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Separation of visual functions within the corpus callosum of monkeys
Brain Research, 49 (1973) 185-189
185
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Separation of visual functions within the corpus callosum of monkeys
CHARLES R. H A M I L T O N AND BETTY A N N BRODY
Division of Biology, California Institute of Technology, Pasadena, Calif. 91109 and Department of Psychology, Stanford University, Stanford, Calif. 94305 (U.S.A.) (Accepted October 3rd, 1972)
The corpus callosum and anterior commissure are crucial for the transfer of information between the two cerebral hemispheres. If these forebrain commissures are severed, and sensory input is lateralized, then each half brain perceives, learns, and remembers independently of the other 13. A discrimination taught to one half is neither initially performed above chance by the other half, nor is it subsequently learned more quickly. Similarly, tasks that require a comparison of stimuli separately received by the two hemispheres appear to depend on intact forebrain commissuresg,lo, 14, although there may be some exceptions~, 15. The few attempts at gross localization of the functional pathways within the cerebral commissures of primates have been successful. For example, transfer of somesthetic information depends on fibers coursing through the body of the corpus callosum11, while transfer of visual information depends on an intact anterior commissure or splenium of the callosum2,7. These behavioral results complement anatomical studies of commissural pathways which show that connections between anterior parietal (somesthetic) regions are located in the midbody of the callosum12, occipital (visual) connections in the spleniumlz, and temporal lobe connections (visual) in the splenium and anterior commissure5,1~,16. Thus there is good correspondence between the sensory modality of a cortical region and the type of information carried by its commissural connections. It seems likely that functional localization in the commissures also exists within one sensory system. For example, the various areas of cortex involved in visual discrimination appear to have anatomically separable interhemispheric connections. The striate border and adjoining prestriate cortex connect through the posterior splenium1~, more distal prestriate and posterior inferotemporal cortex connect through more anterior portions of the splenium and perhaps posterior body of the callosum~2, and anterior inferotemporal cortex connects through the anterior commissureS, xa. It seems logical that differences in the processing of visual information by these various cortical areas should be reflected in differences in the information transferred through their commissural connections. In support of this expectation we have found a differential capability of the anterior and posterior portions of the splenium in conveying
186
SHORT COMMUNICATIONS
visual information between the hemispheres, and suggest that this represents differences in function of the cortical areas interconnected by these two callosal regions. Four rhesus monkeys (Macaca mulatta) underwent midsaggital section of the optic chiasm, anterior commissure, and all of the corpus callosum and hippocampal commissure anterior to the splenium, which we define as the posterior 10 mm of the corpus callosum. In two monkeys the posterior 5 mm of the callosum were also sectioned, leaving the anterior splenium intact, while in the other two monkeys the penultimate 5 mm of the callosum were sectioned, leaving the posterior splenium intact. Histological verification of the locations of the surviving callosal fibers is not available since these animals are being further tested in order to determine more specifically the behavioral functions attributable to these fibers. The nature of the results indicates that the principal surgery, which was done under visual control, was successful, and therefore the histological results will simply pinpoint the precise location of the intact callosal bridges. However, until more detailed information concerning the cortical distribution of the fibers of these callosal regions is available, no exact statements could be made concerning the location of the cortical areas involved. The monkeys were trained in an apparatus designed for testing split-brain monkeys 13. They voluntarily placed their heads between restraining headpieces and peered through eyeholes at the stimuli which could be seen through transparent circular response keys, 1.5 in. in diameter, mounted