significant. The fact that none of these correlations were significant is not surprising considering the small size of the groups. However, there were significant positive correlations between the Combined Word and Combined Figure Recognition Scores for both the right and left temporal groups. They were respectively .79 and .71 (p < .01, r = .623, df = 14). The correlation between these scores for the non-brain-damaged group was not significant (r = .08). These results are not surprising considering the large variations in Recognition Scores within each of the experimental groups and the small variations within the non-brain-damaged group.
DISCUSSION
The results of this investigation clearly indicate the existence of hemispheric asymmetry of visual functions at least with respect to the
Hemispheric lateralization of visual perception
115
material employed in this Experiment. That is, the recognition of the familiar nonmeaningful items (words) was impaired by left temporal lobe damage while the recognition of the unfamiliar, nonmeaningful items (figures) was impaired by right temporal lobe damage. There was a clear difference in the recognition of words and figures, the direction being dependent upon the laterality of the damage. None of the left temporal damaged subjects displayed any indication of receptive aphasia as shown by their ability to comprehend and follow the instructions. As for expression, although a few subjects displayed a mild degree of expressive aphasia, its influence was well controlled by not requiring the subject to respond verbally. Simple confrontation tests which grossly measure the peripheral visual fields detected visual field deficits in three left temporal subjects and five right temporal subjects. A look at the scores of these subjects showed that while their Total Recognition Scores were generally higher than those of subjects without field defects, they were not sufficient either in number or magnitude to account for the large group differences. Although central fields were not tested, Warrington (1962) has shown that they also do not account for the group differences. In addition to the asymmetry described above, the experimental groups performed at a lower level than the non-brain-damaged group. There are two possible explanations for this result: one assuming a general depression in performance as a result of brain damage; another is the influence of anti-convulsant drugs (mysoline, dilantin, phenobarbital) which all of the brain-damaged subjects were receiving at the time of testing. It is not possible to determine here if these results are due to one or both of these factors. Because of Hebb's (1949) suggestion that brain-damage impairs the learning of new material, it was predicted that there would be a differential influence of pre-testing exposure to half of the stimulus items. That is, the items with which the subject was given some experience (training) would be recognized sooner than the nontrained items. It was expected that this would be the case for the nonbrain-damaged group for both words and figures but only true for the left temporal group with the figures and the right temporal group with the words. That is, experience with the figures and with the words would not enhance the performance of the right temporal group and the left temporal group respectively. In fact there was a difference in Recognition Scores between the Familiar and Unfamiliar items with the former being recognized sooner
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than the latter. However, this result was independent of the type of Material and of Groups contrary to that predicted. This result probably does not reflect on the validity of Hebb's hypothesis. It probably indicates that the training session was not adequately designed to tap these differences. One problem which appeared after the fact was the assumption that the time of individual exposures summated. Dainoff and Haber (1967) have demonstrated that in a recognition task one long look at the stimulus was always superior to two or more shorter looks, summing to the same total presentation time. The results were relatively independent of the influence of such factors as age, sex and length of institutionalization as indicated by the correlations obtained. However, because of the small number of subjects in each group these findings were expected. The only correlation factor which approached significance was age on onset of seizures for the subjects in the right temporal group. This factor is considered to be a rough representation of the age of onset of damage and as such indicated an inverse relationship with Combined Recognition Scores of this group for both words and figures. Hebb (1942) has suggested that age of onset of damage is an important variable which deserves investigation particularly with respect to speech development. Specifically, since at early ages the central nervous system has a good deal of plasticity, damage to the speech area may not produce as drastic effects as damage at a later age when various functional qualities become more anatomically restricted. Although the correlations obtained here for the left temporal group are positive, they are very small. However, the correlations for the right temporal group which are sufficient to suggest some relationships, are in the wrong direction. Hebb's (1942» hypothesis would suggest a positive correlation between Recognition Scores and age of onset but negative correlations were obtained. That is, as the age of onset of seizures increased performance on the tasks in this Experiment improved. An unsupported suggestion for these results would necessitate a different relationship between age of onset of damage and performance for each hemisphere. These results tend to support Hebb's suggestions and the author's conviction that such factors as age of onset of damage and duration of damage should be investigated fully. The strikingly large inter-subject variations for the right and left temporal groups and small variations· within the non-brain-damaged group account for the significant correlations between word
