Cortical efferents of the entorhinal cortex and the adjacent parahippocampal region in the monkey (Macaca fascicularis)

Entorhinal cortex (EC) relays information from the hippocampus to the cerebral cortex. The origin of this entorhino-cortical pathway was studied semiquantitatively and topographically with the use of 23 retrograde tracer injections in cortical areas of the frontal, temporal, and parietal lobes of the monkey. To assess possible alternative, parallel pathways, the parahippocampal region, comprised of temporal pole (TP), perirhinal (PRC), and posterior parahippocampal cortices (PPH), was included in the study. The majority of the cortical areas receive convergent projections from EC and the parahippocampal region. Strong EC layer V output is directed to temporal pole, medial frontal and orbitofrontal cortices, and the rostral part of the polysensory area of the superior temporal sulcus (sts). Moderate EC output is directed to the caudal superior temporal gyrus, area TE, and parietal cortex, and little to none to the lateral frontal cortex. With the exception of the projection to the medial frontal cortex, output from TP, PRC, and PPH surpassed that from EC, although with regional differences. TP layers II-III, V-VI project strongly to all areas injected except parietal cortex and caudal superior temporal gyrus, while PRC layers III/V-VI send strong projections to rostral parts of area TE and sts. PPH layers III/V-VI project heavily to parietal cortex and caudal superior temporal gyrus. These results suggest that the medial temporal output is primarily organized hierarchically, but at the same time, it has multiple exits of information. These parallel, alternative routes may influence local circuitry in the cerebral cortex and participate in the consolidation of declarative memory.