Neurons in V1 patch columns project to V2 thin stripes

In the primate, connections between primary visual cortex (V1) and the second visual area (V2) are segregated according to the characteristic pattern of cytochrome oxidase (CO) activity in each of these cortical areas. Patches supply thin stripes, whereas interpatches supply pale stripes and thick stripes. Previously, the projection from patches to thin stripes was reported to arise exclusively from layer 2/3. In this present report, we made injections of a retrograde tracer, cholera toxin-B (CTB-Au), into macaque V2 thin stripes to re-examine the laminar origin of their input from V1. While the great majority of cells indeed resided in layer 2/3, small populations were also present in layers 4A, 4B, and 5/6. The location of CTB-filled cells in each layer was analyzed to determine the relationship with CO patches. Cells in layers 2/3, 4A, and 4B were aggregated into patches, forming columns that project to thin stripes. Surprisingly, cells in layer 5/6 were scattered, seemingly at random. These findings confirm that the main V1 projection to V2 stripes emanates from patches in layer 2/3. However, multiple V1 layers innervate V2 thin stripes, and the projection from layer 5/6 does not respect the patch/interpatch dichotomy.