Can commissurotomized subjects compare digits between the visual fields?

Neuropsychologia, Vol. 32, No. 12, pp. 1475-1486, 1994 Copyright 0 1994 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 002%3932/94 $7.00+0&l

0028-3932(94)00085-9

CAN COMMISSUROTOMIZED SUBJECTS COMPARE DIGITS BETWEEN THE VISUAL FIELDS?

MICHAEL C. CORBALLIS

Department of Psychology, University of Auckland, Private Bag 92019, Auckland, New Zealand

(Received 21 January 1994; accepted 30 May 1994)

Abstract-Sergent (Brain 113, 537-568, 1990) flashed pairs of digits to the opposite visual fields of three commissurotomized subjects (L.B., N.G. and A.A.), and found that they were highly accurate in deciding which digit was the larger, but at little or no better than chance in deciding whether the digits were the same or different. Experiment 1 confirmed that these subjects were better at relative than at sam&ifferent judgements. However their performance on relative judgements was considerably lower than in Sergent’s study and could be. explained largely in terms of the subjects’ use of information available to a single hemisphere. A fourth subject, D.K., with section of the posterior corpus callosum only, had little difficulty with either task despite previous evidence of visual disconnection, and appeared able to transfer the information verbally. In Experiment 2, L.B. was better able to make both relative and same-different judgements when the digits were presented within either the left or right visual fields than when they were presented in opposite fields. These results suggest little, if any, interhemispheric transfer of either shape or numerical information following complete forebrain commissurotomy.

Key Words: commissurotomy; hemispheric differences; visual perception; subcortical vision.

INTRODUCTION

It is now well established that section of the forebrain commissures results in a pervasive disconnection syndrome [l, 171, in which each cerebral hemisphere is essentially unaware of information available to the other. One manifestation of this is that commissurotomized subjects generally perform at little or no better than chance at deciding whether two patterns, projected simultaneously to each visual field, are the same or different [6, 8, 9, 14, 161.

Sergent [13,14; but see also 71 has recently argued that this result may underestimate the ability of these subjects to compare information across the visual fields. In one study [ 143, for example, she flashed pairs of digits, one in each visual field, and had three commissurotomized subjects, L.B., A.A. and N.G., perform a variety of comparisons. As expected, A.A. and N.G. failed to score above chance in deciding whether the digits were the same or different, although L.B. achieved 75% correct. (Curiously, these results contrast with those ofJohnson 193, who found that N.G. did score above chance in a same-different task, while L.B. failed to do so.) However all three subjects scored considerably higher when required to judge whether the left or the right digit was the larger. Indeed L.B. scored 97.22% correct when the larger digit was on the left, and 100% when it was on the right. Sergent then proceeded to show that all three subjects could score well above chance in making same-different judgements when the instructions were changed to focus on the digits’ relative numerical values; that is, the subjects were to indicate whether one or other digit was higher, or whether

1475

1416 M. C. CORBALLIS

the two were equal. When the same task was repeated but with instructions simply to judge whether the digits were the same or different, their scores again reverted to being at or close to chance. These experiments suggested to Sergent that commissurotomized subjects can pass highly accurate numerical information between hemispheres, even if they cannot transfer information about shape.

Interpretation of these results is not entirely straightforward, however. When making relative judgements across the two fields it is possible to score considerably above 50% on the basis of information presented to one field alone. In Sergent’s study 1141, the subjects were presented with pairs of digits, each ranging from 1 to 9. The two digits were never the same, leaving 72 possible “different” combinations, and the task was to decide whether the larger of the two was on the left or on the right. If the subject examines only the digits on the right, say, and responds “right” if that digit exceeds four and “left” otherwise, this will produce a total of 56 correct responses. This is 77.8% correct, well above the “chance” level of 50%. While this cannot fully explain the performance actually achieved by the three subjects, L.B., N.G. and A.A., who scored 98.6,95.8 and 86.1%, respectively, it does warn against directly comparing performance on this task with that on the same-different task.

The same argument applies to other experiments on relative judgements. For example, Sergent [15] had commissurotomized subjects compare pairs of circles, one in each visual field. In one experiment they had to judge which was the larger of two circles, and in another they had to judge which contained the radial line closer to the vertical. There were only three levels of each, and judgements based on just one of the two stimuli could yield an expected accuracy of 83.3% correct, which is precisely the score achieved by A.A. on the size task. The other two subjects, L.B. and N.G., both scored higher, but were also above chance on the sameedifferent versions of these tasks. In another study, L.B. was shown up to four asterisks in random locations in each visual field [6]. When asked to decide whether there were more on the left, more on the right, or the same number on each side, he scored at no better than chance when “more on the left” and “more on the right” were combined and classified as “different” and contrasted with “same” responses. However, on “different” trials when he judged the number on one or other side to be greater he was nearly always correct. L.B.‘s performance could be explained in terms of accurate assessment of the number on one side, together with only limited information as to the number on the other.

