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Effects of early postnatal hemidecortication on spatial discrimination in cats
EXPERIMENTAL
NEUROLOGY
Effects
of
6, 332-339
Early Spatial
BERNICE M.
(1962)
Postnatal
Hemidecortication
Discrimination WENZEL,
R. D.
on
in Cats
TSCHIRGI,
AND
J.
L.
TAYLOR~
University of California School of Medicine, Los Angeles, California University of Illinois, College of Medicine, Chicago, Illinois Received
July
and
6, 1962
Under certain circumstances, major unilateral cortical removal may result in little permanent deficit in motor abilities. To study the effects of early damage on ability to make spatially oriented responses, hemidecortication was done on 9-day old kittens. The two that survived surgery developed normally, and showed no motor deficiency other than permanent loss of the placing reflex in the contralateral limbs. Testing in special learning situations began at 9 weeks. Performance of the hemidecorticate cats was compared with that of normal littermates and surgical controls. The operated animals performed as well as the control in a T-maze. In two reaction-time tests, one measuring visual function and one measuring paw dominance, both of the operated and three of four control animals made faster responses on one side than on the other. The operated cats responded faster on the side ipsilateral to their lesion. It is concluded that cats deprived of one cerebral cortex soon after birth are not essentially different from controls in spatially oriented behavior or in motor ability. Introduction
The literature has contained references for many years to the minimal effects produced by certain types of brain damage in very young animals. Kennard (3) showed that infant monkeys could be subjected to extensive removal of motor cortex without appreciable functional loss, Complete hemidecortication was done by Ramirez de Arrellano (4) in a group of infant monkeys and a group at least 19 months old. The younger group was characterized by fewer and less severe neurological symptoms immediately postoperatively as well as by more rapid improvement, even to a normal condition for most symptoms. Few systematic behavioral studies have been done following lesions in early life. Tsang (6) found 1 The research has been supported by a grant from the National Association for Mental Health. The authors wish to acknowledge the assistance of Ingrid AhrneCollier and Toby Goldstein in testing and maintenance of the colony. 332
POSTNATAL
HEMIDECORTICATION
333
that hemidecortication in young rats resulted in very slight deficits in maze learning. Recently, Akert, Orth, Harlow and Schiltz (1) reported that bilateral frontal lobotomy in infant monkeys does not have the detrimental effect on delayed response learning that typically follows such a lesion in adult monkeys. Similarly, Benjamin and Thompson (2) showed that somatic cortex can be removed bilaterally from young kittens with almost no loss in roughness discrimination 6 months later, while the same operation in adult cats results in a marked decrement. In the experiments reported here, a series of behavioral tests was carried out designed to evaluate the possible effects of a chronic lateral imbalance in cerebral tissue on the ability to orient successfully in horizontal space, i.e., to learn left-right response patterns. These tests were part of a larger program of investigation on the relation between bilaterality of the central nervous system and spatial discrimination. Procedure
The experimental population consisted of six cats-one left and one right hemidecortication, two littermate normal controls, and two surgical controls. Lesions were made by suction, under barbiturate anesthesia, at 8 to 10 days of age. Since all but one of the cats are still alive-aged 3 to 4 years-complete histological examination has not yet been made. Figure 1 shows the brain of the hemidecorticate cat that recently died 3 years and 7 months after operation. From gross examination, it appears that the only parts of the neocortex remaining were very small amounts of orbitofrontal and prepyriform cortex and the olfactory tubercle. The entire hippocampal gyrus was removed. Further details must await sectioning of the brain. The surgical controls were anesthetized and had the scalp and skull opened, but no brain tissue was removed. At 9 weeks, the kittens began formal testing in a T-maze. Each correct turn was reinforced by the opportunity to play outside the maze for about a minute; after a wrong turn they were restrained for the same time. Training consisted of five stagesspread over 5 weeks: (a) six trials with both doors open and exit permitted through either; (b) twenty-four trials with only one door open (on the nonpreferred side if there was one) and exploration permitted until exit; (c) twenty-four trials with both doors closed and the correct turn toward the same side as the open door in Part 2; (d) thirty-six trials with the correct turn reversed; (e) eighteen trials with the correct turn reversed again.