on a vertical panel that was fixed at arm's length. A solid state logic unit automatically controlled the presentation of the stimuli and rewards. Two types of visual tasks, matching to sample and simultaneous two-choice discriminations, were trained with similar pairs of white outline patterns. For the two-choice discrimination two keys were used, placed one above and one below a food-well. For the matching to sample task, three keys were arranged in an equilateral triangle above the food-well9. Pushing the sample, which appeared at the top of the triangle, caused two possible matches to appear on the bottom keys. The monkeys were first trained to perform the matching to sample task within each hemisphere by occluding one eye at a time. Then to determine if the animals could compare visual patterns via the part of the callosum that remained, the sample pattern was presented to one eye while the two possible matches were presented to the other eye. Pairs of polaroid filters, properly oriented before the stimuli and the eyeholes, produced this separation ofinputsg, 1~. Each monkey was tested in this way with several different pairs of stimuli. The results were clear: all monkeys could compare visual stimuli at a level of 90 ~ correct for at least 120 trials. Furthermore they could perform equally well with new patterns which had not been learned previously within each hemisphere. Control tests indicated that each monkey's performance depended on the visual cues and that the polaroids completely separated the inputs as intended. Since monkeys with complete section of the optic chiasm, corpus callosum, and anterior commissure do not perform above chance on such comparison tasks ~, we conclude that either the anterior or posterior 5 mm of the splenium can convey information representing visual patterns. Next, the 4 monkeys were tested for the ability to transfer pattern discriminations
187
SHORT COMMUNICATIONS
from one hemisphere to the other. When the monkey learned a discrimination to the 90 ~o criterion with one eye it was tested with the other eye and if necessary retrained to criterion. Each monkey was tested on 4 problems; the eye trained first was counterbalanced. Again the results were clear: there was good interocular transfer of the discriminations through the posterior splenium but no transfer through the anterior splenium. Table I summarizes the results for the initial level of performance and for the percent savings with the untrained eye. In several cases the same patterns that did not show transfer through the anterior splenium when a discrimination was tested did show transfer when tests of interhemispheric comparison were used. We conclude from these two experiments that the anterior splenium is more restricted in its ability to transmit information than is the posterior splenium. Some information present in the cortical areas interconnected by the posterior splenium must be unavailable to the areas interconnected by the anterior splenium. Several interpretations of the incomplete nature of this information are possible. The designation of a particular stimulus as correct might be carried out in cortical areas anatomically separate from those that process pictorial information. If these areas were not interconnected by the anterior splenium, then information about the reward values of the stimuli would not transfer through this part of the callosum. This would preclude interocular transfer of two-choice discriminations, for which assignment of reward values is necessary, but would not affect interhemispheric comparisons, such as matching to sample, for which reward values should never be associated with particular stimuli. Why reward values should be attached to stimuli at the presumably earlier stages of analysis occurring in areas connected by the posterior splenium poses some problems for this view. Another possibility is that indirect input to the untrained hemisphere via the anterior splenium is insufficient to keep it alert, in contrast to indirect input via the posterior splenium or direct input via the optic tract. Consequently memories for twochoice discriminations would not be formed by the inattentive hemisphere. However
TABLE I AVERAGE INTEROCULAR TRANSFER BY 4 MONKEYS EACH TESTED ON 4 DISCRIMINATIONS Initial T r a n s f e r is the ~ c o r r e c t on the first 40 trials w i t h t he s e c ond e ye ; c h a n c e p e r f o r m a n c e = 50 ~ . Savings = 100 (a - - b)/(a + b) where a = trials to c r i t e r i o n w i t h t he first eye a n d b = trials to criter i o n with the second eye.
