size constancy in monkeys with inferotemporal lesions

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Size constancy in monkeys withinferotemporal lesionsN. K. Humphrey a & L. Weiskrantz aa Psychological Laboratory, University of CambridgePublished online: 29 May 2007.

To cite this article: N. K. Humphrey & L. Weiskrantz (1969) Size constancy in monkeys withinferotemporal lesions, Quarterly Journal of Experimental Psychology, 21:3, 225-238, DOI:10.1080/14640746908400217

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Q. J1 exp. Psychol. (1969) 21, 225-238

SIZE CONSTANCY IN MONKEYS WITH INFEROTEMPORAL LESIONS

N. K. HUMPHREY+ AND L. WEISKRANTZt Psychological Laboratory, University of Cambridge

In a study of size constancy, monkeys were trained with extended practice to choose the larger of two projected stimulus discs independently of distance. When surgical brain lesions were made following pre-operative training, it was found that posterior parietal lesions caused no deficit on this task while inferotemporal lesions caused a severe breakdown from which three out of four animals showed almost no recovery. Mathematical analysis showed that the pattern of performance after inferotemporal lesions could be described as due to a tendency to oscillate between a “correct” and two “incorrect strategies” and that this tendency was also apparent to at least some extent in the unoperated animals. I t is argued that the two incorrect strategies might have resulted from a disturbance of size constancy, such that the animal became unable to use both retinal size and distance information in computing physical size and instead used either retinal size or distance information alone. Parallels are drawn between such an hypothetical perceptual disorder and certain clinical disturbances in man.

Introduction

Removal of the inferotemporal cortex in monkeys creates a mosaic of deficits in the performance of visual tasks which no single theory has been able to piece together. As solutions themselves become fragmented, one simple possibility deserves consideration still. This is that the lesion produces a primary disturbance in the perception of visual form. Such a disturbance, acting to distort visual experience of present events and hence necessarily leaving its mark on associative memory and imagery, could be responsible for many of the deleterious effects on visual behaviour which have been observed.

A lead to the kind of disturbance which might occur can be obtained from human clinical cases. There is a recurrent type of perceptual disorder in man charac- terized by fluctuating changes in the size, shape and position of objects in the visual fields (see Critchley, 1953 ; Willanger and Klee, 1966). Patients report that the perceived -,vorld undergoes dramatic transformations: objects may grow suddenly large or small, they may move or tilt to one side and their contours may become unstable and undulating. The illusory changes sometimes occur sporadically but are likely to be brought on especially by prolonged fixation (Willanger and Klee, 1966). The disorder in man is associated with several types of central pathology. I t can occur in cases of organic brain damage, where the lesion lies in the posterior

England. t Present address: The Institute of Experimental Psychology, I South Parks Road, Oxford,

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226 N. K. HUMPHREY AND L. WEISKRANTZ

parieto-temporal or occipital cortex. But very similar symptoms are found in cases of temporal lobe epilepsy (Penfield and Jasper, 1954), the early stages of schizophrenia (Chapman, 1966), recovery from perceptual isolation (Heron, Doane and Scott, 1956), and mescalin intoxication (Mayer-Gross, Slater and Roth, 1960); they can be produced also by direct electrical stimulation of parietal or temporal cortex (Hecaen, Penfield, Bertrand and Malmo, 1956).

The research to be described here looked for evidence in inferotemporal monkeys of a perceptual disorder potentially comparable to such disturbance in man. It was designed in particular to test a hypothesis about the functional origin of the disorder should it be present. This is, that it is due to a failure of perceptual constancy for size and form, i.e. a failure in the process whereby the observational context in which an object is seen-the observer’s distance, position of his eyes, angle of regard-is taken into account in assessing the properties of the object from its retinal image.

Methods Outline of the experiment

Monkeys were trained on a visual task requiring the judgement of size independently of distance. Their performance was compared before and after surgical brain lesions in the hope of detecting some change attributable to a failure of constancy.

The visual task was modelled on that used by Holway and Boring (1941) in their classical study of size constancy in man. Holway and Boring asked their subjects to make size matches between a variable and a standard disc projected on screens at different distances down two long corridors. For the present study, instead of a size match a size discrimination was required, the monkeys being trained to choose the larger of two projected discs.

There were two operated groups, the experimental group with bilateral inferotemporal lesions and a control group with bilateral posterior parietal lesions.

