MODULAR ORGANIZATION OF OCCIPITOTEMPORAL PATHWAYS - CORTICAL CONNECTIONS BETWEEN VISUAL AREA-4 AND VISUAL AREA-2 AND POSTERIOR INFEROTEMPORAL VENTRAL AREA IN MACAQUE MONKEYS

The modular organization of cortical pathways linking visual area 4 (V 4) with occipital visual area 2 (V2) and inferotemporal posterior infe rotemporal ventral area (PITv) was investigated through an analysis of the patterns of retrogradely labeled cell bodies after injections of tracers into V4 and PITv. Although cytochrome oxidase or other stains have failed to yield reliable independent anatomical markers for corti cal modules beyond V1 and V2, V4 and PITv seem to have modular compart ments with specific patterns of cortico-cortical connectivity. Tracer injections of V4 labeled cells in V2 (1) thin stripes exclusively, (2) interstripes exclusively, or (3) specific combinations of interstripe and thin stripe subcompartments. These labeling patterns suggest (1) that there is a complicated organization of inputs to V4, (2) that pro jections from V2 to V4 display a submodular selectivity, and (3) that projections from V2 to V4 display some degree of cross-stream converge nce. Consistent with this framework, extensive regions of PITv provide feedback projections to interstripe-recipient portions of V4, whereas more restricted portions of PITv provide feedback to thin stripe-reci pient portions of V4. Similarly, the feedforward projection from V4 to PITv often arose from multiple cell clusters across a wide expanse of V4. When distinguishable fluorescent tracers were injected into two P ITv sites separated by 3-5 mm, a variety of projection patterns was ob served in V4. In most cases, labeled cells were found in multiple, int erdigitating, nonoverlapping clusters of 1-3 mm width, whereas in othe r cases the two labeled fields were highly intermixed. These results s uggest that V4 and PITv contain functional modules that can be charact erized by the specific patterns of segregated and convergent projectio ns they receive from lower cortical areas. These specific patterns of intercortical input, in conjunction with intrinsic cortical circuitry, may endow extrastriate cortical neurons with new and more complex rec eptive field properties.