Corticocortical projections to the monkey temporal lobe with particular reference to the visual processing pathways.

Cortico-cortical connections occurring within the temporal lobe and afferent projections to the temporal cortex particularly from the prefrontal and parahippocampal areas were studied in the monkey by means of retrograde axonal transport of horseradish peroxidase (HRP) or wheat-germ-agglutinin-conjugated HRP (WGA-HRP). In particular, 0.1-0.3 microliter of 50% HRP or 5% WGA-HRP was injected into various parts of the temporal cortex, i.e. the rostral (TEr), the caudal (TEc), and the most caudal (TEO) parts of the inferotemporal cortex, the superior temporal gyrus, and the temporal pole (TG), and in the upper bank of the inferior arcuate sulcus in the frontal lobe. Labeled cells, which represent cells of origin of association fibers projecting to the injection site, appeared in various cortical regions. The main findings of the present study are the following. The temporal pole (TG) receives fibers almost exclusively from the most rostral part of the TE. The rostral part of the TE receives many fibers from both the caudal part of the TE and the TEO. The caudal part of the TE receives fibers from the TEO, and the TEO from the prestriate cortex (OA and OB). Taking these findings together, the morphological basis of the "step-wise" progression of visual impulses from the prestriate cortex to the TEO, TE and finally to the TG is clearly presented. The superior temporal gyrus (TA or area 22) receives most fibers from the dorsolateral frontal gyrus, while the inferotemporal cortex (TE or areas 21 and 20) receives most fibers from the ventrolateral frontal gyrus (inferior frontal convexity). Both the temporal pole (TG) and the inferotemporal cortex (TE) receives a fair number of fibers from the parahippocampal region (TH and TF).