Laminar distribution of neurons projecting from area V1 to V2 in macaque and squirrel monkeys.

The present study reevaluates the sublaminar distribution and cellular morphology of neurons projecting from area V1 to V2. Observations are based on retrogradely transported HRP, Phaseolus vulgaris leucoagglutinin (PHA-L), or biocytin after injections made in area V2 of three squirrel monkeys and eight macaques. With material prepared in the coronal or horizontal tissue planes, it is clear that projection neurons in V1, in both species, are concentrated in layer 4B and in a single band (150-250 microns wide) restricted to the upper subdivision of layer 3 (layer 3A). There are also labeled neurons, but fewer in number, in layers 3B and 4A, and occasionally in layers 2 and 5. Golgi-like labeling from PHA-L or biocytin confirmed that most of the projection neurons in layer 3A are pyramidal. As reported for several other corticocortical systems, these pyramidal neurons differ in soma size, soma shape, and dendritic geometry. These results emphasize the complex organization of layer 3, and the distributed nature of efferent projections from area V1. Given the selective connectivity of vertical interlaminar networks, these results specifically suggest that information transmitted to area V2 from neurons in layer 3A reflects more highly processed, convergent input than that originating from either layer 3B or 4B.