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Demonstration of ocular dominance columns in a New World primate by means of monocular deprivation
Brain Research, 207 (1981) 453-458 © Elsevier/North-Holland Biomedical Press
453
Demonstration of ocular dominance columns in a New World primate by means of monocular deprivation
E .J. DeBRUYN and V. A. CASAGRANDE* Departments of Anatomy and Psychology, IZanderbilt University, Nashville, Tenn. 37232 (U.S.A.)
(Accepted October 2nd, 1980) Key words: primate - - autoradiography - - cortex - - ocular dominance column - - monocular de-
privation We investigated the pattern of transneuronally transported [3I-I]prolineto the visual cortex of normal and deprived marmosets (Callithrix ]acchus). In the normal marmoset a continuous band of label was evident within layer IV of striate cortex. Following injection of the dosed eye of deprived marmosets, distinct fluctuations of label were apparent over the base of layer IV in both hemispheres. These results support the hypothesis that the presence of ocular separation of geniculo-striate afferents is a feature common to all primates.
Recent anatomical investigations into the organization of the afferent pathway from the lateral geniculate nucleus to striate cortex indicate that the pattern of termination of geniculo-cortical fibers varies considerably between species 1,5,11,13,17,2°,22,23, 25. In primates, differences have been discovered which apparently reflect the evolutionary separation of Old and New World species. Thus, in Old World primates, the afferents representing the input from each eye are segregated into vertical bands or ocular dominance columns (ODCs) in cortex4,S,lOA1,13,15, 23, while in 4 species of the New World Cebid familylO,15,17, 24 and in the marmoset 2z these afferents overlap. Based on the results of these morphological studies, it seems reasonable to assume that there exists a basic difference in the organization of ocular input to striate cortex in New vs Old World primates. This assumption, however, is complicated by recent evidence indicating the presence of well defined ODCs in the spider monkey 7, and hints of ocular segregation of inputs in the owl 19 and squirrel monkeys 14. The results of the last two investigations14,19, which conflict with earlier reports on the owl 17 and squirrel monkeylO,XS, 24, have led us to believe that at least part of the discrepancy between the studies of Old and New World primates may lie in differences in the degree of segregation rather than in differences in the basic organizational patterns of the two groups. Our reasoning is as follows: in the weakly segregated system which appears to be present in the squirrel monkey, the terminal arbors of geniculocortical axons representing one eye may overlap such that transported label from two * To whom correspondence should be addressed.
454 columns may meet and completely obscure the intervening ODCs of opposite ocularity; thus, the impression is one of a continuous pattern of termination (Fig. 1A). Therefore, if the amount of cortical space occupied by terminals representing each eye could be made unequal, it might be possible to demonstrate the columnar nature of these inputs. In light of recent evidence3,4,6,15,16, which suggests that monocular lid suture reduces the size of deprived geniculo-cortical terminal arbors while increasing the size of non-deprived arbors, we chose to investigate the effects of lid suture on the geniculo-cortical organization of a New World species which normally exhibits an overlapping pattern of inputs, the common marmoset (Callathrixjacchus). For this investigation, two juvenile marmosets (78-47 and 78-48) raised with a monocular suture from birth to 5 months of age received injections of 2.5 mCi of [3H]proline (spec. act. 20 Ci/mmol) into the vitreous of their deprived eyes. In addition, 2 normal adult marmosets (78-36 and 79-4) received monocular injections of 5.0 and 2.5 mCi of [3H]proline respectively (spec. act. 100 and 23.7 Ci/mmol). Following a survival period of two weeks, 3 of the animals (78-36, 78-47, and 79-4) were perfused through the heart with 0.9 ~o saline followed by 10 ~ formol saline. The brains were blocked in either the frontal plane(78-47, 79-4) or in a plane tangential to the pial surface of striate cortex (78-36) and stored at 4 °C in 10~o formol saline to which 