Evidence from V1 Connections for Both Dorsal and Ventral Subdivisions of V3 in Three Species of New World Monkeys

We used patterns of connections of primary visual cortex (V1) to reevaluate differing proposals on the organization of extrastriate cortex in three species of New World monkeys. Several fluorescent tracers and the bidirectional tracer cholera toxin B subunit (CTB) were injected into dorsal V1 (representing the lower visual quadrant) and ventral V1 (representing the upper visual quadrant) of titi, squirrel, and owl monkeys. Labeled cells and terminals were plotted on brain sections cut parallel to the surface of flattened cortex and were related to architectonic boundaries. The results provided compelling evidence for both dorsal V3 with dorsal V1 connections and ventral V3 with ventral V1 connections. The connection pattern indicated that V3 represents the visual hemifield as a mirror image of V2. In addition, V3 could be recognized by a weak banding pattern in brain sections processed for cytochrome oxidase. V1 has connections with at least 12 subdivisions of visual cortex, with half of the connections involving V2 and 20% V3. Comparable results were obtained from all three species, suggesting that visual cortex is similarly organized.