Segregation and convergence of specialised pathways in macaque monkey visual cortex.

At the level of cortical area V2, the various visual inputs to the cortex have reorganised to form 3 distinct channels. Anatomically these are embodied in the thick and thin dark stripes, and paler interstripes characteristic of cytochrome oxidase architecture. Do the outputs of these compartments remain segregated at higher levels of processing, or are they in turn combined and repackaged? To examine this question we have injected distinct orthograde tracers into the functionally distinct areas V4 and V5 of one hemisphere in 3 macaque monkeys (Macaca fascicularis). V4 is known to receive input from both thin stripes and interstripes of V2, but some parts of V4 receive only interstripe afferents, others receive a relatively greater contribution from the thin stripes. Thus V4 itself is thought to possess subcompartments of at least two distinct types, acting to extend the blob-thin stripe and interblob-interstripe pathways through V1 and V2. The experiments reported here reveal no further divergence between these channels: both types of V4 subcompartment make rather similar patterns of connection with further visual areas and subcortical structures. In contrast to V4, area V5 receives input from the thick stripes of V2. V4 and V5 are weakly interconnected, at best, and there is limited direct convergence in their two sets of ascending connections. For instance, both areas send output to area LIP; but V4 targets the dorsal half of the area, and V5 the ventral half, with some minor overlap. Projections to the superior temporal sulcus are also mainly separate, although we found instances of direct convergence in areas FST and possibly V4t. Segregation is also the rule for subcortical connections to the pulvinar from these two areas. In summary, the segregated outputs of V2 can remain largely distinct through at least two subsequent stages of cortical processing.