3 0 ~ sucrose had been added until they sank. They were then sectioned frozen at 30 #m and one section out of every five was mounted on chrome alum gelatinized slides and processed by standard autoradiographic procedures. Following exposure times of 6-12 weeks, the sections were developed and lightly counterstained with cresyl violet. In addition to the eye injection, the fourth animal (78-48) received an intravenous injection of 40 #Ci of [14C]2-deoxyglucose (spec. act. 337 Ci/mmol) on the day of sacrifice. Following visual stimulation of the deprived eye and processing by previously described methods 21, the sections were washed to remove the deoxyglucose and processed as in the other cases. The results of our normal cases confirm the observation of Spatz 22 that the transported label forms a continuous band within layer IV* of area 17 in both hemispheres with a second, faint band over layer IIIB (Fig. 2A and B). Unlike his findings, however, we did not observe a difference in the density of label in the central as opposed to peripheral representation of the visual field. It is possible, however, that the larger quantities of tracer used in the present study obscured these differences. In contrast to the results in normal marmosets, the pattern of transport in our deprived animals showed distinct 200-300 tzm wide fluctuations in the density of label over the base of layer IV of area 17 (Fig. 2C-G), a pattern of segregation which is strikingly similar to the ODCs found in Old World primates, and particularly to the weaker segregation found in the prosimian Galago4,8,1~. These variations were slightly more prominent in the hemisphere ipsilateral to the deprived (injected) eye, and were not apparent in the upper tier of layer IV, where the label formed a continuous band.
* Our investigations of the cortical organization of the marmoset have led us to believe that the numbering system of H~isslerand Wagner9 provides the best description of the laminar organization of area 17 in this animal. Thus, this scheme is used in the present report.
455
A
cx
B
IV
IPSI CONTRA LGN
Fig. 1. A schematic diagram illustrating the hypothesized pattern of geniculo-striate input in the marmoset and the proposed mechanism for the production of columns in the deprived animal based on the results of the present study. A represents the condition found in the normal animal. The terminal arbors of geniculo-striate axons from both contralaterally- and ipsilaterally-innervated geniculate laminae overlap such that each set totally obscures its counterpart, thus giving the impression of a total overlap of input. In the deprived case (B), the arbors representing the deprived (ipsilateral) eye are reduced in size, and therefore form the typical columnar pattern reminiscent of that seen in Old World primates. For the sake of argument we have pictured the open eye (contralateral) afferents as correspondingly larger, although we do not have results concerning the actual distribution of label following an open eye injection. Abbreviations: CONTRA, contralaterally-innervated geniculate laminae; CX, cortex; IPSI, ipsilaterally-innervated geniculate laminae; LGN, lateral geniculate nucleus; IV, cortical layer IV.
Additionally, the faint banding in layer IIIB, which forms a 'honeycomb' pattern as has been described in other primates 10, was also present. Finally, the patches or columns in layer IV were more distinct in case 78-48 than in case 78-47, a difference which may be attributed to the fact that the suture in the latter case was incomplete, and thus, the effects of deprivation were not as strong. We believe that the segregated pattern of input seen in the deprived animals could only be produced from a continuous pattern if ocular input from the two eyes of the marmoset normally alternates as pictured in Fig. 1A. It is unlikely that these columns are artifacts produced by higher order effects of deprivation, since monocular lid suture does not produce changes in the pattern of label in the mouse 5 or in the tree shrew 2, species which also show a normal pattern of overlapping ocular input to striate cortex. Moreover, Hubel and Wiesel's recent demonstrations that: (1) weak variation in label density occurs predominantly in one sublayer of squirrel monkey visual cortex (layer IVca); and (2) fluctuations in ocular dominance occur with periodicity of 250 /zm in this species are consistent with the restricted vertical extent and narrow width of the columns produced by lid suture in the marmoset. We should stress, however, that differences do exist between the two species, as the fluctuations in label in the marmoset occur in the base of layer IV, whereas those in the squirrel monkey appear in the equivalent of the upper tier of this layer. Moreover, the columns exhibited by the deprived marmosets are presumably smaller than the 'hidden' hypothetical alternations of left and right eyes inputs of a normal marmoset (Fig. 1B). Thus, in the normal