A recent study by Seymour et al. [16] of interhemispheric comparisons following callosotomy fails to confirm some of Sergent’s basic findings. In deciding which visual field contained the larger of two digits one subject, J.W., scored no better than expected on the basis of the information on one side only. J.W. and two further subjects, V.P. and D.R. were tested under conditions contrived to negate the strategy of basing the decision on a single digit, and their performances fell away to chance, even though V.P. has some sparing of fibres in the splenium. Moreover J.W. was unable to score at better than chance in judging whether digits in the two fields were the same or different even when encouraged to adopt a strategy based on the relative values of the digits-a strategy that Sergent [ 141 claimed was successful for her three commissurotomized subjects. These three subjects had undergone section of the corpus callosum only, whereas Sergent’s subjects had also undergone section of other commissures, including the anterior commissure, but it is difficult to see how this could

explain the discrepancy in results. The present study re-examines the interhemispheric comparison of digits in the three

subjects (L.B., N.G. and A.A.) tested by Sergent [14], as well as in a fourth subject (D.K.) with posterior callosotomy. The main aim was to verify and extend Sergent’s [14]

VISUAL INTEGRATION AFTER COMMISSUROTOMY 1477

observations on performance on relative judgements about stimuli in the two fields, and in

particular to investigate the extent to which performance could be explained in terms of the information available to a single hemisphere. L.B. was also tested on same-different and relative judgements about pairs of digits that were presented wholly in the left or right visual fields.

EXPERIMENT 1 Mrtlzod

Subjects. Three of the subjects were L.B., a 40.year-old man, N.G., a 59-year-old woman, and A.A., a 42-year-old man, who had undergone section of the corpus callosum and the anterior and hippocampal commissures in 1963, 1964 and 1965, respectively. The testing was carried out in February 1993. Details of the neurological history and status of these subjects are available elsewhere [4]. and magnetic resonance imaging has confirmed that their forebrain commissures are completely sectioned 131.

The fourth subject was D.K., a 27-year-old man. At the age of 18 he had undergone surgery for an arteriovenous malformation deep in the medial face of the right occipital lobe, resulting in section of the posterior third to one half of the corpus callosum, loss of some occipital tissue and possible damage to the fornix. Two years after the operation his Verbal and Performance IQs on the Wechsler Adult Intelligence Scale were assessed at I I6 and 110, respectively. He showed no signs of hemineglect when asked to copy a simple drawing, and scored above the 75th percentile on the Rey Complex Figure. He has a left homonymous field defect in the upper left quadrant, but otherwise appears to have normal vision. He is employed and drives a car.

Although no brain scans are available, D.K. shows marked signs of visual disconnection. In a previous study [7] he scored at no better than chance on a lexical decision task involving four-letter strings flashed bilaterally, but was highly accurate if the strings were flashed in the right visual field (RVF) and well above chance when they were gashed in the left visual field (LVF); his performance on these tasks was comparable to that of L.B. His ability to make lexical decisions about LVF letter strings contrasts with his very poor ability to name words flashed in the LVF; he is reasonably accurate, however, at naming single letters in the LVF [12, and unpublished experiments]. Corballis and Trudel [7] also showed him to be at chance on a task requiring judgements of whether diagonal lines llashed simultaneously in the two visual fields were aligned or not, but highly accurate when the lines were flashed in the same field. His performance under the bilateral condition was considerably worse than that of L.B.

Equipment und stimuli. Digits, ranging from 2 to 8, were flashed on a VGA screen controlled by a computer. The digits 1 and 9 were excluded since their presence might have encouraged a strategy of examiningjust one digit in the case of relative judgements (see below); if the subject detects a nine, for example, it can be immediately inferred that it must be the larger of any pair. The subjects were not informed of these omissions. The digits were 1.2 cm high and were white against a black background. The subjects sat with their chins on a chinrest so that their eyes were 57 cm from the screen, which meant that 1 cm on the screen corresponded to 1 deg of visual angle. They recorded their responses by pressing either the N or the M key on the keyboard. The digit pairs were presented for 100 msec, with the digits centred 4.9 cm to the left and right of the centre of the screen.

Tusks. There were two tasks: &me-different judgemenrs. In this task, the digits were presented in all possible combinations, so that there were 42 .‘different” combinations and 7 “same” combinations. Each “same” combinations was presented 6 times, to make a total of 42. This made a total of 84 presentations per block of trials, and these were presented in a different random order on each block.

