The projection from V1 to extrastriate area 21a: a second patchy efferent pathway colocalizes with the CO blob...

The different patchy organizations of neurons projecting from primary visual cortex (area 17) to the various extrastriate areas may contribute to functional differences in the output to each of these areas. The pattern of neurons projecting to extrastriate area 21a was examined using large injections of retrograde tracers and compared to the pattern shown by neurons projecting to the lateral suprasylvian area (LS). Patches of neurons projecting to 21a showed a bimodal laminar distribution, with numerous labeled cells in the upper and lower third of layer 3 bracketing a sparsely labeled central third; LS-projecting neurons were confined to the lower and middle thirds of layer 3. The 21a projecting cells were relatively tighter in their clustering pattern than the LS projecting cells, i. e. the difference in labeling density between patch and interpatch zones was greater for 21a-projecting cells than for LS-projecting cells. As previously shown for the LS-projecting cells, patches of 21a-projecting cells colocalized with CO blob columns in area 17. Combined with our earlier results, this study shows that the CO blob compartments in area 17 give rise to at least two distinct efferent pathways, one projecting to LS and the other to 21a, and furthermore suggest that each pathway may carry unique information to its extrastriate target.