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457 Fig. 2. The pattern of geniculo-striate input in the marmoset. A and B are dark-field autoradiographs of normal marmoset (78-36) visual cortex ipsilateral and contralateral to the injected eye, respectively. Note the uniform density of label over layer iV and the fainter label over layer IIIB. Compare this pattern with that in C-G. C, D, E and F are dark-field autoradiographs (sections 41, 71, 41 and 69, respectively) of the visual cortex of deprived marmoset 78-48 ipsilateral (C and E) and contralateral (D and F) to the injected (deprived) eye. Distinct patches of increased label density, 200-300 #m wide, are evident at the base of layer IV. Label in the upper part of layer IV and in layer IIIB is similar to the pattern seen in normal cases. G is a series of line drawings showing the distribution of columns from caudal (section 50) to rostral (section 121)in the visual cortex of deprived marmoset 78-48. Abbreviations: IIIA, IIIB, IIIC, IV, V, VI, cortical layers; 17, 18, cortical areas 17 and 18.
m a r m o s e t such alternations are presumably larger than those found in the squirrel monkey. A last c o m m e n t concerns the actual degree o f overlap o f left and right eye inputs to cortex in New World primates. One possibility suggested by Hubel and WieseU 4 is that the apparent overlap is due to increased 'spillover' o f label is within the less well defined geniculate laminae o f New World primates. While it is likely that some fluctuation m a y be obscured by spread of label to inappropriate geniculate layers, such an explanation alone would not account for the difference observed in our normal and deprived animals since spread o f label in the geniculate nucleus did not appear to differ between cases. In conclusion, our results provide evidence for the existence of an alternating pattern o f left and right eye inputs to the striate cortex o f the m a r m o s e t : a pattern which segregates into ocular dominance like columns under deprivation conditions. These results also suggest that the presence of ocular separation o f geniculo-striate afferents is a feature c o m m o n to all primates, and the morphological differences between Old and New World species reflect only differences in the degree o f separation. Finally, we should emphasize again that in contrast to primates, other mammalian species (e.g. mouse 5 and tree shrewm2) m a y exhibit fundamental differences in the segregation o f ocular input to cortex since monocular deprivation produces no apparent changes in the degree o f overlap between ocular inputs. This research was supported by Grants EY-01778 and 1 K07 EY-00061 to V.A.C. and IT32-MH15452 to E.J.D. We gratefully acknowledge the contribution of the following individuals: A n n e Marie Gibler for help in the histology; Dr. Judy Brunso-Bechtold, Valarie Wise, Sherre Florence, Cheryl Moise, and Dr. Jon Kaas for helpful criticism o f the manuscript. Special thanks to Dr. Eric Haseltine and Elizabeth Birecree for help in all phases of the project. 1 Casagrande, V. A. and Harting, J. K., Transneuronal transport of tritiated fucose and proline in the visual pathways of the tree shrew, Tupaia glis, Brain Research, 96 (1975) 367-372. 2 Casagrande, V. A., Norton, T. T., Guillery, R. W. and Harting, J. K., Studies of binocular competition in the development of visual pathways of the tree shrew, Neurosci. Abstr., 2 (1976) 1072. 3 Casagrande, V. A., Joseph, R. and Florence, S. L., Effects of monocular deprivation on geniculostriate connections in prosimian primates, Anat. Rec., 190 (1978) 359. 4 Casagrande, V. A., Raczkowski, D. and Diamond, I. T., Effects of visual deprivation on the development of visual pathways in the galago, Neurosci. Abstr., 3 (1977) 558.
