The organization of geniculostriate projections in Tupaia was studied using three separate methods, anterograde transport from the lateral geniculate, retrograde transport from the striate cortex, and reconstruction of single geniculostriate axons. The results show that each layer of the lateral geniculate body has a unique pattern of projections to the striate cortex, and each pattern consists of a major and a minor target. The two ipsilateral layers project to thin subtiers of layer IV: the major target of geniculate layer 1 is the top of IVa; the major target of geniculate layer 5 is the base of IVb. The minor target of layer 1 is the major target of layer 5. Two of the contralateral layers can be matched to the ipsilateral layers. Layers 1 and 2 are a matched pair and project to IVa; layers 4 and 5 are a matched pair and project to IVb. Thus, projections of a matched pair overlap. The remaining two contralateral layers, 3 and 6, project chiefly to cortical layer III. Layer 3 projects to layers IIIb and I and seems to be the counterpart of the parvocellular C layers in the cat and the intercalated layers in primates. Layer 6 projects to the base of IIIc in a zone contiguous with IVa. This contiguity raises the issue of whether the base of IIIc might actually be a part of layer IV. If this were the case, the two tiers of layer IV which are separated by a conspicuous cleft might be considered two subdivisions of layer IVb.
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