A very interesting aspect of hippocampal anatomy is the presence of two pathways projecting from the entorhinal cortex (EC) to the CA3 region — one directly via the perforant path (PP), and the other through the dentate gyms (DG) using the mossy fibers of the granule cells. This implies that the place fields of the CA3 arise from the joint influence of EC and DG. We hypothesize that the DG plays a modulatory role in this scheme, serving to enhance discrimination during the learning of new place codes. Drawing in part on some receny experimental findings, we model a mechanism whereby DG neurons accomplish pattern separation by modulating the balance of dendritic and somatic inhibition in granule cells. Our results are consistent with a variety of observations in the literature, including the following: 1) DG lesions do not abolish CA3 place fields but disrupt spatial memory; 2) Even similar environments produce different place fields in CA3 but not in the EC. We show that DG modulation allows the model hippocampus to control spatial discrimination, and produces realistic place fields.
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