Intrinsic cortical connections in macaque visual area V2: evidence for interaction between different functional streams.

Area V2 of macaque visual cortex represents an important but poorly understood stage in visual processing. To provide a better understanding of the region, we studied the organization of its intrinsic cortical connections by making focal (200-300 microns) iontophoretic microinjections of the tracer biocytin. Alternate tissue sections were tested for biocytin, cytochrome oxidase (CO), or Cat-301 immunoreactivity to localize biocytin label relative to the three stripelike compartments that characterize this area. Biocytin-labeled pyramidal neurons of layers 2/3, and, to a lesser extent, layer 5, provided laterally spreading axon projections that terminated in discrete patches (250-300 microns diameter), primarily in layers 1-3. Any injected locus in V2 projected to 10-15 similarly sized patches, up to 4 mm from the injection site, and distributed in an elongated field orthogonal to the stripe compartments. We noted prominent patchy connections within, as well as between, individual compartments, perhaps reflecting functional substructures within stripes. Each stripe compartment projected to all three compartments but with different relative frequencies; CO-rich compartments projected mainly to other CO-rich compartments (75%), whereas CO-poor compartments projected equally to CO-rich and CO-poor compartments. We therefore emphasize the existence of substantial interconnections among all three V2 compartments. As further evidence for crosstalk between visual channels, we also noted an input to the V2 "thick" CO stripes from V1 cells in layer 4A as a distinct population in addition to the neurons of layer 4B. Thus, the CO stripe architecture may not be a marker for strictly segregated parallel visual pathways through V2.