334
WENZEL,
TSCHIRGI,
AND
TAYLOR
In order to obtain an additional measure of the ability to learn spatial relations, provided training is given .to overcome the sensory deficit, the second formal learning situation was a visual test to compare reaction times to lights at various peripheral points. The cats were trained to look straight
FIG. 1. Dorsal, ventral and 7 months. The lesion
and lateral views of brain had been made at 9 days
of cat that died at age 3 years of age on the right side.
POSTNATAL
HEMIDECORTICATION
335
ahead until a buzzer was sounded simultaneously with presentation of a light in the periphery. The buzzer was always directly in front of the cat, and served as a signal to look at the light. As soon as the latter response occurred, food reinforcement was given. Reaction time was measured for stimuli at 15” and 30’ to the left and right on the horizontal plane, in daily sessions of twenty trials each. Finally, another test was given to check on the possibility of sidedness in a situation where lateral vision was less critical. The cats were trained with food reinforcement to place each forepaw on a stimulus object when the latter appeared just in front of the paw. Time was recorded between the presentation of the stimulus and the placing of the paw. When response time was stable with each paw, test trials were introduced in which the stimulus was presented in the center and the cat could place either paw. Daily testing sessions of twenty trials each were held for 12 days, with four test trials inserted at random in the series on alternate days. At the conclusion of these tests, the cats were from 1% to 2 years old. Results
At no time after recovery from anesthesia did the kittens show any motor difficulties in feeding or crawling, nor was any circling behavior observed. The animals with lesions began to walk and run as early and as well as the control animals. They began to climb about 2 weeks later than the controls but eventually became just as skillful. Their ability to manipulate a rolling or bouncing ball was indistinguishable from that of the controls. Repeated neurological examinations revealed an initial loss of placing and hopping reflexes contralateral to the lesions, with subsequent partial recovery of hopping. Slight paresis and slightly increased extension in both contralateral limbs, and a loss of the contralateral visual field have remained permanently. No other sensory deficits have been noted. The results of testing in the T-maze are presented in Table 1. They show that the choices of the kittens with lesions in Part 1 were almost completely unilateral toward the side of the intact visual field, presumably a reflection of their visual deficit. However, after the special training of Part 2, in which they were forced to explore until they found the other door, the performance of the kittens with lesions became as good as, or better than, that of the controls. All animals learned equally well in Part 3, and only the surgical controls showed special difficulty in the reversal situation of Part 4. Clearly, no deficit was shown by the kittens with
336
WENZEL,
lesions in ability reverse it.
TSCHIRGI,
AND
to learn a left-right
TAYLOR
discrimination,
TABLE
nor in ability
to
1
T-MAZE RESULTS. PERCENTAGE OF CHOICES IN PART 1, AND PERCENTAGE OF ERRORS IN PART 2 ARE SHOWN FOR EACH CAT. MEAN PERCENTAGE OF ERRORS Is GIVEN FOR EACH GROUP IN PARTS $4 AND 5.
Part 1 Left Right Hemidecorticates Normal
Mean controls
Mean Surgical controls
83 0
Left
17
Part 2 Right
Part 3 Part 4 Part 5
63
100
75
50 50
50 50
25 2.5
75 7.5
10.5
51
42
1.5
46
42
15
81
14
13 17 13 63
Mean
In the early stages of training in the visual test, the cats with lesions tended to have very long reaction times on their blind side, but with further experience they learned to respond well on both sides. Table 2 shows the results obtained after enough training to produce a stable level of responding. All means are based on at least nine sessions. Although the hemidecorticate cats were somewhat slower in looking at lights on the side contralateral to their lesion, the striking fact in the data is that three TABLE MEAN
RESPONSE
TIME
2
IN SECONDS FOR EACH
CAT IN VISUAL
Left visual field 30”
Hemidecorticates Normal
controls
Surgical controls
TEST
Right visual field 1.5”
15”
30”
1.11
0.85
1.18
1.23
1.16
1.12
0.51
0.54
0.69 0.63
0.62 0.69
1.11
1.13
0.46
0.48
1.14
1.15
0.48
0.44
0.54 0.47
0.53
0.54
One hemidecorticate cat (see text) Initial training Spontaneous reversal
1.11
0.85
1.18
1.23
1.17
1.19
0.68
0.61
Special training
0.76
0.79
0.74
0.66
POSTNATAL
337
HEMIDECORTICATION
of the four control cats also showed visual sidedness, and, in general, less regular data. On the whole, the behavior of the experimental and control animals was very similar. Table 2 also contains data concerning an incidental finding in the visual test, viz., a unique response pattern shown by the cat with left hemidecortication after many daily testing sessions had occurred. She
PAW
HEMICECORTICATES
PAW
NORMAL
PAW
CONTROLS
PAW
SURGlCAL
PAW
CONTROLS
FIG. 2. Mean latencies for each cat in paw test. Shaded bars represent left and right paws on standard trials; white bars represent left and right paws on test trials. Figures in white bars show the number of times each paw was used.