As soon as the experimenter initiated a trial, a small fixation cross appeared in the centre of the screen, to be replaced after 500 msec by the pair of digits. Subjects were instructed to fixate on the cross, and to respond with the forefinger and middle finger ofone hand by pressing the M key iftheyjudged the digits to be the same, and the N key if they judged them to be different. Each subject was given 10 practice trials comprising digit pairs drawn at random from experimental trials, and L.B., N.G. and D.K. were then given two blocks of 84 trials, one with each hand; L.B. and N.G. started with the right hand and D.K. with the left. A.A. was given only one block of 84 trials, and used his left hand. Relatiur judgements. In this task, the digit pairs were the same as under the “different” condition of the sameedifferent task; they were never the same. There were therefore a total of 42 trials per block, presented in random order. The conditions of presentation were otherwise the same as for the same-different task,

The subjects were told to press the N key if the digit on the left was the larger, and the M key if the digit on the right was the larger. This choice of keys ensured response compatibility. All subjects were again given 10 practice trials. L.B. was then given four blocks of trials, using his left hand, right hand, right hand, and left hand, respectively. N.G. was also given four blocks, but with the hands reversed (i.e. right, left, left and right). A.A. and D.K. were given only two blocks, first with the right hand then with the left hand.


Results and Discussion

Same-diferent judgements. The responses (“same” and “different”) of the subjects were subjected to individual x2 analyses to determine the effects of hand and digit combination (same vs different). L.B. failed to show a significant effect of digit combination [x2( 1) = 2.391, while for N.G. the association was significant only according to a directional test [x2(1)=3.43, 0.05 < P<O.lO]. A.A. used only his left hand, and with that hand the effect of digit combination was not significant [x2(1)=0.57]; the association was in fact slightly negative. For D.K., by contrast, the association was highly significant [x2(1)= 83.95, P<O.OOl]. That is, only D.K. scored significantly above chance on the task.

L.B.‘s responses were also significantly dependent on the hand used [x2(1)=9.57, P < O.OOl], reflecting a strong bias to respond “same” with the right hand and “different” with the left, perhaps due to a bias to respond with the index finger. D.K., however, showed a strong overall bias to respond “same” [x2( 1) = 7.7 1, P < 0.011. Although A.A. used only his left hand, his responses with that hand showed a highly significant bias toward “same” responses [x2(1)=21.0, P<O.OOl]. There were no other significant effects. Table 1 shows

Table 1. Percentage of correct responses made by the four commissurotomized subjects on the same-different task, as a

function of stimulus combination and hand used, in Experiment 1

L.B. N.G. A.A. D.K.

Right hand Same Different

Left hand Same Different

Overall

62.3 50.0 100.0 52.4 59.5 78.6

40.5 64.3 71.4 90.5 71.4 54.8 21.4 69.0 56.0 57.1 46.4 84.5

percentage correct as a function of same vs different pairs, and of the hand used, for each subject. The mean reaction times (RTs) were 1075 msec for L.B., 1061 msec for N.G., 933 msec for A.A., and 682 msec for D.K.

These results confirm that L.B., N.G. and A.A. all perform poorly in making same-different judgements. In that N.G. scored slightly better than L.B., the results resemble those of Johnson [9] rather than Sergent [14]. However D.K. clearly had relatively little difficulty with the task. It was noted however that he appeared to be having difficulty early in the first block until he suddenly remarked that he could perform the task by naming the digits, and from that point he often did so audibly. This probably explains why he performed slightly better with the right hand, which he used on the second block of trials, although the difference was not in fact significant [x’(l)= 1.601. Subsequent tests, to be the subject of a later report, suggest that D.K. can report pairs of digits and letters in opposite visual fields fairly (but not completely) accurately, but cannot transfer information about shape.

Relative judgements. The responses (“left greater “, “right greater”) were again analysed for each subject to determine whether they were systematically influenced by the digit combination (left greater vs right greater), and by the hand used. All four subjects showed a significant effect of digit combination [L.B.: x2(1)=52.66, P<O.OOl; N.G.: x2(1)= 11.20, P~O.001; A.A.: x2(1)=7.47, P~0.01; D.K.: x2(1)=65.23, P<O.OOl]. In addition, N.G.


showed a strong bias to respond “right greater” [x2( 1) = 10.5, P<O.OOl]. There were no other significant effects. The accuracy of the four subjects, as a function of whether the larger digit was on the left or on the right, and of the hand used, is shown in Table 2. Their mean RTs were 659 msec for L.B., 1124 msec for N.G., 922 msec for A.A., and 518 msec for D.K.

These data confirm Sergent’s [14] finding that L.B., N.G. and A.A. perform better in terms of overall accuracy in making relative judgements about digits in the two visual fields than in judging their sameness or difference. Their actual accuracy scores, however, were considerably lower than those reported by Sergent, but similar to that reported for the subject J.W. by Seymour et al. [16]. Only D.K. performed at greater than 90% correct, which was the level reached by L.B. and N.G. in Sergent’s study. Again, however, it appeared that D.K. performed the task by actually vocalizing the digits.