Anterior splenium intact
Posterior splenium intact
Subject
Initial transfer
Savings
Subject
lnitial transfer
Savings
OTT CSS
61 48
--27 2
ANT ECT
80 88
65 72
Mean
54
--12
Mean
84
68
188
SHORT COMMUNICATIONS
performance on the interhemispheric matching task would not suffer since each hemisphere would remain alert because it was activated by direct visual input. A more intriguing interpretation of our behavioral results derives from recent anatomical and physiological studies of the cortical visual areas. If the posterior splenium conveys unprocessed sensory datalbetween the hemispheres, then its capability for interocular transfer is simply explained. The second hemisphere can process this data, learn, and remember on its own just as if it received direct input. Interhemispheric matching easily occurs since the relevant sensory information is available to either side. The properties of receptive fields of single units in the posterior splenium of the cat are consistent with this interpretation and have led Berlucchi et al. to offer a similar suggestion 1. The inability of the anterior splenium to transfer two-choice discriminations while still supporting interhemispheric comparisons could result from a loss of information regarding the position of the stimuli. Recent studies indicate that the precise topographical representation of the visual fields on the striate cortex progressively degenerates as the visual fields are repeatedly mapped onto prestriate and inferotemporal cortex 3,17-19. This can be seen in several anatomical studies that show, for example, that spatially discrete regions of area 17 project to overlapping areas of cortex in the lunate gyrus 3,17,18. Physiological studies provide evidence that the receptive fields are not organized topographically in at least one of these areas 4, or in the inferotemporal cortex s. Furthermore, single unit recordings in the anterior lunate gyrus 4 and in inferotemporal cortex s reveal large receptive fields, within which the location of an adequate stimulus is relatively unimportant for driving the unit. If accurate localization of objects within the visual fields depends on an accurate topographical representation of the visual field on the cortex, then positional information would characterize the more posterior striate and prestriate visual areas and not those of the anterior lunate gyrus or posterior inferotemporal cortex. The latter areas would be concerned more with properties of patterns and objects than with their positions within the visual field. Therefore the callosal connections of these areas, presumably passing through the anterior splenium 12, should be incapable of transmitting information regarding position of objects in the visual field. For the matching to sample task tested with separated inputs this is unimportant since the hemisphere which views the two matching stimuli need only know which sample is present, not its location in the visual field. By contrast, discrimination between two patterns presented simultaneously requires knowledge of their spatial positions. For example, a monkey must know on which screen the positive stimulus appeared in order to associate that stimulus with a correct spatially directed response and with the presence of a reward. If the position of the positive stimulus were not available, the untrained hemisphere would only know that a response directed toward a particular place in space was rewarded, but not which stimulus was present at that position. According to this view, then, the information transmitted through the anterior splenium fails to support interocular transfer of two-choice discriminations because it does not specify the relative spatial positions of the stimuli in the visual field. These speculations suggest an important role for topographic representations in perception.
SHORT COMMUNICATIONS
189
"~Ve t h a n k S. B. T i e m a n , H. L. W i n t er , and G. H a g i w a r a for assistance. T h e research was s u p p o r t e d by N I H G r a n t NS-06501.
1 BERLUCCHI,G., GAZZANIGA,M. S., AND RIZZOLATTI,G., Microelectrode analysis of transfer of visual information by the corpus callosum, Arch. itaL Biol., 105 (1967) 583-596. 2 BLACK,P., AND MYERS, R. E., Visual function of the forebrain commissures in the chimpanzee, Science, 146 (1964) 799-800. 3 CRAGG, B. G., AND AtNSWORTH,A., The topography of the afferent projections in the circumstriate visual cortex of the monkey studied by the Nauta method, Vision Res., 9 (1969) 733-747. 4 DUBNER,R., AND ZEKI, S. M., Response properties and receptive fields of cells in an anatomically defined region of the superior temporal sulcus in the monkey, Brain Research, 35 (1971) 528-532. 5 EBNER,F. F., AND MYERS, R. E., Commissural connections in the neocortex of monkey, Anat. Rec., 142 (1962) 229. 6 GAVALAS,R. J., ANDSPERRY,R. W., Central integration of visual half-fields in split-brain monkeys, Brain Research, 15 (1969) 97-106. 