DISTRIBUTION, MORPHOLOGY, AND GAMMA-AMINOBUTYRIC-ACID IMMUNOREACTIVITY OF HORIZONTALLY PROJECTING NEURONS IN THE MACAQUE INFERIOR TEMPORAL CORTEX

In al ea TE of the macaque inferior temporal cortex, horizontal axons running parallel to the pial surface mediate interactions between late rally displaced sites across the cortex. We examined the spatial distr ibution and the types of cells that give rise to these horizontal axon s, which are important factors in determining the nature of the latera l interactions in TE. Intracortical injections of retrograde tracers l abeled columnar clusters of cells and cells diffusely scattered within TE. The clusters were 0.35 +/- 0.11 mm (mean +/- SD) in diameter and were laterally distributed up to 6 mm from the injection site. Labeled cells were found in layers 2 to 6, with only a few labeled cells seen in layer 4. The clustering of labeled cells in layers 5 and 6 was loo ser than that in layers 2 and 3. Intracellular staining of the retrogr adely labeled cells revealed that the majority of them were typical or modified pyramidal cells, both within and between the clusters. Only a few nonpyramidal interneurons were also stained at the fringe of the tracer injection site. Consistent with these results, only a small pr oportion of the retrogradely labeled cells exhibited gamma-aminobutyri c acid (GABA)-like immunoreactivity, mostly found within I mm from the injection site. The results indicate that direct horizontal interacti ons in TE are predominantly mediated by pyramidal or modified pyramida l cells in layers 2, 3, 5, and 6 and are primarily excitatory in natur e. The contribution of GABAergic interneurons to direct horizontal int eractions is restricted to only short-distance projections. (C) 1998 W iley-Liss, Inc.