Subjects

assigned to the control group and four, ITI-IT4, to the experimental group.

Surgery Subpial cortical excisions were made by

aspiration with a fine sucker. At the end of the experiment the brains were perfused with formalin, embedded in paraffin, sectioned at zo p and every fifth section stained with thionine.

Seven young experimentally naive rhesus monkeys were subjects. Three, P I - P ~ , were

Anaesthesia was induced with Nembutal.

Histological constructions of the lesions are shown in Figure I .

Apparatus The stimuli were presented as circular discs of light, back-projected on to two ground glass

screens (ZI x 16 cm.) in two well-lit “corridors” formed by walls of heavily textured cloth (Figs. z and 3). The screens and projectors were mounted on trolleys which could be pulled along rails, thus varying the distances of the stimuli from the animal. The sizes of the discs were controlled by iris diaphragms in the projectors. The animal sat in a transport cage in front of two Perspex windows (20 x 1z4cm.) looking on to the two corridors.

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MONKEYS WITH INFEROTEMPORAL LESIONS 227

I -T1 I-T3

I-T2 1-T4

a a : :

C b a P1

C b a PZ

C b a

P 3

FIGURE I .

Except when the projection screens were closest to him the animal was unable to see them both at once so that to compare the stimuli he had to look down one corridor after the other. He made his choice of the larger disc by pressing the window in front of the appropriate corridor. A correct choice was rewarded by a peanut delivered automatically to a trough just below the window; an incorrect choice was mildly punished by the room lights being switched off for a few seconds. In between trials a wooden screen covered the windows. At the beginning of a trial the screen was raised allowing the animal to see the stimuli. When he had made his choice the screen was lowered-after a short delay in the case of a correct choice, immediately in the case of an incorrect one. The sizes of stimuli used were 4, z and I-cm. diameter. They were paired either as 4 cm. against z cm. or z cm. against I cm., so that the larger disc was always twice the diameter of the smaller. The distances used covered the range from 30 to 120 cm. from the animal (it being assumed for these measurements that he sat with his head almost up against the bars of the cage).

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2 28 N. K. HUMPHREY AND L. WEISKRANTZ

1

FIGURE z. General view of the testing apparatus.

Pre-operative testing schedule Thirty trials, with correction, were given per day on 5 or 6 days a week. The correct

window (the window in front of the larger disc) and the stimulus pair used (4 cm. vs. 2 cm. or 2 cm. vs. I cm.) were determined by random schedules. The discrimination proved to be difficult to learn and a long period of shaping was required. Initially (pre-op., Stage I), the stimuli were presented at the near ends of the corridors, up against the windows, and the animals were trained to a criterion of 27/30 correct on I day. The stimuli were then presented at equal distances increasingly further down the corridors and the animals trained again to a criterion of 27/30 correct whatever the distance. Trials in which the stimuli were at unequal distances (pre-op., Stage 11) were then introduced according to the following schedule (see Table I). Each block of 30 trials contained 18 “equal distance trials” and 12 “unequal distance trials.” The unequal distance trials were presented alternately with

TABLE I Schedule of trials within a single go-trial block

Positions of stimuli (cm.) Class of trial large small Number per block

Equal L4X L2X

s4x s2x

30-120 I 2 0

I 2 0 60 30 60 30

30-120

30 60 30

I20 I20 60

I8 2 2

2 2

2

2

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FIGURE 3. Stimulus disc as seen through one of the windows.

equal distance trials in the middle of the block. The distances of the stimuli on the equal distance trials were varied randomly over the whole range from 3 0 to IZO cm. The distances on the unequal distance trials were chosen as shown in the table so as to give 4 trials in which the larger stimulus was twice the distance of the smaller (LzX), z trials in which the larger stimulus was 4 times the distance of the smaller (LqX), 4 trials in which the smaller stimulus was twice the distance of the larger (SzX), and z trials in which the smaller stimulus was 4 times the distance of the larger (SqX), the order of these trials being random. The animals were trained on this schedule to a criterion of better than 85 per cent over 7 days on each of the three main classes of trial, “equal distance,” “large far” (L4X and LzX) and “small far” (S4X and SzX), taken separately.