showed a sudden spontaneous reversal in the relative speed of reaction on the two sides, so that her faster responseswere being made toward her absent visual field. Special training was given to test the reliability of this reversal and all that was accomplished was to eliminate any differential. Figure 2 presents the results in the paw-placing test, showing the mean latencies for both the 240 regular trials and the 24 test trials, and the frequency of use of each paw on the test trials. The trends in these data are very similar to those of the visual test. Not only did the
338
WENZEL,
TSCHIRGI,
AND
TAYLOR
hemidecorticate cats show faster times and greater use on a given side, but the same three control cats did, also. Discussion
Although few experiments have been done with the specific purpose of measuring the effect of very early, extensive, cortical lesions on later ability to learn a variety of tasks, the existing literature is in essential agreement that behavioral loss is extremely small or even undetectable. The statement is often made that the loss is “astonishingly” small, expressing a tacit conviction that such a gross tissue deficit should leave an equally gross behavioral deficit. The plasticity of the young nervous system easily triumphs over such a naive correlation, however, and creates a functional entity out of the raw material remaining after massive lesions. The significant feature of the present results is certainly the absence of any differences in ability to orient properly to the tasks and to make the appropriate spatial discriminations among details of the input array. Only in such a relatively subtle measure as the frequency of use of each paw in the ambiguous situation included in the paw-placing test do we get a hint of the nature of the “new” brain that these animals possess. Theirs is a brain in which the processing of information concerned with one side of the body is accomplished less quickly, and hence presumably by a more indirect route, than is information about the other side. But even here, the cats with lesions differed only in degree, suggesting that in the normal course of development one cerebral hemisphere becomes relatively more effective in processing spatial information. Another instance of this phenomenon was reported recently by Trevarthen (5) who found that split-brain monkeys, trained to perform two contradictory visual tasks simultaneously, usually showed unequal learning and retention in the two halves of the brain. The monkeys were free to choose either hand for making their responses. In a majority of cases, the task presented to the eye contralateral to the preferred hand was learned and retained better, indicating the superior functioning of one hemisphere. Three principal conclusions have emerged from the observations reported here. First, in general agreement with others, we can say that extensive unilateral removal of cerebral tissue very early in fife leaves no gross deficits in motor, sensory or intellective functions either immediately after operation or at later times. Second, the hemidecorticate cats are just as skillful as normal cats in learning various types of left-right dfs-
POSTNATAL
HEMIDECORTICATION
339
criminations despite the lateral imbalance of the cerebrum. Third, normal cats tend to show a sidedness in their behavior which is almost as great as that of the cats with lesions, suggesting that early removal of one hemisphere does not create strong sidedness, but may intensify certain aspects of sidedness which normally exist even with both hemispheres intact. References
K., 0. S. ORTH, H. F. HARLOW, and K. A. SCHILTZ. 1960. Learned behavior of rhesus monkeys following neonatal bilateral prefrontal lobotomy. Science 132: 1944-1945. 2. BENJAMIN, R. M., and R. F. THOMPSON. 1959. Differential effects of cortical lesions in infant and adult cats on roughness discrimination. Exptl. Neural. 1: 1.
AKERT,
305-321.
3. KENNARD, M. A. 1938. Reorganization of motor function in the cerebral cortex of monkeys deprived of motor and premotor areas in infancy. J. Neurophysiol 1: 477-496. 4. RAMIREZ DE ARELLANO, M. 1961. Hemidecortication in monkeys. Comparison of rate and degree of recovery of neurological deficit as related to age at time of operation. Excerpta Med. Intern. Congr. Ser. No. 38: 150-151. 5. TREVARTHEN, C. B. 1962. Double visual learning in split-brain monkeys. Science 136: 258-259. 6. TSANG, Y.-C. 1937. Maze learning in rats hemidecorticated in infancy. J. Cow@.