Table 2. Percentage correct of the four commissurotomized subjects in judging the relative values of digits, as a function of which visual field

contained the larger digit and of hand used, in Experiment 1

L.B. N.G. A.A. D.K.

Right hand Right digit larger Left digit larger Overall (Right hand)

Left hand Right digit larger Left digit larger Overall (Left hand)

Overall (Both hands)

76.2 83.3 81.0 90.5 73.8 54.8 38.1 95.2 75.0 69.0 59.5 92.9

83.3 65.9 76.2 95.2 78.6 45.2 61.9 95.2 80.1 55.4 69.0 95.2 77.6 62.2 64.3 94.1

As noted earlier, it is possible to achieve a score of well above 50% by responding on the basis of the digits in one visual field alone. In the present experiment a consistent strategy of attending to the digit on one side, say the right, and responding “right greater” if that digit exceeded 3 and “left greater” otherwise, would achieve a score of 78.6% correct. Of the three subjects also tested by Sergent, only L.B. scored at about this level. On the basis of this criterion, therefore, there is no evidence that these three subjects were actually able to compare the digits across the visual fields at all.

In order to assess the relative contributions of the two digits to the decisions more precisely, the data were also subjected to discriminant-function analysis, in which the functions relating the numerical values of the two digits that maximally discriminated the subjects’ “left greater” and “right greater” responses were computed. These functions take the form (a + b,R + b,L), where R and L represent the numerical values of digits on the right and left, respectively. The constants a, b,, and h, were scaled such that perfect performance would be represented by a =O, h, = 1, and h,= - 1, and are shown for each sujbect, separately for each hand, in Table 3. Also shown are the Fvalues indicating whether or not the functions discriminated significantly between the two responses.

In all cases the RVF digit was weighted positively and the LVF digit negatively, as expected if the subjects were indeed comparing the two. However the weights were not in all cases significantly different from zero. Moreover the function did not discriminate significantly in the case of N.G. when she used her left hand, nor for A.A. when he used his right hand; in these cases, performance was even worse than one would expect had the


Table 3. Results of discriminant-function analysis of relative judgements in Experiment 1

Subject Hand U h, ht. d.f. F

L.B. Right ~ 2.042 0.864* ~ 0.439* 2. 81 Left -2.333 0.929* ~ 0.429* 2. 81

N.G. Right -0.125 0.496* -0.271 2, 81 Left 0.960 0.160 - 0.208 2.801

A.A. Right 0 0.436 -0.136 2, 39 Left ~ I.250 0.693* -0.343* 2, 39

D.K. Right -0.917 1.021* -0.871* 2. 39 Left - 2.000 1.164* -0.764* 2, 39

21.66* 25.43*

6.04* 1 .I4 1.96 5.6x*

42.82* 52.78*

*P<o.ol. tN.G. made one “illegal” response in which she pressed a key other than N or M.

judgement been based on the digit in one visual field only. For N.G.‘s right-handed performance, only the RVF weight was significantly greater than zero, so it cannot be reliably concluded that the digits were actually compared. In all other cases, both digits contributed significantly, although the RVF digit received the larger weight.

In the case of D.K., since his overall performance was above the maximum attainable by focusing on one visual field only, it is clear that both digits contributed to the judgement. However the higher weights for the digit on the right suggest that, regardless of the hand used, the decision was controlled by the left hemisphere, which received accurate (direct) information about the value of the RVF digit, but rather less accurate information about the value of the LVF digit. L.B.‘s accuracy was very close to the maximum attainable from a single visual field (78.6%) so it remains conceivable that his decisions Kerr based on just one of the two digits, more often the one on the right, but sometimes the one on the left.

If the subjects were indeed responding largely on the basis of a single digit, then a further prediction follows: They should be highly accurate when a digit representing a high number is paired with one representing a low number, but not above chance when two high or low digits are paired. To test this, the digit 5 was excluded and digits above 5 were designated “high” and those below 5 “low”. Table 4 shows the percentages correct for each of the four categories defined by the combinations of high and low digits on the left and right. For L.B. the prediction is clearly confirmed: Hc was barely above chance when the two digits wcrc either both high or both low, but around 90% correct when high and low digits were paired; this difference was highly significant [x’(l) = 18.64, P<O.OOl]. The prediction is weakly confirmed for A.A. [12(l)=3.96, P-cO.051, but not for N.G. [x2(1)=0.16, n.s.]. Curiously, only N.G. scored significantly above chance when low digits or high digits were paired

Table 4. Perccntagc correct on rclativc judgements for each combtnation of low (< 5) and high ( > 5) dIgit\ in the left and right visual fields, for L.B.. N.G. and A.A.

in Experiment I

LVI; RVF L.B. N.G. A.A

Low Low 5x.3 70.8 50.0 High High 54.2 5x.3 50.0 Low High 91.7 X0.6 80.3 High Low 88.9 55.6 66.7


[x2(1)=4.08, P<O.O5], suggesting that only she of the fully commissurotomized subjects may have been capable of some (weak) interhemispheric transfer; she also performed best of the three on same-different judgements.