458 5 Drager, U. C., Observations on monocular deprivation in mice, J. Neurophysiol., 41 (1) (1978) 28-42. 6 Florence, S. L. and Casagrande, V. A., Effects of visual deprivation on the geniculo-striate pathways of the tree shrew, J.S.C. reed. Ass., 74 (1978) 48. 7 Florence, S. L. and Casagrande, V. A., A note on the evolution of ocular dominance columns in primates, Invest. OphthaL, Suppl. (1978) 291-292. 8 Glendening, K. K., Kofron, E. A. and Diamond, I. T., Laminar organization of projections of the lateral geniculate nucleus to the striate cortex in Galago, Brain Research, 105 (1976) 538-546. 9 H/issler, R. and Wagner, A., Experimentelle und morphologische befunde fiber die vierfache corticale projektion des visuellen systems. In Proc. 8th Int. Congr., Neurol. (Wein), Vol. III, 1965, pp. 77-96. 10 Hendrickson, A. E., Wilson, J, R. and Ogren, M. P., The neuroanatomical organization of pathways between the dorsal lateral geniculate nucleus and visual cortex in new and old world primates, J. comp. Neurol., 182 (1978) 123-136. 11 Hitchcock, P. F. and Hickey, T. L., Ocular dominance columns: evidence for their presence in humans, Brain Research, 182 (1) (1980) 176-179. 12 Hubel, D. H., An autoradiographic study of the retinocortical projections in the tree shrew (Tupaia glis), Brain Research, 96 (1975) 41-50. 13 Hubel, D. H. and Wiesel, T. N., Functional architecture of macaque monkey visual cortex, Proc. roy. Soc. B, 198 (1977) 1-59. 14 Hubel, D. H. and Wiesel, T. N., Distribution of inputs from the two eyes to striate cortex of squirrel monkeys, Neurosci. Abstr., 4 (1978) 632. 15 Hubel, D. H., Wiesel, T. N. and LeVay, S., Functional architecture of area 17 in normal and monocularly deprived macaque monkeys, CoM Spr. Harb. Symp. quant. Biol., 40 (1976) 581-589. 16 Hubel, D. H., Wiesel, T. N. and LeVay, S., Plasticity of ocular dominance columns in monkey striate cortex, Phil. Trans B, 278 (1977) 377-409. 17 Kaas, J. H., Lin, C.-S. and Casagrande, V. A., The relay of ipsilateral and contralateral retinal input from the lateral geniculate nucleus to striate cortex in the owl monkey: a transneuronal transport study, Brain Research, 106 (1976) 371-378. 18 LeVay, S., Stryker, M. P. and Shatz, C. J., Ocular dominance columns and their development in layer IV of the cat's visual cortex: a quantitative study, J. comp. NeuroL, 179 (1978) 223-244. 19 Rowe, M. H., Benevento, L. A. and Rezak, M., Some observations on the patterns of segregated geniculate inputs to the visual cortex in new world primates: an autoradiographic study, Brain Research, 159 (2) (1978) 371-378. 20 Shatz, C. J., Lindstrom, S. and Wiesel, T. N., The distribution of afferents representing the right and left eyes in the cat's visual cortex, Brain Research, 131 (1977) 103-116. 21 Skeen, L. C., Humphrey, A. L., Norton, T. T. and Hall, W. C., Deoxyglucose mapping of the orientation column system in the striate cortex of the tree shrew, Tupaia glis, Brain Research, 142 (3) (1978) 538-545. 22 Spatz, W. B., The retino-geniculo-cortical pathway in Callathrix. II. The geniculo-cortical projection, Exp. Brain Res., 36 (1979) 401-410. 23 Tigges, J. and Tigges, M., Ocular dominance columns in the striate cortex of chimpanzee (Pan troglodytes), Brain Research, 166 (1979) 386-390. 24 Tigges, J., Tigges, M. and Perachio, A. A., Complementary laminar terminations of afferents to area 17 originating in area 18 and in the lateral geniculate nucleus in squirrel monkey, J. comp. NeuroL, 176 (1977) 87-100. 25 Weber, J. T., Casagrande, V. A. and Harting, J. K., Transneuronal transport of [3H]proline within the visual system of the grey squirrel, Brain Research, 129 (1977) 346-352.