Psychol.
24: 221-248.
NEUROLOGY
Effects
of
6, 332-339
Early Spatial
BERNICE M.
(1962)
Postnatal
Hemidecortication
Discrimination WENZEL,
R. D.
on
in Cats
TSCHIRGI,
AND
J.
L.
TAYLOR~
University of California School of Medicine, Los Angeles, California University of Illinois, College of Medicine, Chicago, Illinois Received
July
and
6, 1962
Under certain circumstances, major unilateral cortical removal may result in little permanent deficit in motor abilities. To study the effects of early damage on ability to make spatially oriented responses, hemidecortication was done on 9-day old kittens. The two that survived surgery developed normally, and showed no motor deficiency other than permanent loss of the placing reflex in the contralateral limbs. Testing in special learning situations began at 9 weeks. Performance of the hemidecorticate cats was compared with that of normal littermates and surgical controls. The operated animals performed as well as the control in a T-maze. In two reaction-time tests, one measuring visual function and one measuring paw dominance, both of the operated and three of four control animals made faster responses on one side than on the other. The operated cats responded faster on the side ipsilateral to their lesion. It is concluded that cats deprived of one cerebral cortex soon after birth are not essentially different from controls in spatially oriented behavior or in motor ability. Introduction
The literature has contained references for many years to the minimal effects produced by certain types of brain damage in very young animals. Kennard (3) showed that infant monkeys could be subjected to extensive removal of motor cortex without appreciable functional loss, Complete hemidecortication was done by Ramirez de Arrellano (4) in a group of infant monkeys and a group at least 19 months old. The younger group was characterized by fewer and less severe neurological symptoms immediately postoperatively as well as by more rapid improvement, even to a normal condition for most symptoms. Few systematic behavioral studies have been done following lesions in early life. Tsang (6) found 1 The research has been supported by a grant from the National Association for Mental Health. The authors wish to acknowledge the assistance of Ingrid AhrneCollier and Toby Goldstein in testing and maintenance of the colony. 332
POSTNATAL
HEMIDECORTICATION
333
that hemidecortication in young rats resulted in very slight deficits in maze learning. Recently, Akert, Orth, Harlow and Schiltz (1) reported that bilateral frontal lobotomy in infant monkeys does not have the detrimental effect on delayed response learning that typically follows such a lesion in adult monkeys. Similarly, Benjamin and Thompson (2) showed that somatic cortex can be removed bilaterally from young kittens with almost no loss in roughness discrimination 6 months later, while the same operation in adult cats results in a marked decrement. In the experiments reported here, a series of behavioral tests was carried out designed to evaluate the possible effects of a chronic lateral imbalance in cerebral tissue on the ability to orient successfully in horizontal space, i.e., to learn left-right response patterns. These tests were part of a larger program of investigation on the relation between bilaterality of the central nervous system and spatial discrimination. Procedure
The experimental population consisted of six cats-one left and one right hemidecortication, two littermate normal controls, and two surgical controls. Lesions were made by suction, under barbiturate anesthesia, at 8 to 10 days of age. Since all but one of the cats are still alive-aged 3 to 4 years-complete histological examination has not yet been made. Figure 1 shows the brain of the hemidecorticate cat that recently died 3 years and 7 months after operation. From gross examination, it appears that the only parts of the neocortex remaining were very small amounts of orbitofrontal and prepyriform cortex and the olfactory tubercle. The entire hippocampal gyrus was removed. Further details must await sectioning of the brain. The surgical controls were anesthetized and had the scalp and skull opened, but no brain tissue was removed. At 9 weeks, the kittens began formal testing in a T-maze. Each correct turn was reinforced by the opportunity to play outside the maze for about a minute; after a wrong turn they were restrained for the same time. Training consisted of five stagesspread over 5 weeks: (a) six trials with both doors open and exit permitted through either; (b) twenty-four trials with only one door open (on the nonpreferred side if there was one) and exploration permitted until exit; (c) twenty-four trials with both doors closed and the correct turn toward the same side as the open door in Part 2; (d) thirty-six trials with the correct turn reversed; (e) eighteen trials with the correct turn reversed again.
