Divergent backward projections from the anterior part of the inferotemporal cortex (area TE) in the macaque

Abstract

The organization of backward projections from the anterior part of the inferotemporal cortex (area TE) to the posterior part of the inferotemporal cortex (area TEO) was studied in the macaque monkey by using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). The objectives of the study were to investigate this backward projection and to compare it with 1) the backward projections that have been described previously in the early sensory areas and 2) the forward projection from area TEO to area TE. After a single iontophoretic injection of PHA-L into area TE in three monkeys, a dense distribution of labeled terminals was observed in area TEO and in the ventral bank of the superior temporal sulcus (area PITd) that adjoined area TEO. A less dense distribution was observed in areas V4, V2, and V1. Clusters of labeled terminals in areas TEO and PITd extended more than 4 mm along the cortical surface. The forward projections from area TEO to area TE also were studied for comparison by reanalyzing two previous cases (Saleem et al. ¿1993 Cerebral Cortex 3:454-464). These projections (from area TEO to area TE) were more focal than the terminations that occurred in area TEO after injections into area TE. Nine single axons projecting from area TE to areas TEO/PITd were reconstructed through serial sections. These showed variable, complex branching patterns with multiple arbors (1-12). Arbors were localized in layers 1-3 for four axons, in layer 1 for one axon, layers 5 and 6 for two axons, and in both layers 1-3 and layers 5-6 for two axons. Axons with horizontally elongated arbors confined to layer 1 were not predominant. The size of the individual arbors of these axons along their long axes tended to be larger (1.56 +/- 1.24 mm) than those of TEO-to-TE forward axons (<0.6 mm). Thus, the authors conclude that, like other backward systems described to date, those from area TE to areas TEO/PITd are divergent. However, single axons have more variable laminar patterns of terminal distribution than those in the other backward systems.