453
Demonstration of ocular dominance columns in a New World primate by means of monocular deprivation
E .J. DeBRUYN and V. A. CASAGRANDE* Departments of Anatomy and Psychology, IZanderbilt University, Nashville, Tenn. 37232 (U.S.A.)
(Accepted October 2nd, 1980) Key words: primate - - autoradiography - - cortex - - ocular dominance column - - monocular de-
privation We investigated the pattern of transneuronally transported [3I-I]prolineto the visual cortex of normal and deprived marmosets (Callithrix ]acchus). In the normal marmoset a continuous band of label was evident within layer IV of striate cortex. Following injection of the dosed eye of deprived marmosets, distinct fluctuations of label were apparent over the base of layer IV in both hemispheres. These results support the hypothesis that the presence of ocular separation of geniculo-striate afferents is a feature common to all primates.
Recent anatomical investigations into the organization of the afferent pathway from the lateral geniculate nucleus to striate cortex indicate that the pattern of termination of geniculo-cortical fibers varies considerably between species 1,5,11,13,17,2°,22,23, 25. In primates, differences have been discovered which apparently reflect the evolutionary separation of Old and New World species. Thus, in Old World primates, the afferents representing the input from each eye are segregated into vertical bands or ocular dominance columns (ODCs) in cortex4,S,lOA1,13,15, 23, while in 4 species of the New World Cebid familylO,15,17, 24 and in the marmoset 2z these afferents overlap. Based on the results of these morphological studies, it seems reasonable to assume that there exists a basic difference in the organization of ocular input to striate cortex in New vs Old World primates. This assumption, however, is complicated by recent evidence indicating the presence of well defined ODCs in the spider monkey 7, and hints of ocular segregation of inputs in the owl 19 and squirrel monkeys 14. The results of the last two investigations14,19, which conflict with earlier reports on the owl 17 and squirrel monkeylO,XS, 24, have led us to believe that at least part of the discrepancy between the studies of Old and New World primates may lie in differences in the degree of segregation rather than in differences in the basic organizational patterns of the two groups. Our reasoning is as follows: in the weakly segregated system which appears to be present in the squirrel monkey, the terminal arbors of geniculocortical axons representing one eye may overlap such that transported label from two * To whom correspondence should be addressed.
454 columns may meet and completely obscure the intervening ODCs of opposite ocularity; thus, the impression is one of a continuous pattern of termination (Fig. 1A). Therefore, if the amount of cortical space occupied by terminals representing each eye could be made unequal, it might be possible to demonstrate the columnar nature of these inputs. In light of recent evidence3,4,6,15,16, which suggests that monocular lid suture reduces the size of deprived geniculo-cortical terminal arbors while increasing the size of non-deprived arbors, we chose to investigate the effects of lid suture on the geniculo-cortical organization of a New World species which normally exhibits an overlapping pattern of inputs, the common marmoset (Callathrixjacchus). For this investigation, two juvenile marmosets (78-47 and 78-48) raised with a monocular suture from birth to 5 months of age received injections of 2.5 mCi of [3H]proline (spec. act. 20 Ci/mmol) into the vitreous of their deprived eyes. In addition, 2 normal adult marmosets (78-36 and 