Intrinsic connections in the macaque inferior temporal cortex

Intrinsic connections in the inferior temporal cortex were analyzed by making extracellular injections of biocytin in Japanese macaques. Analysis was focused mainly on the dorsal part of area TE, in which a functional columnar organization has been shown. Interlaminar connections were analyzed in coronal section after laminar-specific microinjections, and intralaminar connections were examined from tangential sections. After injections at various depths in the dorsal TE, both axons and cell bodies were strongly labeled above or below the injection site in a columnar appearance. Axons from layer 3 ran in bundles towards the white matter and gave off prominent collaterals in layer 5. Ascending axons from lower to upper layers were also present (e.g., layers 4, 5, and 6 to layer 3). In tangential sections, there were abundant axons running parallel to the pia mater. These horizontal axons, particularly those in layers 2 and 3, produced patches of terminals 0.5 +/- 0.1 mm (mean +/- s.d.) in size and cylindrical in shape, spanning layers 1-3 or even to layers 4 and 5. In the tangential plane, they were distributed in an anisotropic manner around the injection. The farthest patch appeared at 4 mm from the injection site. The center-to-center distance between nearest-neighbor patches was 0.7 +/- 0.3 mm. These patches were found only within the dorsal TE and did not extend into the lower bank of the superior temporal sulcus or into the ventral part of area TE. Area TEO, which is a major afferent source to area TE, had axonal patches with spacing similar to those in area TE but with smaller sizes (0.4 +/- 0.1 mm). The results show that intrinsic horizontal axons both in area TE and in area TEO arborize in a patchy manner, as has been reported for several other cortical areas. In are TE, the size and spacing of the terminal patches match those of columns with similar stimulus selectivity. Thus, these patches may be related to the functional modularity in area TE. Vertical connections across layers and cylindrical patches of horizontal axons most likely contribute to the shared stimulus selectivity among cells within a column.