Conclusions

Although the three subjects with complete forebrain commissurotomy performed better on relative judgements than on same-different judgements in terms of percentage correct, the advantage dissolves when one considers that it is possible to perform at well above 50% by basing relative judgements on the value ofjust one of the digits. With one exception (N.G., left hand), the absolute values of the weights were higher for RVF than for LVF digits, indicating that relative judgements were based primarily on the RVF digit, and therefore on information projected to the left hemisphere.

However the fact that the RVF digit was always weighted positively and the LVF digit negatively in the discriminant function raises the possibility that some degree of interhemispheric comparison of numerical information was occurring-although the weights were not in all cases significant. It is also possible that control was shifted to the LVF digit (right hemisphere) on at least some trials, so that interhemispheric transfer may never have occurred. Indeed the analysis shown in Table 4 strongly suggests that L.B.‘s (and to a lesser extent A.A.‘s) performance can be explained this way; he was highly accurate when a high digit was paired with a low one, but barely above chance when two high or two low

digits were paired. It is also worth noting that performance on relative judgements can be raised to over 90%

with the transfer of merely binary information. For example, suppose the LVF digit is coded as “large” if above 4 and “small” otherwise, and this code is relayed from the right to the left hemisphere. The decision rule might then be: (1) if the left digit is large, respond “right greater” if the right digit is above 6 and “left greater” otherwise, and (2) if the left digit is small, respond “right greater” if the right digit is above 3 and “left greater” otherwise. Consistently applied, this rule would lead to an accuracy of 92.9%, which is in fact higher than the performance achieved by the three subjects with complete forebrain commissurotomy.

The data therefore fail to support Sergent’s contention [14] that numerical information can be transferred interhemispherically in the absence of the forebrain commissures. Ironically, only N.G. showed evidence of weak transfer, yet her performance on relative judgements was considerably below that of L.B., whose performance could be explained in terms of information projected to a single hemisphere. The present results are not comparable with those of Sergent 1141, however, in that L.B., N.G. and A.A. here scored considerably lower on the relative-judgement task. It is conceivable that this was due to different stimulus conditions. Although the digits were presented closer to the fixation point in the present experiment (4.9 vs 7.5 deg), they were evidently smaller (1.2 deg vertically vs 3.0 deg horizontally [sic]) and were flashed for a shorter duration (100 vs 150 msec). The fact that D.K. scored highly on both tasks suggests that the different stimulus conditions may not have been critical. Even so, it seemed reasonable to assess performance under conditions likely to enhance discriminability. This, in part, was the aim of Experiment 2.

EXPERIMENT 2

In this experiment, L.B.‘s performance on both same-different and relative judgements is examined under conditions in which the digits to be compared were moved closer together


(2.8 cm instead of 9.8 cm apart) and were presented unilaterally as well as bilaterally. Unilateral presentation provides information as to whether the tasks can be performed wholly in a single hemisphere, and provide a baseline for determining whether any difficulty with bilateral presentation is due to poor discriminability of the digits.

Method

Subject. The subject for this experiment was L.B. Equipment and stimuli. The equipment was the same as in Experiment 1. The stimulus pairs were the same except

that they were presented in different locations. There were three conditions: (1) LVFcondition, in which the digits were centred 4.2 deg and 1.4 deg to the left of the centre of the screen; (2) Bilateral condition, in which they were centred 1.4 deg to the left and 1.4 deg to the right; and (3) RVF condition, in which they were centred 1.4 deg and 4.2 deg to the right.

Tasks. Same-diDrent judgements. All “same” combinations and half of the possible “different” combinations of the digits 2 through 8 were presented under all three location conditions. The “different” combinations comprised the following pairs: (2,3), (2,5), (2,7), (3,4), (3,6), (3,8), (4,5), (4,7), (5,6), (5,8), (6.7) and (7,8). Under each location condition each “different” pair was presented twice, once with the larger digit on the left and once with the larger digit on the right, making a total of 24 “different” pairs. The “same” pairs were presented 3 times, making a total of 21, in each location condition. There were thus 135 presentations per block, randomly ordered. Each trial began with the appearance of the fixation cross for 500 msec, followed by a pair of digits for 100 msec. The subject again pressed the M key for “same” and the N key for “different”judgements, using the forefinger and middle finger of one hand. He used his right hand for one block of trials and then his left hand for another block. Relative judgements. In this task, only “different” pairs were presented. Each of the 42 possible combinations was presented in each location, making a total of 126 trials per block, which were presented in random order. L.B. again pressed the M key for “right greater” and the N key for “left greater”. He was given one block with his right hand followed by another with his left hand.