Hemispheric lateralization
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and figure Recognition Scores for the former group and lack of significance for the latter group. The variations within the experimental groups probably reflects two things, the degree of damage and the degree of dominance. There is no reason to assume that the degree of damage or dominance is equal for each subject. Luria ( 1966) examined this inequality and concluded that "correspondingly, with lesions of the dominant hemisphere, speech (as well as its related functions) is disturbed to a different degree in different subjects and may re-develop unequally being restored comparatively satisfactorily in some and hardly at all in others. These findings cannot entirely be explained by the severity of the lesion (the size of the focus, the presence of complicating factors, etc.). It is evident that the degree of dominance of one hemisphere in relation to lateralized processes such as speech varies considerably from case to case and this factor introduces a considerable element of diversity into the local pathology of higher cortical function. This may also account for the fact that circumscribed lesions in the same location may produce symptoms of unequal severity in different individuals " (p. 89). These conclusions are as relevant for the right hemisphere as for the left hemisphere. The most important results of course are those which indicate a deficit in the recognition of words by subjects with left temporal lobe damage and in the recognition of figures by subjects with right temporal lobe damage. Results similar to the ones here have generally been explained on the basis of verbal mediation (e.g. Milner, 1958). Weinstein (1962) referred to "non-verbal" achievements depending more on the right than left hemisphere. McFie and Zangwill (1960), Boller and De Renzi (1967) and others have also described this asymmetry in relation to "non-verbal" functions. Kimura (1963) found the perception of unfamiliar material to be impaired by right temporal damage and suggested that, "as material becomes more verbal, its perception depends more on the dominant hemisphere, since the final identification of the material is more intimately bound up with the centers for speech" (p. 269). The only apparent model to account for hemispheric asymmetry is based upon the assumption that there are differences in functions between the hemispheres which are independent of the sensory modality involved. The unfortunate aspect of this model is that it implies a neurological system which must act as a filter mechanism to inhibit left temporal activity in the perception of "non-verbal"
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material and right temporal activity in the perception of "verbal" material. In summary, it appears that although both hemispheres partiClpate in visual recognition tasks, differences in function can be demonstrated with highly specific tasks. Demonstrating these differences has required testing subjects with well localized and discrete cerebral lesions. However, when the important variables have been adequately defined, it should be possible to detect these differences in hemispheric functions by employing non-brain-damaged subjects.
SUMMARY
The existence of hemispheric asymmetry of function with special reference to the function of the right temporal lobe was investigated. Two types of material (nonsense words and nonsense figures) were selected on the basis of low association (a') value, to be presented to three groups of subjects, non-brain-damaged, left temporal ana right temporal. The subjects were tested in a tachistoscopic like task in which the material to be identified was presented at varying exposure durations. Identifications were made by selecting the appropriate item from a visual array of test and non-test items. Prior to testing the subject was given some experience with half of the test items. Each subject was tested on two occasions, once with the words and once with the figures. The left temporal subjects displayed a deficit in the iuentification of the words and the right temporal subjects displayed a deficit in the identification of the figures. Both experimental groups were inferior to the non-brain-damaged group. Prior experience with some of the test material enhanced performance but contrary to that predictec. this effect was not related to the type of material and the groups. These results indicated the probable existence of a hemispheric asymmetry of function independent of the sensory modality involved. This asymmetry was discussed in light of suggestions that the right temporal lobe is responsible for non-verbal functioning and the left temporal lobe for verbal functioning.
Acknowledgment. The research presented here was carried out in partial fulfillment of the Ph. D. degree at York University, Toronto. The author is grateful to the members of his Supervising Committee, Drs. 1. Howard and P. Herzberg, and in particular to the Chairman of the Committ~e,_ Dr. D. Randall.
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