79-4) received monocular injections of 5.0 and 2.5 mCi of [3H]proline respectively (spec. act. 100 and 23.7 Ci/mmol). Following a survival period of two weeks, 3 of the animals (78-36, 78-47, and 79-4) were perfused through the heart with 0.9 ~o saline followed by 10 ~ formol saline. The brains were blocked in either the frontal plane(78-47, 79-4) or in a plane tangential to the pial surface of striate cortex (78-36) and stored at 4 °C in 10~o formol saline to which 3 0 ~ sucrose had been added until they sank. They were then sectioned frozen at 30 #m and one section out of every five was mounted on chrome alum gelatinized slides and processed by standard autoradiographic procedures. Following exposure times of 6-12 weeks, the sections were developed and lightly counterstained with cresyl violet. In addition to the eye injection, the fourth animal (78-48) received an intravenous injection of 40 #Ci of [14C]2-deoxyglucose (spec. act. 337 Ci/mmol) on the day of sacrifice. Following visual stimulation of the deprived eye and processing by previously described methods 21, the sections were washed to remove the deoxyglucose and processed as in the other cases. The results of our normal cases confirm the observation of Spatz 22 that the transported label forms a continuous band within layer IV* of area 17 in both hemispheres with a second, faint band over layer IIIB (Fig. 2A and B). Unlike his findings, however, we did not observe a difference in the density of label in the central as opposed to peripheral representation of the visual field. It is possible, however, that the larger quantities of tracer used in the present study obscured these differences. In contrast to the results in normal marmosets, the pattern of transport in our deprived animals showed distinct 200-300 tzm wide fluctuations in the density of label over the base of layer IV of area 17 (Fig. 2C-G), a pattern of segregation which is strikingly similar to the ODCs found in Old World primates, and particularly to the weaker segregation found in the prosimian Galago4,8,1~. These variations were slightly more prominent in the hemisphere ipsilateral to the deprived (injected) eye, and were not apparent in the upper tier of layer IV, where the label formed a continuous band.
* Our investigations of the cortical organization of the marmoset have led us to believe that the numbering system of H~isslerand Wagner9 provides the best description of the laminar organization of area 17 in this animal. Thus, this scheme is used in the present report.
455
A
cx
B
IV
IPSI CONTRA LGN
Fig. 1. A schematic diagram illustrating the hypothesized pattern of geniculo-striate input in the marmoset and the proposed mechanism for the production of columns in the deprived animal based on the results of the present study. A represents the condition found in the normal animal. The terminal arbors of geniculo-striate axons from both contralaterally- and ipsilaterally-innervated geniculate laminae overlap such that each set totally obscures its counterpart, thus giving the impression of a total overlap of input. In the deprived case (B), the arbors representing the deprived (ipsilateral) eye are reduced in size, and therefore form the typical columnar pattern reminiscent of that seen in Old World primates. For the sake of argument we have pictured the open eye (contralateral) afferents as correspondingly larger, although we do not have results concerning the actual distribution of label following an open eye injection. Abbreviations: CONTRA, contralaterally-innervated geniculate laminae; CX, cortex; IPSI, ipsilaterally-innervated geniculate laminae; LGN, lateral geniculate nucleus; IV, cortical layer IV.