Results and discussion

Same-diflerent judgements. A three-way x2 analysis was first undertaken to determine the effect of hand, location (LVF, bilateral and RVF), and digit combination (same vs different) on response (“same” vs “different”). There was a significant effect of digit combination [x’(l)= 138.83, P<O.OOl], indicating that, overall, L.B. had little difficulty with the task. However there was also a significant interaction between location and digit combination [x2(2)=28.32, P<O.OOl]; the effect of digit combination was much greater for LVF [x2(1)=78.41, P<O.OOl] and RVF [x2(1)=82.14, P<O.OOl] presentations than for bilateral presentations. The effect of digit combination was still significant with bilateral presentation [x2(1)=6.12, P<O.O5], although only when the right hand was used [right hand: x2(1)=5.47, P~0.05; left hand: x’(l)= 1.621. As in Experiment 1, when presentation was bilateral there was a strong effect of hand on response [x2( 1) = 7.65, P-C O.Ol], with a bias to respond “same” with the right hand and “different” with the left hand. Again, this may reflect a bias toward responding with the index finger rather than the middle finger. His mean RTs were 6.5 1 msec for LVF presentation, 957 msec for bilateral presentation, and 698 msec for RVF presentation.

These effects are apparent from Table 5, which shows percentage correct as a function of hand, location, and digit combination. Although there was a response-compatibility effect. with better performance for the right hand in the RVF and for the left hand in the LVF, it was not significant [x2(1)=0.54].

In this experiment L.B. was able to perform at better than chance with bilateral presentation, although only when he used his right hand. However his performance with unilateral presentation was in marked contrast, and was close to perfect, regardless of hand, indicating that his difficulty with bilateral presentation was not due to poor discriminability. It is also of interest that he had no difficulty making same different judgements when the digits were presented wholly in the LVF, and thus to the right cerebral hemisphere.


Table 5. L.B.‘s percentage correct for samedilferent and relative judgements as a function of digit pairings, location, and hand used, in Experiment 2

Hand Digit pair LVF Location Bilateral RVF

Right Same Different Overall

Same-different task 95.2 16.2 100.0 91.7 58.3 100.0 93.3 66.1 100.0

Left Same loo.0 38.1 95.2 Different loo.0 19.2 95.8 Overall 100.0 60.0 95.6

Both Overall 96.7 63.3 97.8

Right Right greater Left greater Overall

Relative-judgement task 71.4 57.1 100.0 66.1 66.7 100.0 69.0 61.9 100.0

Left Right greater 90.5 85.7 100.0 Left greater 100.0 16.2 90.5 Overall 95.2 81.0 95.2

Both Overall 82.1 71.4 91.6

Relative judgements. A three-way x2 analysis was again carried out to determine the effects to hand, location, and digit combination on L.B.‘s responses. There was a significant overall effect of digit combination [x2(1)= 114.69, P<O.OOl], but again this interacted significantly with location [x2(2)= 10.27, P<O.Ol]. Performance was poorest when presentation was bilateral, but it was still significant [x2(1)= 15.43, P<O.OOl]; it was higher with LVF presentation [x2( 1) = 34.73, P < O.OOl] and almost perfect with RVF presentation [x2( 1) = 76.36, P<O.OOl]. Although the triple interaction between hand, location, and digit combination just failed to reach significance [x2(2)= 5.74, P<O.lO], it can be seen from the data in Table 5 that there was again something of a response-compatibility effect, with the right hand giving a RVF advantage and the left hand a LVF advantage. Moreover, the effect of digit combination with bilateral presentation was significant when he used his left hand [x2(l)= 16.24, P<O.OOl] but not when he used his right hand [x2(1)=2.40]. His mean RTs were 950 msec for LVF presentation, 1125 msec for bilateral presentation, and 734 msec for RVF presentation.

These results show that L.B.‘s performance was if anything slightly lower with bilateral presentation than it was in Experiment 1, and lower with bilateral than with unilateral presentation. These results suggest that his poor performance with bilateral presentation in Experiment 1 was not attributable to a difficulty in discriminating the digits themselves. Moreover his overall performance with bilateral presentation is below the level attainable by basing the judgement on the digit in a single visual field, although with his left hand he just exceeded that level.

Table 6 shows the relative weights of the two digits according to a discriminant-function analysis. With LVF presentation, the larger weight was associated with the left digit, and with right-hand performance the weight of the right digit did not differ significantly from zero. With bilateral presentation, it is again evident that the judgement was based more on

14x4 M. C. CORBALLIS

Table 6. Discriminant-function analysis of L.B.‘s performance on relative judgements in Experiment 2

Location Hand ‘I h, h,. Fi

LVF

Bilateral

RVF

Right Left Right Left Right Left

2.666 0.250 - 0.750’ 5.66* PO.083 0.885* -0.936* 34.3x* -6.332 0.921* 0.279 7.48* -0.980 0.893* -0.642* 16.74*

0 I .ooo* - I .ooo* 62.40* ~ I.666 1.128* 0.729* 42.37*

*P<o.oI. l_d.f.=2, 39

the right (left-hemisphere) digit; indeed, with right-hand performance, the weight accorded the left digit is actually positive, though not significant. With RVF presentation, performance was perfect when L.B. used his right hand.