7 GAZZANIGA, M. S., Interhemispheric communication of visual learning, Neuropsychologia, 4 (1966) 183-189. 8 GROSS,C. G., ROCHA-MIRANDA,C. E., ANDBENDER,D. B., Visual properties of neurons in inferotemporal cortex of the macaque, J. Neurophysiol., 35 (1972) 96-111. 9 HAMtLTON,C. R., HtLLYARD,S. A., AND SPERRY,R. W., Interhemispheric comparison of color in split-brain monkeys, Exp. Neurol., 21 (1968) 486-494. 10 LEE-TEr~G,E., AND SPERRY, R. W., Intermanual stereognostic size discrimination in split-brain monkeys, J. comp. physiol. Psychol., 62 (1966) 84-89. 11 MYERS,R. E., discussion. In V. B. MOUNTCASTLE(Ed.), lnterhemispheric Relations and Cerebral Dominance, Johns Hopkins Press, Baltimore, Md., 1962, pp. 117-129. 12 PANDYA, D. N., KAROL, E. A., AND HEILBRONN,D., The topographical distribution of interhemispheric projections in the corpus callosum of the rhesus monkey, Brain Research, 32 (1971) 31-43. 13 SPERRY, R. W., Mental unity following surgical disconnection of the cerebral hemispheres, Harvey Lect., 62 (1968) 293-323. 14 SPERRY, R. W., AND GREEN, S. M., Corpus callosum and perceptual integration of visual halffields, Anat. Rec., 148 (1964) 339. 15 TR~VARTHEN,C. B., TWO mechanisms of vision in primates, Psychol. Forsch. 31 (1968) 299-377. 16 WrtITLOCK,D. G., AriD NAUTA,W. J. H., Subcortical projections from the temporal neocortex in Macaca mulatta, J. comp. Neurol., 106 (1956) 183-212. 17 ZEKLS. M., Representation of central visual fields in prestriate cortex of monkey, Brain Research, 14 (1969) 271-291. 18 ZEKL S. M., Convergent input from the striate cortex (area 17) to the cortex of the superior temporal sulcus in the rhesus monkey, Brain Research, 28 (1971) 338-340. 19 ZEKI, S. M., Cortical projections from two prestriate areas in the monkey, Brain Research, 34 (1971) 19-35.
185
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Separation of visual functions within the corpus callosum of monkeys
CHARLES R. H A M I L T O N AND BETTY A N N BRODY
Division of Biology, California Institute of Technology, Pasadena, Calif. 91109 and Department of Psychology, Stanford University, Stanford, Calif. 94305 (U.S.A.) (Accepted October 3rd, 1972)
The corpus callosum and anterior commissure are crucial for the transfer of information between the two cerebral hemispheres. If these forebrain commissures are severed, and sensory input is lateralized, then each half brain perceives, learns, and remembers independently of the other 13. A discrimination taught to one half is neither initially performed above chance by the other half, nor is it subsequently learned more quickly. Similarly, tasks that require a comparison of stimuli separately received by the two hemispheres appear to depend on intact forebrain commissuresg,lo, 14, although there may be some exceptions~, 15. The few attempts at gross localization of the functional pathways within the cerebral commissures of primates have been successful. For example, transfer of somesthetic information depends on fibers coursing through the body of the corpus callosum11, while transfer of visual information depends on an intact anterior commissure or splenium of the callosum2,7. These behavioral results complement anatomical studies of commissural pathways which show that connections between anterior parietal (somesthetic) regions are located in the midbody of the callosum12, occipital (visual) connections in the spleniumlz, and temporal lobe connections (visual) in the splenium and anterior commissure5,1~,16. Thus there is good correspondence between the sensory modality of a cortical region and the type of information carried by its commissural connections. It seems likely that functional localization in the commissures also exists within one sensory system. For example, the various areas of cortex involved in visual discrimination appear to have anatomically separable interhemispheric connections. The striate border and adjoining prestriate cortex connect through the posterior splenium1~, more distal prestriate and posterior inferotemporal cortex connect through more anterior portions of the splenium and perhaps posterior body of the callosum~2, and anterior inferotemporal cortex connects through the anterior commissureS, xa. It seems logical that differences in the processing of visual information by these various cortical areas should be reflected in differences in the information transferred through their commissural connections. In support of this expectation we have found a differential capability of the anterior and posterior portions of the splenium in conveying
186
SHORT COMMUNICATIONS