Post-operative testing schedule Testing was resumed on the seventh day after operation in all animals except P3, who did

not recover sufficiently until the eleventh day. Initially (post-op., Stage I), the operated animals were given only equal distance trials, distance varying over the whole range. All except IT4 were retrained to a criterion of 27/30 on these before going on to unequal distance trials. For reasons which will become clear later (the necessary failure on equal distance trials of an animal adopting the “distance strategy”) it was decided to start IT4 on unequal distance trials after only 3 days although he had not reached the criterion. The

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unequal distance trials (post-op., Stage 11) were introduced according to the same schedule as before operation. For the first 7 days of testing with unequal distance trials both correct and incorrect choices on these trials were rewarded; thereafter, selective reinforcement was reintroduced and maintained. Testing was continued until the animal reattained the pre-operative criterion of 85 per cent over 7 days on the three main classes of trial, or until 35 days testing were completed.

Results Pre-operative discrimination learning

All animals took a very long time to reach the final criterion, total days to criterion ranging from 20 for IT1 to 162 for IT4 (see Table 11). The initial discrimination with the stimuli up against the windows was learned in a few hundred

TABLE I1 Total days to criterion (excluding the criterion run) at different pre- and post-operative stages

Pre-op. Post-op * Subject Stage I Stage I1 Stage I Stage I1

PI 46 8

p3 89 30 PZ 45 26

2 I

I0

3 I 0

IT1 20 0 I 2 7 IT2 67 21 2 > 35 IT3 85 39 2 > 35 IT4 48 "4 > 3 > 35

trials, but when the stimuli were moved back along the corridors performance dropped almost to a chance level. This stage, involving only equal distance trials, was eventually mastered after considerably more training, but when the final stage involving unequal distance trials began performance relapsed in all animals except IT1 and PI. The relapse occurred on both equal and unequal distance trials although the animals had previously performed well on the former. With further training performance improved, more or less in parallel on both kinds of trial, but in several animals it remained just short of criterion for many days: IT4, for instance, scored over 75 per cent on all three major classes of trial for over 35 days before reaching 85 per cent.

Post-operative general behaviour The three posterior parietal animals were obviously severely im-

paired in the first hours following operation. They stumbled around their cages, bumping into the walls and hitting their heads on protruding objects; if they tried to steady themselves by catching hold of the wire, they reached too short, too far or in the wrong direction, missed their target and fell ; they appeared quite insensitive to hard pinches on one or both sides of their bodies and often let their limbs droop lifelessly; they made no attempt to reach for food and showed no interest in it if it

(i) ParietaZs.

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was put into their hands although they took it greedily if it was pressed against their lips. At the same time, automatic movements-grooming, etc.-were nearly normal, and visual recognition, judged by reactions to familiar objects and people, was unimpaired. Recovery from these gross behavioural disorders was rapid. PI and Pz appeared more or less normal after 4 days and P3 after 8 days, although all three remained slow and clumsy and showed severe misreaching for some days more.

None of the inferotemporal animals showed any obvious signs of disturbance after the operation.

Post-operative discrimination performance Despite a residual motor disturbance which handicapped them in

picking nuts out of the troughs, PI and Pz regained criterion on equal distance trials almost at once, taking I and z days, respectively. P3 took 10 days but this apparent impairment was due at least in part to the much greater difficulty this animal had in picking out nuts, which for the first z days made him leave more than half of them behind and so upset the reinforcement schedule. When the unequal distance trials were introduced the animals showed almost no deficit and the pre-operative criterion was achieved in 3, I and o days, respectively (see Table 11).

(ii) Inferotemporals. IT1 performed very erratically at first on equal distance trials and for 10 days refused to complete 30 trials, although he regained criterion eventually in I Z days. IT2 and IT3 regained criterion at this stage quickly, taking z days each. IT4 performed at near chance level for 3 days and, as mentioned earlier, was not taken to criterion. When the unequal distance trials were introduced all four animals performed very badly. IT1 achieved criterion only after

(ii) Inferotemporals.

(i) Parietals.

100

80

:a L

40

g 100

5 8 0 Z ................

.................... ...................... ......I ,..’ i €01 ..*’ 40 I-T3 , I I ,

...........................................

-14 -7 7 14 21 28 35 -14 -7 7 14 21 28 35 Time (days)

FIGURE 4. Performance curves for ITI, 2, 3“and 4. Separate curves show performance on “equal distance,” “large far” (L2X and L4X) and small far” (SzX and S4X) trials. Days - 14 and -7 correspond to the period just before operation and days +7 through + 3 5 to the period of post- operative testing at Stage 11. -, Equal distance; .... large far; .... small far.