Additionally, the faint banding in layer IIIB, which forms a 'honeycomb' pattern as has been described in other primates 10, was also present. Finally, the patches or columns in layer IV were more distinct in case 78-48 than in case 78-47, a difference which may be attributed to the fact that the suture in the latter case was incomplete, and thus, the effects of deprivation were not as strong. We believe that the segregated pattern of input seen in the deprived animals could only be produced from a continuous pattern if ocular input from the two eyes of the marmoset normally alternates as pictured in Fig. 1A. It is unlikely that these columns are artifacts produced by higher order effects of deprivation, since monocular lid suture does not produce changes in the pattern of label in the mouse 5 or in the tree shrew 2, species which also show a normal pattern of overlapping ocular input to striate cortex. Moreover, Hubel and Wiesel's recent demonstrations that: (1) weak variation in label density occurs predominantly in one sublayer of squirrel monkey visual cortex (layer IVca); and (2) fluctuations in ocular dominance occur with periodicity of 250 /zm in this species are consistent with the restricted vertical extent and narrow width of the columns produced by lid suture in the marmoset. We should stress, however, that differences do exist between the two species, as the fluctuations in label in the marmoset occur in the base of layer IV, whereas those in the squirrel monkey appear in the equivalent of the upper tier of this layer. Moreover, the columns exhibited by the deprived marmosets are presumably smaller than the 'hidden' hypothetical alternations of left and right eyes inputs of a normal marmoset (Fig. 1B). Thus, in the normal
CC~
~ll
o~
qn
i
J~
0
oD v
457 Fig. 2. The pattern of geniculo-striate input in the marmoset. A and B are dark-field autoradiographs of normal marmoset (78-36) visual cortex ipsilateral and contralateral to the injected eye, respectively. Note the uniform density of label over layer iV and the fainter label over layer IIIB. Compare this pattern with that in C-G. C, D, E and F are dark-field autoradiographs (sections 41, 71, 41 and 69, respectively) of the visual cortex of deprived marmoset 78-48 ipsilateral (C and E) and contralateral (D and F) to the injected (deprived) eye. Distinct patches of increased label density, 200-300 #m wide, are evident at the base of layer IV. Label in the upper part of layer IV and in layer IIIB is similar to the pattern seen in normal cases. G is a series of line drawings showing the distribution of columns from caudal (section 50) to rostral (section 121)in the visual cortex of deprived marmoset 78-48. Abbreviations: IIIA, IIIB, IIIC, IV, V, VI, cortical layers; 17, 18, cortical areas 17 and 18.
m a r m o s e t such alternations are presumably larger than those found in the squirrel monkey. A last c o m m e n t concerns the actual degree o f overlap o f left and right eye inputs to cortex in New World primates. One possibility suggested by Hubel and WieseU 4 is that the apparent overlap is due to increased 'spillover' o f label is within the less well defined geniculate laminae o f New World primates. While it is likely that some fluctuation m a y be obscured by spread of label to inappropriate geniculate layers, such an explanation alone would not account for the difference observed in our normal and deprived animals since spread o f label in the geniculate nucleus did not appear to differ between cases. In conclusion, our results provide evidence for the existence of an alternating pattern o f left and right eye inputs to the striate cortex o f the m a r m o s e t : a pattern which segregates into ocular dominance like columns under deprivation conditions. These results also suggest that the presence of ocular separation o f geniculo-striate afferents is a feature c o m m o n to all primates, and the morphological differences between Old and New World species reflect only differences in the degree o f separation. Finally, we should emphasize again that in contrast to primates, other mammalian species (e.g. mouse 5 and tree shrewm2) m a y exhibit fundamental differences in the segregation o f ocular input to cortex since monocular deprivation produces no apparent changes in the degree o f overlap between ocular inputs. This research was supported by Grants EY-01778 and 1 K07 EY-00061 to V.A.C. and IT32-MH15452 to E.J.D. We gratefully acknowledge the contribution of the following individuals: A n n e Marie Gibler for help in the histology; Dr. Judy Brunso-Bechtold, Valarie Wise, Sherre Florence, Cheryl Moise, and Dr. Jon Kaas for helpful criticism o f the manuscript. Special thanks to Dr. Eric Haseltine and Elizabeth Birecree for help in all phases of the project. 1 Casagrande, V. A. and Harting, J. K., Transneuronal transport of tritiated fucose and proline in the visual pathways of the tree shrew, Tupaia glis, Brain Research, 96 (1975) 367-372. 2 Casagrande, V. A., Norton, T. T., Guillery, R. W. and Harting, J. K., Studies of binocular competition in the development of visual pathways of the tree shrew, Neurosci. Abstr., 2 (1976) 1072. 3 Casagrande, V. A., Joseph, R. and Florence, S. L., Effects of monocular deprivation on geniculostriate connections in prosimian primates, Anat. Rec., 190 (1978) 359. 4 Casagrande, V. A., Raczkowski, D. and Diamond, I. T., Effects of visual deprivation on the development of visual pathways in the galago, Neurosci. Abstr., 3 (1977) 558.
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