334
WENZEL,
TSCHIRGI,
AND
TAYLOR
In order to obtain an additional measure of the ability to learn spatial relations, provided training is given .to overcome the sensory deficit, the second formal learning situation was a visual test to compare reaction times to lights at various peripheral points. The cats were trained to look straight
FIG. 1. Dorsal, ventral and 7 months. The lesion
and lateral views of brain had been made at 9 days
of cat that died at age 3 years of age on the right side.
POSTNATAL
HEMIDECORTICATION
335
ahead until a buzzer was sounded simultaneously with presentation of a light in the periphery. The buzzer was always directly in front of the cat, and served as a signal to look at the light. As soon as the latter response occurred, food reinforcement was given. Reaction time was measured for stimuli at 15” and 30’ to the left and right on the horizontal plane, in daily sessions of twenty trials each. Finally, another test was given to check on the possibility of sidedness in a situation where lateral vision was less critical. The cats were trained with food reinforcement to place each forepaw on a stimulus object when the latter appeared just in front of the paw. Time was recorded between the presentation of the stimulus and the placing of the paw. When response time was stable with each paw, test trials were introduced in which the stimulus was presented in the center and the cat could place either paw. Daily testing sessions of twenty trials each were held for 12 days, with four test trials inserted at random in the series on alternate days. At the conclusion of these tests, the cats were from 1% to 2 years old. Results
At no time after recovery from anesthesia did the kittens show any motor difficulties in feeding or crawling, nor was any circling behavior observed. The animals with lesions began to walk and run as early and as well as the control animals. They began to climb about 2 weeks later than the controls but eventually became just as skillful. Their ability to manipulate a rolling or bouncing ball was indistinguishable from that of the controls. Repeated neurological examinations revealed an initial loss of placing and hopping reflexes contralateral to the lesions, with subsequent partial recovery of hopping. Slight paresis and slightly increased extension in both contralateral limbs, and a loss of the contralateral visual field have remained permanently. No other sensory deficits have been noted. The results of testing in the T-maze are presented in Table 1. They show that the choices of the kittens with lesions in Part 1 were almost completely unilateral toward the side of the intact visual field, presumably a reflection of their visual deficit. However, after the special training of Part 2, in which they were forced to explore until they found the other door, the performance of the kittens with lesions became as good as, or better than, that of the controls. All animals learned equally well in Part 3, and only the surgical controls showed special difficulty in the reversal situation of Part 4. Clearly, no deficit was shown by the kittens with
336
WENZEL,
lesions in ability reverse it.
TSCHIRGI,
AND
to learn a left-right
TAYLOR
discrimination,
TABLE
nor in ability
to
1
T-MAZE RESULTS. PERCENTAGE OF CHOICES IN PART 1, AND PERCENTAGE OF ERRORS IN PART 2 ARE SHOWN FOR EACH CAT. MEAN PERCENTAGE OF ERRORS Is GIVEN FOR EACH GROUP IN PARTS $4 AND 5.
Part 1 Left Right Hemidecorticates Normal
Mean controls
Mean Surgical controls
83 0
Left
17
Part 2 Right
Part 3 Part 4 Part 5
63
100
75
50 50
50 50
25 2.5
75 7.5
10.5
51
42
1.5
46
42
15
81
14
13 17 13 63
Mean
In the early stages of training in the visual test, the cats with lesions tended to have very long reaction times on their blind side, but with further experience they learned to respond well on both sides. Table 2 shows the results obtained after enough training to produce a stable level of responding. All means are based on at least nine sessions. Although the hemidecorticate cats were somewhat slower in looking at lights on the side contralateral to their lesion, the striking fact in the data is that three TABLE MEAN
RESPONSE
TIME
2
IN SECONDS FOR EACH
CAT IN VISUAL
Left visual field 30”
Hemidecorticates Normal
controls
Surgical controls
TEST
Right visual field 1.5”
15”
30”
1.11
0.85
1.18
1.23
1.16
1.12
0.51
0.54
0.69 0.63
0.62 0.69
1.11
1.13
0.46
0.48
1.14
1.15
0.48
0.44
0.54 0.47
0.53
0.54
One hemidecorticate cat (see text) Initial training Spontaneous reversal
1.11
0.85
1.18
1.23
1.17
1.19
0.68
0.61
Special training
0.76
0.79
0.74
0.66
POSTNATAL
337
HEMIDECORTICATION
of the four control cats also showed visual sidedness, and, in general, less regular data. On the whole, the behavior of the experimental and control animals was very similar. Table 2 also contains data concerning an incidental finding in the visual test, viz., a unique response pattern shown by the cat with left hemidecortication after many daily testing sessions had occurred. She
PAW
HEMICECORTICATES
PAW
NORMAL
PAW
CONTROLS
PAW
SURGlCAL
PAW
CONTROLS
FIG. 2. Mean latencies for each cat in paw test. Shaded bars represent left and right paws on standard trials; white bars represent left and right paws on test trials. Figures in white bars show the number of times each paw was used.