7 days and the other three failed to do so in 35 days. Figure 4 shows the inferotemporals’ performance at Stage I1 in terms of scores over successive 7-day periods on each of the three main classes of trial. It will be seen that there was not a general breakdown, performance on different classes of trial being affected to quite different extents. IT1 The pattern of deficit varied from animal to animal.

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showed a drop in his score on the large far trials but relatively little change on equal distance and small far trials. IT2 showed a similar drop on large far trials with little change on equal distance and small far trials. IT3 showed a big drop on small far trials, a smaller drop on equal distance trials and if anything an improve- ment on large far trials. IT4 showed a big drop on small far trials and smaller drops on both equal distance and large far trials (it is noteworthy that this animal who had never scored under 75 per cent in the 35 days immediately preceding operation never scored over 75 per cent post-operatively). The performance of IT1 recovered quite quickly but IT2, IT3 and I T 4 showed a remarkably stable impairment. When the scores on the unequal distance trials are broken down into total post-operative scores on the classes LqX, LzX, S4X and SzX (see Table 111) the differences between the animals and the heterogeneity of their scores are more marked still.

TABLE 111 Average scores (per cent correct) throughout post-operative Stage 11

Subject Equal L4X LZX s 4 x szx ITI 94 69 79 81 91

IT3 82.5 85.5 92 61.5 65 IT4 70 49 69 5 1 51

IT2 90 46 67 87 88.5

Analysis of the inferotemporals’ performance The post-operative performance of the inferotemporals, each animal showing a

particular rather regular distribution of scores, could not easily be attributed to a non-specific disability. The evidence suggests-and this impression was supported by observation of the animals during the testing sessions-that even in making errors they continued to respond discriminatively to the stimuli, but that the basis for their discrimination had changed. This being so, then (i) Why should the patterns of performance have been just such as they were?, (ii) Why, in three animals at least, should these patterns have remained so stable over 35 days’ testing, apparently uninfluenced by differential reinforcement?

In line with the theoretical aim of this study, the possibility was examined of a defect in perceptual size constancy. The hypothesis was that post-operatively the inferotemporals continued to respond to the perceived size of the stimuli, choosing the stimulus which looked the larger, but that now perceived size no longer corre- sponded to physical size because of some error in the computation of size from the information at the retina. This hypothesis was made the basis for a specific analysis.

Veridical perception of size depends on combining information about both the retinal image size of an object and its distance according to the rule

The reasoning went as follows.

perceived size = K (image size x distance).

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MONKEYS WITH INFEROTEMPORAL LESIONS 23 3

If the mechanism which normally applies this rule were to be put out of action, a form of compromise might be adopted, viz. perceived size might be made depen- dent not on both image size and distance but on one alone with the other arbitrarily fixed. The two possible strategies would be (i) to use information about image size and assume an arbitrary value for distance, giving

perceived size = image size x constant

or (ii) to use information about distance and assume an arbitrary value for image size, giving

I n an animal whose constancy mechanism were to be functioning badly but not permanently out of order, perception might fluctuate between the use of the proper rule and these two partial strategies. Suppose the “image size strategy” were to be adopted a proportion p of the time, the “distance strategy’’ a proportion q of the time and the “correct strategy,” combining both size and distance information, a proportion I-p-q of the time, then it should be possible to make precise predictions about performance on a size discrimination involving known sizes and distances. The predicted scores for the different classes of trial in the present experiment would be as follows.

Here the image size of the larger stimulus is twice that of the smaller and the distances are equal. Hence with the image size strategy the animal would score IOO per cent, with the distance strategy he would score at chance, i.e. 50 per cent, and with the correct strategy he would score IOO per cent. His average score would thus be

perceived size = constant x distance.

Equal distance.

per cent correct = I O O ~ + soq + IOO (I-p-q) = I00 - 504.

L4X. Here the image size of the larger stimulus is half that of the smaller and the distance of the larger is 4 times that of the smaller. Hence with the image size strategy the animal would score o per cent, with the distance strategy he would score IOO per cent and with the correct strategy he would score IOO per cent. His average score would thus be

per cent correct = op + Iooq + IOO (I-p-q) = I00 - 100p.

L2X. Here the image size of the larger stimulus is equal to that of the smaller and the distance of the larger is twice that of the smaller. Hence with the image size strategy the animal would score 50 per cent, with the distance strategy he would score IOO per cent and with the correct strategy he would score IOO per cent. His average score would thus be

per cent correct = 50p + Iooq + IOO (I-p-q) = I00 - sop.