Analysis of performance with bilateral presentation again showed that L.B. was much

more accurate when a “high”digit (i.e. greater than 5) was paired with a “low” one (less than 5) than when two high or two low digits were paired; the percentages correct were 62.5 and 83.3, respectively, which differed significantly [I’( 1) = 13.30, P<O.OOl]. Again, then, there is little evidence that L.B. actually compared the digits on the two sides.

Conclusions

This experiment fails to resolve the question of why the three subjects with complete forebrain commissurotomy (L.B., N.G. and A.A.) performed so much better on relative judgements in Sergent’s [14] study than in Experiment 1. The best performed of those subjects, L.B. still managed only 71.4% correct on relative judgements with bilateral presentation in the present experiment, compared with 98.6 in Sergent’s study. Moreover, the dip in performance with bilateral compared with unilateral presentation in the present experiment is more suggestive of disconnection than of interhemispheric integration. However the present study suggests that transfer is no more efficient in the context of relative judgements than in the context of sameedifferentjudgements, especially when it is recognized that subjects can score nearly 80% correct on the basis ofjust one of the two presented digits.

Finally it is of interest to note that when the digit pairs were flashed wholly in the LVF, L.B. was actually poorer at relative judgements (82.1% correct) than at same-different judgements (96.7% correct), reversing the difference obtained with bilateral presentation. The reason for this may be that same-different judgements can be based on shape alone, which is well within the competence of the right hemisphere. Relative judgements, by contrast, require that the digits be identified and the numerical value of each determined, a task not so easily accomplished by the right hemisphere.

GENERAL DISCUSSION

Sergent 114, 151 reported that subjects with complete forebrain commissurotomy were much better at judging relative quantity of stimuli in the two visual fields than at judging their sameness or difference. The same pattern emerges from the present data, although performance on relative judgments was considerably lower than in Sergent’s [ 141 study. Part of the advantage of relative judgements lies in the fact that it is possible to score well above


50% by considering only one of the two quantities, whereas this strategy cannot produce greater than 50% accuracy on same-different judgements. When this possibility is taken into account, there was no convincing evidence that those subjects with complete commissurotomy scored better on relative judgements than on sameedifferent judgements. These results essentially confirm those reported by Seymour et al. [ 161 for the callosotomized subject J.W.

It remains unclear why the subjects performed so much better on relative judgements in Sergent’s [14] study. L.B., N.G. and A.A. scored 98.6,95.8 and 86.1% correct, respectively, in her study, compared with 77.6,62.3 and 64.3 in Experiment 1. In Seymour et al.‘s study, J.W. scored 75.8% correct. It is unlikely that the discrepancy was due to poorer discriminability of the digits in Experiment 1, since in Experiment 2 L.B. scored even more poorly with bilateral presentation when the digit pairs were presented closer to the fixation point, but was highly accurate when they were presented unilaterally. Moreover, the dimensions and retinal eccentricities of the stimuli in Seymour et a/.‘~ study closely matched those in Sergent’s. It might be noted that Sergent included the digits 1 and 9, for which the task of making relative judgements becomes a trivial one, but this is still not sufficient to explain the discrepancy.

Although the present results suggest an overall pattern of visual disconnection, they do not rule out the possibility that there was some weak interhemispheric transfer, especially in the case of N.G. Even if this were so, however, there is no need to assume that the information transferred is precise, since transfer of merely binary information can in principle raise performance on relative judgements to well above the performance achieved by any of the three fully commissurotomized subjects. In their study of comparisons of numerosity between visual fields in a commissurotomized subject, Corballis and Sergent [6] also found that the transfer of binary information was sufficient to explain the results, and Sergent [ 141 reported ready transfer of binary (odd vs even) information in tasks requiring comparison of digit categories. Crude information might be transmitted by simple cross-cueing [2], or more likely by a subcortical route involving the superior colliculi [S], although there is also a case for supposing that the pretectum may be involved [lo, it]. D.K., with only partial commissurotomy, performed better than the other commissurotomized subjects, both in same-different judgements and relative judgements of digits. These results were unexpected, since D.K. proved worse than L.B. in judging whether or not diagonal lines in the two visual fields were aligned; moreover, like L.B., he failed to score at better than chance in lexical decisions about letter strings that straddled the midline [7]. It is possible that D.K. has in the meantime benefited from practice. However his superior performance in the present study may also reflect the fact that his callosal section was restricted to the posterior third to one half, and that this allowed the transfer of numerical information.