visual information between the hemispheres, and suggest that this represents differences in function of the cortical areas interconnected by these two callosal regions. Four rhesus monkeys (Macaca mulatta) underwent midsaggital section of the optic chiasm, anterior commissure, and all of the corpus callosum and hippocampal commissure anterior to the splenium, which we define as the posterior 10 mm of the corpus callosum. In two monkeys the posterior 5 mm of the callosum were also sectioned, leaving the anterior splenium intact, while in the other two monkeys the penultimate 5 mm of the callosum were sectioned, leaving the posterior splenium intact. Histological verification of the locations of the surviving callosal fibers is not available since these animals are being further tested in order to determine more specifically the behavioral functions attributable to these fibers. The nature of the results indicates that the principal surgery, which was done under visual control, was successful, and therefore the histological results will simply pinpoint the precise location of the intact callosal bridges. However, until more detailed information concerning the cortical distribution of the fibers of these callosal regions is available, no exact statements could be made concerning the location of the cortical areas involved. The monkeys were trained in an apparatus designed for testing split-brain monkeys 13. They voluntarily placed their heads between restraining headpieces and peered through eyeholes at the stimuli which could be seen through transparent circular response keys, 1.5 in. in diameter, mounted on a vertical panel that was fixed at arm's length. A solid state logic unit automatically controlled the presentation of the stimuli and rewards. Two types of visual tasks, matching to sample and simultaneous two-choice discriminations, were trained with similar pairs of white outline patterns. For the two-choice discrimination two keys were used, placed one above and one below a food-well. For the matching to sample task, three keys were arranged in an equilateral triangle above the food-well9. Pushing the sample, which appeared at the top of the triangle, caused two possible matches to appear on the bottom keys. The monkeys were first trained to perform the matching to sample task within each hemisphere by occluding one eye at a time. Then to determine if the animals could compare visual patterns via the part of the callosum that remained, the sample pattern was presented to one eye while the two possible matches were presented to the other eye. Pairs of polaroid filters, properly oriented before the stimuli and the eyeholes, produced this separation ofinputsg, 1~. Each monkey was tested in this way with several different pairs of stimuli. The results were clear: all monkeys could compare visual stimuli at a level of 90 ~ correct for at least 120 trials. Furthermore they could perform equally well with new patterns which had not been learned previously within each hemisphere. Control tests indicated that each monkey's performance depended on the visual cues and that the polaroids completely separated the inputs as intended. Since monkeys with complete section of the optic chiasm, corpus callosum, and anterior commissure do not perform above chance on such comparison tasks ~, we conclude that either the anterior or posterior 5 mm of the splenium can convey information representing visual patterns. Next, the 4 monkeys were tested for the ability to transfer pattern discriminations
187
SHORT COMMUNICATIONS
from one hemisphere to the other. When the monkey learned a discrimination to the 90 ~o criterion with one eye it was tested with the other eye and if necessary retrained to criterion. Each monkey was tested on 4 problems; the eye trained first was counterbalanced. Again the results were clear: there was good interocular transfer of the discriminations through the posterior splenium but no transfer through the anterior splenium. Table I summarizes the results for the initial level of performance and for the percent savings with the untrained eye. In several cases the same patterns that did not show transfer through the anterior splenium when a discrimination was tested did show transfer when tests of interhemispheric comparison were used. We conclude from these two experiments that the anterior splenium is more restricted in its ability to transmit information than is the posterior splenium. Some information present in the cortical areas interconnected by the posterior splenium must be unavailable to the areas interconnected by the anterior splenium. Several interpretations of the incomplete nature of this information are possible. The designation of a particular stimulus as correct might be carried out in cortical areas anatomically separate from those that process pictorial information. If these areas were not interconnected by the anterior splenium, then information about the reward values of the stimuli would not transfer through this part of the callosum. This would preclude interocular transfer of two-choice discriminations, for which assignment of reward values is necessary, but would not affect interhemispheric comparisons, such as matching to sample, for which reward values should never be associated with particular stimuli. Why reward values should be attached to stimuli at the presumably earlier stages of analysis occurring in areas connected by the posterior splenium poses some problems for this view. Another possibility is that indirect input to the untrained hemisphere via the anterior splenium is insufficient to keep it alert, in contrast to indirect input via the posterior splenium or direct input via the optic tract. Consequently memories for twochoice discriminations would not be formed by the inattentive hemisphere. However