S4X. Here the image size of the larger is 8 times that of the smaller and the Hence with the image size distance of the larger is a quarter that of the smaller.

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N. K. HUMPHREY AND L. WEISKRAZLN 234

strategy the animal would score IOO per cent, with the distance strategy he would score o per cent and with the correct strategy he would score IOO per cent. His average score would thus be

per cent correct = IOOP + oq + IOO (I-p-q) = I00 - 100q.

S2X. Here the image size of the larger is 4 times that of the smaller and the distance of the larger is a half that of the smaller. Hence with the image size strategy the animal would score IOO per cent, with the distance strategy he would score o per cent and with the correct strategy he would score IOO per cent. His average score would thus be

per cent correct = IOOP + oq + IOO (I-p-q) = I00 - 100q.

In summary, the predicted average scores would be

Equal IOO - 5oq L4X 100- 100p

L2X I 0 0 - sop s 4 x I00 - 100q s2x I00 - 100q.

With this hypothetical analysis in mind, an attempt was made to find values of p and q which, substituted in the equations, give expected scores close to the observed average scores over the post-operative period. This meant trying to fit the scores on j i v e different classes of trial by assigning values to two independent variables (with the constraint that p + q < I): a good fit should be obtainable only if the animal’s performance did conform to the predicted pattern.

TABLE IV Observed scores (0) for post-operative Stage 11 compared with expected scores ( E ) calculated

for the values of p and q shown

Equal L4X LZX s4x s z x Subject p q 0 E 0 E 0 E 0 E 0 E

IT1 0.36 0-14 94 93 69 64 79 82 81 86 91 86 IT2 0.58 0.15 go 92-5 46 42 67 71 87 85 88-5 85 IT3 0.14 0.36 82.5 82 85.5 86 92 93 61-5 64 65 64 IT4 0.50 0.50 70 75 49 50 69 75 51 5 0 5 1 50

The result of the attempt is shown in Table IV; the same is shown diagrammatically in Figure 5 . It will be seen that the rank order of the expected scores is right in each case, while the mean absolute discrepancy between expected and observed scores is under 3 per cent. The closeness of this fit clearly gives strong support to the hypothesis.

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-0 50 100 0 50 too Expected scores

FIGURE 5 . The same data as in Table IV, shown diagrammatically. The diagonals are the"1inesof perfect fit."

Analysis of the pre-operative performance Although the analysis was developed to explain the performance of the infero-

temporals, its success prompted an attempt to treat the pre-operative data in the same way. As already described, several animals before operation took a long time to master the stage of the problem involving unequal distance trials, despite having reached criterion with equal distance trials alone. It is at least a possibility that their poor performance was due to a natural difficulty with the discrimination, similar to the inferotemporal deficit but much less severe. This was tested by trying to find values of p and q which fit with the pre-operative scores. Only the final 14 days up to and including those to the final criterion were considered, since in several animals earlier performance appeared very erratic, and in any case some required little more than 14 days altogether. Taking each animal individually, the best fit obtainable is in several cases very poor. This could be due, however, to the relatively small number of trials involved in 14 days testing, being insufficient for the statistical norms to emerge. If the data from all 7 animals are pooled to give more weight to the scores (as is permissible since the equations are linear), the fit is unexpectedly good (Table V). Further consideration was therefore given to the

A, Equal; ., LqX; 0, LzX; 0, S4X; 0, S2X.

TABLE V Observed scores (0) pooled for all animals over theJina1 14 days before operation compared

with expected scores ( E ) calculated for the values of p and q shown

Equal L4X L2X s 4 x s2x P 4 O E O E O E O E O E

0.24 0.17 89.5 91.5 78.5 76 86.5 88 82 83 85 83

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one animal, IT4 ,whose performance by contrast with that of the others was fairly stable for much longer before he reached criterion. When IT4% scores over the 35 days up to and including those to criterion are considered, the fit turns out to be very good (Table VI). I t seems therefore that at least the terminal performance of

TABLE VI Observed scores (0) for IT4 over the final 35 days before operation compared with expected

scores ( E ) calculated for the values of p and q shown

Equal L4X LZX s 4 x s2x P 4 O E O E O E O E O E

the un-operated animals followed the same pattern as the performance of the inferotemporals.

