Entorhinal cortex of the monkey: IV. Topographical and laminar organization of cortical afferents.

The nonhuman primate entorhinal cortex is the primary interface for information flow between the neocortex and the hippocampal formation. Based on previous retrograde tracer studies, neocortical afferents to the macaque monkey entorhinal cortex originate largely in polysensory cortical association areas. However, the topographical and laminar distributions of cortical inputs to the entorhinal cortex have not yet been comprehensively described. The present study examines the regional and laminar termination of projections within the entorhinal cortex arising from different cortical areas. The study is based on a library of 51 (3)H-amino acid injections that involve most of the afferent regions of the entorhinal cortex. The range of termination patterns was broad. Some areas, such as the medial portion of orbitofrontal area 13 and parahippocampal areas TF and TH, project widely within the entorhinal cortex. Other areas have a more focal and regionally selective termination. The lateral orbitofrontal, insular, anterior cingulate, and perirhinal cortices, for example, project only to rostral levels of the entorhinal cortex. The upper bank of the superior temporal sulcus projects mainly to intermediate levels of the entorhinal cortex, and the parietal and retrosplenial cortices project to caudal levels. The projections from some of these cortical regions preferentially terminate in the superficial layers (I-III) of the entorhinal cortex, whereas others project more heavily to the deep layers (V-VI). Thus, some of the cortical inputs may be more influential on the cortically directed outputs of the hippocampal formation or on gating neocortical information flow into the other fields of the hippocampal formation rather than contributing to the perforant path inputs to other hippocampal fields. Copyright 2008 Wiley-Liss, Inc.