TABLE I AVERAGE INTEROCULAR TRANSFER BY 4 MONKEYS EACH TESTED ON 4 DISCRIMINATIONS Initial T r a n s f e r is the ~ c o r r e c t on the first 40 trials w i t h t he s e c ond e ye ; c h a n c e p e r f o r m a n c e = 50 ~ . Savings = 100 (a - - b)/(a + b) where a = trials to c r i t e r i o n w i t h t he first eye a n d b = trials to criter i o n with the second eye.
Anterior splenium intact
Posterior splenium intact
Subject
Initial transfer
Savings
Subject
lnitial transfer
Savings
OTT CSS
61 48
--27 2
ANT ECT
80 88
65 72
Mean
54
--12
Mean
84
68
188
SHORT COMMUNICATIONS
performance on the interhemispheric matching task would not suffer since each hemisphere would remain alert because it was activated by direct visual input. A more intriguing interpretation of our behavioral results derives from recent anatomical and physiological studies of the cortical visual areas. If the posterior splenium conveys unprocessed sensory datalbetween the hemispheres, then its capability for interocular transfer is simply explained. The second hemisphere can process this data, learn, and remember on its own just as if it received direct input. Interhemispheric matching easily occurs since the relevant sensory information is available to either side. The properties of receptive fields of single units in the posterior splenium of the cat are consistent with this interpretation and have led Berlucchi et al. to offer a similar suggestion 1. The inability of the anterior splenium to transfer two-choice discriminations while still supporting interhemispheric comparisons could result from a loss of information regarding the position of the stimuli. Recent studies indicate that the precise topographical representation of the visual fields on the striate cortex progressively degenerates as the visual fields are repeatedly mapped onto prestriate and inferotemporal cortex 3,17-19. This can be seen in several anatomical studies that show, for example, that spatially discrete regions of area 17 project to overlapping areas of cortex in the lunate gyrus 3,17,18. Physiological studies provide evidence that the receptive fields are not organized topographically in at least one of these areas 4, or in the inferotemporal cortex s. Furthermore, single unit recordings in the anterior lunate gyrus 4 and in inferotemporal cortex s reveal large receptive fields, within which the location of an adequate stimulus is relatively unimportant for driving the unit. If accurate localization of objects within the visual fields depends on an accurate topographical representation of the visual field on the cortex, then positional information would characterize the more posterior striate and prestriate visual areas and not those of the anterior lunate gyrus or posterior inferotemporal cortex. The latter areas would be concerned more with properties of patterns and objects than with their positions within the visual field. Therefore the callosal connections of these areas, presumably passing through the anterior splenium 12, should be incapable of transmitting information regarding position of objects in the visual field. For the matching to sample task tested with separated inputs this is unimportant since the hemisphere which views the two matching stimuli need only know which sample is present, not its location in the visual field. By contrast, discrimination between two patterns presented simultaneously requires knowledge of their spatial positions. For example, a monkey must know on which screen the positive stimulus appeared in order to associate that stimulus with a correct spatially directed response and with the presence of a reward. If the position of the positive stimulus were not available, the untrained hemisphere would only know that a response directed toward a particular place in space was rewarded, but not which stimulus was present at that position. According to this view, then, the information transmitted through the anterior splenium fails to support interocular transfer of two-choice discriminations because it does not specify the relative spatial positions of the stimuli in the visual field. These speculations suggest an important role for topographic representations in perception.