Discussion

The evidence of the present study may be summarized as follows: ( I ) Normal monkeys could be trained to choose the larger of two projected stimuli independently of distance, although they took a long time to reach a consistently high level of performance. (2) Large posterior parietal lesions, after pre-operative training, caused no deficit on this task. (3) Inferotemporal lesions, by contrast, caused a severe breakdown, from which three out of four animals showed almost no recovery. (4) The pattern of performance after inferotemporal lesions can be described as due to a tendency to oscillate between a “correct” and two “incorrect strategies.” (5) This tendency was also apparent to at least some extent in the unoperated animals.

The analysis of the deficit in terms of correct and incorrect strategies rose directly out of the hypothesis of a breakdown in perceptual size constancy. On this hypothesis the strategies represent alternative modes of perceptual organization, size perception being made to depend either on both retinal size and distance information or on retinal size or distance information alone. A problem is raised, however, by the finding that the analysis applies not only to the performance of the inferotemporals but to the performance of the unoperated animals as well. Although the pre-operative “deficit” was relatively slight, the fact that it followed the same pattern as the post-operative deficit suggests that it had the same cause and that therefore it too should be attributed to a failure of perceptual constancy. This suggestion, without external support, is hard to take. I t is certainly unlikely that healthy free- ranging monkeys can be subject under natural conditions to sporadic errors in size perception. Two considerations, however, make the possibility of a perceptual disorder in the unoperated monkeys of the present study not implausible. First, these monkeys were manifestly not normal free-ranging animals, but captive animals confined to small laboratory cages where their scope for “active” visual

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237 MONKEYS WITH INFEROTEMPORAL LESIONS

exploration, such as is required for the maintenance of adequate perception was severely restricted (cf. Held and Hein, 1964). Second, the situation in which the animals were tested, looking through windows at meaningless projected stimuli at the ends of corridors giving rather limited depth cues, was highly artificial and such as might emphasize any potential perceptual instability.

The necessity for special pleading to account for the performance of the “normal” monkeys must make the hypothesis that the inferotemporals suffered a specific impairment of perceptual constancy rather less attractive. I t is, however, hard to see how the results can be otherwise explained. There have certainly been other theories of the inferotemporal defect which, resting on earlier evidence for a disturbance in perception, have proposed explanations in terms of visual field defects, or impairment of selective attention or perceptual categorization (for a recent summary see Wilson, 1968). The results of the present experiment do not at all easily fit into the framework of these previous theories. It seems possible, rather, that the hypothesis of impaired constancy may itself be able to cover many of the earlier findings, while having the advantage of accounting in detail for the disturbance of size perception found in this study. This hypothesis has moreover the advantage of tying in the inferotemporals’ deficit with an established clinical syndrome in man, for abnormalities of size perception occur quite commonly among the symptoms of perceptual disturbance referred to already. The character of the clinical disorder can be illustrated from case records, as, for instance, in a case of diffuse cortical damage after head injury where “On maintained fixation the objects, after a latent period of 5 to 6 sec., apparently grew rapidly to more than double their original size” (Willanger and Klee, 1966). Similar phenomena, in other pathological conditions, are reported by Penfield and Jasper (1954)) Chapman (1966) and Heron et al. (1956), among others. In the absence of proper experimental study of such cases, parallels between man and monkey must remain conjectural. But on the present evidence the hypothesis of a breakdown in constancy after inferotemporal lesions in monkeys, with a parallel in such human cases may be considered at least in intriguing possibility.

Still to consider is the failure to find any specific impairment in the control animals of this study, with parietal lesions. In view of the manifest spatial disorientation of these animals in the first few days after operation, it is somewhat surprising that they should have shown no deficit at all in size constancy. I t should be noted however that the most obvious signs of disorientation had dis- appeared by the time formal testing was started, so that a transient perceptual disturbance may have been missed. I t is perhaps wisest simply to record our negative finding, while withholding judgement on the more general question of what, if any, permanent deficit was present in these animals.

It is a pleasure to acknowledge the help of Dr. A. Cowey, who performed the surgery on four of the monkeys, and also the technical assistance of Mr. R. Hutchison and Miss S. Hopkins. Professor 0. L. Zangwill made research facilities available in Cambridge and gave valuable encouragement. The work was supported by an apparatus grant from the Medical Research Council (965/96/B) and a Research Scholarship to the first author from the Science Research Council.

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Received 4 March 1969

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size constancy in monkeys with inferotemporal lesions

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