COMMENTARY Functional Neuroanatomy of the Parahippocampal Region: The Lateral and Medial Entorhinal Areas

The entorhinal cortex (EC) serves a pivotal role in corti- cohippocampal interactions, but a complete description of its extrinsic connections has not been presented. Here, we have summarized the cortical, subcortical, and hippocampal connections of the lateral ento- rhinal area (LEA) and the medial entorhinal area (MEA) in the rat. We found that the targets and relative strengths of the entorhinal connec- tions are strikingly different for the LEA and MEA. For example, the LEA receives considerably heavier input from the piriform and insular corti- ces, whereas the MEA is more heavily targeted by the visual, posterior parietal, and retrosplenial cortices. Regarding subcortical connections, the LEA receives heavy input from the amygdala and olfactory struc- tures, whereas the MEA is targeted by the dorsal thalamus, primarily the midline nuclei and also the dorsolateral and dorsoanterior thalamic nuclei. Differences in the LEA and MEA connections with hippocampal and parahippocampal structures are also described. In addition, because the EC is characterized by bands of intrinsic connectivity that span the LEA and MEA and project to different septotemporal levels of the den- tate gyrus, special attention was paid to the efferents and afferents of those bands. Finally, we summarized the connections of the dorsocaudal MEA, the region in which the entorhinal ''grid cells'' were discovered. The subregional differences in entorhinal connectivity described here provide further evidence for functional diversity within the EC. It is hoped that these findings will inform future studies of the role of the EC in learning and memory. V C 2007 Wiley-Liss, Inc.