Functional-Specific Projections from V2 to V4 in Macaque Monkeys

Previous studies have revealed modular projections from area V2 to area V4 in macaques. Specifically, V2 neurons in cytochrome oxidase (CO)-rich thin and CO-sparse pale stripes project to distinct regions in V4. However, how these modular projections relate to the functional subcompartments of V4 remains unclear. In this study, we injected retrograde fluorescent tracers into different functional domains (color, orientation, and direction) in V4 that were identified with intrinsic signal optical imaging. We examined the labeled neurons in area V2 and their locations with respect to the CO patterns. Covariation was observed between the functional properties of the V4 injection sites and the numbers of labeled neurons in particular CO stripes. This covariation indicates that the color domains in V4 mainly received inputs from V2 thin stripes, whereas V4 orientation domains received inputs from pale (major) and thick (minor) stripes. Although there are motion-sensitive domains in both V2 and V4, our results did not reveal a functional-specific feedforward projection between them. These results confirmed previous findings of modular projections from V2 to V4 and provided functional specificity for these anatomical projections. Together, these findings indicate that color and form remain separate in the ventral midlevel visual processing.