showed a sudden spontaneous reversal in the relative speed of reaction on the two sides, so that her faster responseswere being made toward her absent visual field. Special training was given to test the reliability of this reversal and all that was accomplished was to eliminate any differential. Figure 2 presents the results in the paw-placing test, showing the mean latencies for both the 240 regular trials and the 24 test trials, and the frequency of use of each paw on the test trials. The trends in these data are very similar to those of the visual test. Not only did the
338
WENZEL,
TSCHIRGI,
AND
TAYLOR
hemidecorticate cats show faster times and greater use on a given side, but the same three control cats did, also. Discussion
Although few experiments have been done with the specific purpose of measuring the effect of very early, extensive, cortical lesions on later ability to learn a variety of tasks, the existing literature is in essential agreement that behavioral loss is extremely small or even undetectable. The statement is often made that the loss is “astonishingly” small, expressing a tacit conviction that such a gross tissue deficit should leave an equally gross behavioral deficit. The plasticity of the young nervous system easily triumphs over such a naive correlation, however, and creates a functional entity out of the raw material remaining after massive lesions. The significant feature of the present results is certainly the absence of any differences in ability to orient properly to the tasks and to make the appropriate spatial discriminations among details of the input array. Only in such a relatively subtle measure as the frequency of use of each paw in the ambiguous situation included in the paw-placing test do we get a hint of the nature of the “new” brain that these animals possess. Theirs is a brain in which the processing of information concerned with one side of the body is accomplished less quickly, and hence presumably by a more indirect route, than is information about the other side. But even here, the cats with lesions differed only in degree, suggesting that in the normal course of development one cerebral hemisphere becomes relatively more effective in processing spatial information. Another instance of this phenomenon was reported recently by Trevarthen (5) who found that split-brain monkeys, trained to perform two contradictory visual tasks simultaneously, usually showed unequal learning and retention in the two halves of the brain. The monkeys were free to choose either hand for making their responses. In a majority of cases, the task presented to the eye contralateral to the preferred hand was learned and retained better, indicating the superior functioning of one hemisphere. Three principal conclusions have emerged from the observations reported here. First, in general agreement with others, we can say that extensive unilateral removal of cerebral tissue very early in fife leaves no gross deficits in motor, sensory or intellective functions either immediately after operation or at later times. Second, the hemidecorticate cats are just as skillful as normal cats in learning various types of left-right dfs-
POSTNATAL
HEMIDECORTICATION
339
criminations despite the lateral imbalance of the cerebrum. Third, normal cats tend to show a sidedness in their behavior which is almost as great as that of the cats with lesions, suggesting that early removal of one hemisphere does not create strong sidedness, but may intensify certain aspects of sidedness which normally exist even with both hemispheres intact. References
K., 0. S. ORTH, H. F. HARLOW, and K. A. SCHILTZ. 1960. Learned behavior of rhesus monkeys following neonatal bilateral prefrontal lobotomy. Science 132: 1944-1945. 2. BENJAMIN, R. M., and R. F. THOMPSON. 1959. Differential effects of cortical lesions in infant and adult cats on roughness discrimination. Exptl. Neural. 1: 1.
AKERT,
305-321.
3. KENNARD, M. A. 1938. Reorganization of motor function in the cerebral cortex of monkeys deprived of motor and premotor areas in infancy. J. Neurophysiol 1: 477-496. 4. RAMIREZ DE ARELLANO, M. 1961. Hemidecortication in monkeys. Comparison of rate and degree of recovery of neurological deficit as related to age at time of operation. Excerpta Med. Intern. Congr. Ser. No. 38: 150-151. 5. TREVARTHEN, C. B. 1962. Double visual learning in split-brain monkeys. Science 136: 258-259. 6. TSANG, Y.-C. 1937. Maze learning in rats hemidecorticated in infancy. J. Cow@.
Psychol.
24: 221-248.