SHORT COMMUNICATIONS
189
"~Ve t h a n k S. B. T i e m a n , H. L. W i n t er , and G. H a g i w a r a for assistance. T h e research was s u p p o r t e d by N I H G r a n t NS-06501.
1 BERLUCCHI,G., GAZZANIGA,M. S., AND RIZZOLATTI,G., Microelectrode analysis of transfer of visual information by the corpus callosum, Arch. itaL Biol., 105 (1967) 583-596. 2 BLACK,P., AND MYERS, R. E., Visual function of the forebrain commissures in the chimpanzee, Science, 146 (1964) 799-800. 3 CRAGG, B. G., AND AtNSWORTH,A., The topography of the afferent projections in the circumstriate visual cortex of the monkey studied by the Nauta method, Vision Res., 9 (1969) 733-747. 4 DUBNER,R., AND ZEKI, S. M., Response properties and receptive fields of cells in an anatomically defined region of the superior temporal sulcus in the monkey, Brain Research, 35 (1971) 528-532. 5 EBNER,F. F., AND MYERS, R. E., Commissural connections in the neocortex of monkey, Anat. Rec., 142 (1962) 229. 6 GAVALAS,R. J., ANDSPERRY,R. W., Central integration of visual half-fields in split-brain monkeys, Brain Research, 15 (1969) 97-106. 7 GAZZANIGA, M. S., Interhemispheric communication of visual learning, Neuropsychologia, 4 (1966) 183-189. 8 GROSS,C. G., ROCHA-MIRANDA,C. E., ANDBENDER,D. B., Visual properties of neurons in inferotemporal cortex of the macaque, J. Neurophysiol., 35 (1972) 96-111. 9 HAMtLTON,C. R., HtLLYARD,S. A., AND SPERRY,R. W., Interhemispheric comparison of color in split-brain monkeys, Exp. Neurol., 21 (1968) 486-494. 10 LEE-TEr~G,E., AND SPERRY, R. W., Intermanual stereognostic size discrimination in split-brain monkeys, J. comp. physiol. Psychol., 62 (1966) 84-89. 11 MYERS,R. E., discussion. In V. B. MOUNTCASTLE(Ed.), lnterhemispheric Relations and Cerebral Dominance, Johns Hopkins Press, Baltimore, Md., 1962, pp. 117-129. 12 PANDYA, D. N., KAROL, E. A., AND HEILBRONN,D., The topographical distribution of interhemispheric projections in the corpus callosum of the rhesus monkey, Brain Research, 32 (1971) 31-43. 13 SPERRY, R. W., Mental unity following surgical disconnection of the cerebral hemispheres, Harvey Lect., 62 (1968) 293-323. 14 SPERRY, R. W., AND GREEN, S. M., Corpus callosum and perceptual integration of visual halffields, Anat. Rec., 148 (1964) 339. 15 TR~VARTHEN,C. B., TWO mechanisms of vision in primates, Psychol. Forsch. 31 (1968) 299-377. 16 WrtITLOCK,D. G., AriD NAUTA,W. J. H., Subcortical projections from the temporal neocortex in Macaca mulatta, J. comp. Neurol., 106 (1956) 183-212. 17 ZEKLS. M., Representation of central visual fields in prestriate cortex of monkey, Brain Research, 14 (1969) 271-291. 18 ZEKL S. M., Convergent input from the striate cortex (area 17) to the cortex of the superior temporal sulcus in the rhesus monkey, Brain Research, 28 (1971) 338-340. 19 ZEKI, S. M., Cortical projections from two prestriate areas in the monkey, Brain Research, 34 (1971) 19-35.