The projection from v1 to extrastriate area 21a: A second patchy efferent pathway that colocalizes with the CO blob columns in cat visual cortex

The different patchy organizations of neurons projecting from primary visua l cortex (area 17) to the various extrastriate areas may contribute to func tional differences in the output to each of these areas. The pattern of neu rons projecting to extrastriate area 21a was examined using large injection s of retrograde tracers and compared to the pattern shown by neurons projec ting to the lateral suprasylvian area (LS). Patches of neurons projecting t o 21 a showed a bimodal laminar distribution, with numerous labeled cells i n the upper and lower third of layer 3 bracketing a sparsely labeled centra l third; LS-projecting neurons were confined to the lower and middle thirds of layer 3. The 21a projecting cells were relatively tighter in their clus tering pattern than the LS projecting cells, i.e. the difference in labelin g density between patch and interpatch zones was greater for 21a-projecting cells than for LS-projecting cells. As previously shown far the LS-project ing cells, patches of 21a-projecting cells colocalized with CO blob columns in area 17. Combined with our earlier results, this study shows that the C O 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 sugges t that each pathway may carry unique information to its extrastriate target .