The discriminant-function analyses afforded an opportunity to assess the relative roles of the two hemispheres in making relative judgements. In making cross-field judgements, greater weight was generally given to the digit in the RVF, implying left-hemispheric control, although this in turn was somewhat influenced by the hand used. In Experiment 2, moreover, L.B. was more accurate when the digits were presented wholly to the RVF than to the LVF. These results suggest that judging the relative values of digits is preferentially left- hemispheric. However L.B. was able to perform both same-different and relative judgements at well above chance when the digits were presented wholly to the LVF, and at a higher level than when the digits were presented bilaterally. This implies that his right hemisphere is at least capable of performing these tasks, albeit at a lower level of efficiency than the left. With LVF presentation however he was more accurate at making same-different than relative


judgementd, reversing the difference obtained with bilateral presentation, suggesting that the right hemisphere has some difficulty in extracting and comparing numerical values.

In summary, the data from this study can be explained without supposing that there is any interhemispheric transfer of numerical information following complete forebrain commissurotomy. The conservative conclusion is that relative judgements were based for the most part on the digit in the RVF, and occasionally on the digit in the LVF. A less conservative conclusion is that both digits contributed on at least some trials, but that the information provided by the LVF digit was no greater than binary, at best; that is, in making the judgements the subjects had access to accurate information about the size of the digit in the RVF but only crude information as to whether the digit in the LVF was “high” or “low”. Taken overall, the data from this study, that of Seymour et al. [16], and that of Corballis and Trudel [7] suggest that interhemispheric integration of perceptual information following forebrain commissurotomy is rather more limited than suggested by Sergent [13-l 51.

A~knowlrdyements~This research was supported in part by a grant from the Auckland University Research Committee. I thank Tony Lambert for help with the testing, and Eran and Dahlia Zaidel for making facilities available. I also thank the subjects for their cheerful and willing cooperation.

REFERENCES

1. Bogcn, J. E. Split-brain syndromes. In Hundhook gfCIinical Neuroloyy, P. J. Vinken, G. W. Bruyn and H. L. Klawans (Editors), Vol. 45, pp. 99-106. Elsevier, Amsterdam, 1985.

2. Bogen, J. E. Partial hemispheric independence with the neocommissures intact. In Brain Circuits and Functions @the Mind, C. Trevarthen (Editor), pp. 215-230. Cambridge University Press, Cambridge, 1990.

3. Bogen, J. E., Schultz, D. H. and Vogel, P. J. Completeness of callosotomy shown by magnetic resonance imaging in the long term. Arch. Neural. 25, 1203 1205, 1988.

4. Bogen, J. E. and Vogel, P. J. Neurologic status in the long term following complete cerebral commissurotomy. In Lrs Syndromes de Disconnexion Calleuse Chez /‘Hornme, F. Michel and B. Schott (Editors), pp. 2277251. Hopital Neurologique, Lyon, 1975.

5. Carlson, N. R. Physiology qj’ Behurior, 4th Edition. Allyn & Bacon, London, 1991. 6. Corballis, M. C. and Sergent, J. Judgments of numerosity by commissurotomized subjects. Neurops~~holoylia

30, 865 876, 1992. 7. Corballis, M. C. and Trudel, C. I. The role of the forebrain commissures in interhemispheric integration.

Neuropsychology 7, 1 19, 1993. 8. Fendrich, R. and Gazzaniga, M. S. Evidence of fovea1 splitting in a commissurotomy patient. Nruropsycholoyia

27, 273 281. 1989. 9. Johnson, L. E. Bilateral visual cross-integration by human forebrain commissurotomy subjects. Nrurops~~cho-

loqiu 22, 1677175, 1984. IO. Legg, C. R. Is subcortical vision necessarily mediated by the superior colliculus? Brhu~~. Brain Sci. 6,455, 1983. 11, Pasik, T. and Pasik, P. Extrageniculostriate vision in primates. Neuro-Opthahn. 1, 95~ 119, 1980. 12. Schaapveld, P. B. Neuropsychological Sequelae in Two Cases of Partial Commissurotomy. Unpublished

Masters Thesis. University of Auckland, Auckland. New Zealand, 1988. 13. Sergent, J. A new look at the human split brain. Brain 110, 1375 1392, 1987. 14. Sergent, J. Furtive incursions into bicameral minds: Integrating and coordinating role ofsubcortical structures.

Bruin 113, 537~-568. 1990. 15. Scrgent, J. Processing of spatial relations within and between the disconnected cerebral hemispheres. Brain 114,

1025 1043, 1991. 16. Seymour. S. E., Reuter-Lorenz. P. A. and Gazzaniga, M. S. The disconnection syndrome: Basic findings

reaffirmed. Brain 117, 105- 115. 1994. 17. Sperry. R. W. Some effects of disconnecting the cerebral hemispheres. Science 217, 1223 1226, 1982.

Documents

Can commissurotomized subjects compare digits between the visual fields?