Intrinsic lattice connections of macaque monkey visual cortical area V4.

We made focal iontophoretic as well as larger pressure injections (n = 30; 19 used for most analyses) of the tracer biocytin in visual area V4 of six macaque monkeys. The resulting transported label enabled mapping of intrinsic inter- and intralaminar connections within the region. We found that pyramidal neurons of layers 2 and 3 make extensive lateral projections within area V4, with oval or circular patches of terminals in layers 1-3. Any small patch of tissue (approximately 250 microns wide) injected in the superficial layers appeared to connect reciprocally to patches scattered up to 3 mm around the injection. The patches of terminal label measure 250-450 microns across, spaced roughly 600 microns (range, 450-1300 microns) center to center, and where most densely packed they occupy 33% of the cortical area with approximately 3-4 patches/mm2. Small injections in layers 4 and 6 did not produce contributions to these patchlike lattice connections, while injections in layer 5 gave very weak rising contributions to the superficial layer patch system. Large pressure injections of biocytin gave wider spread and more densely labeled patches, but their size and spacing appeared much the same as with small injections. The V4 pattern of label was compared to the patterns seen in areas V1 and V2 after similar-sized injections of biocytin; patches in V1 and V2 were slightly smaller (roughly 250-300 microns across) and more closely spaced (425-450 microns center to center), consistent with earlier measures in these areas using HRP. The peak areal occupancy of patches was approximately the same, 27-33% in each region. We interpret these findings as indicating a functional repeat distance of 450-600 microns in area V4 (fixed tissue measures) with a patchy, discontinuous layout. Given the dimensions of the V4 intrinsic connectional system demonstrated here, it seems unlikely that it relates directly to the topography of specific afferent terminations or efferent neuron groups, which appear from other studies to have a larger scale of repeat (2-3 mm). However, patches in V4 tend to aggregate, forming clusters extended in the mediolateral direction, suggesting a possible relation to coarser connection zones.