The hippocampus is a popular site for electrophysiology research because most of its primary cell types are separated into easily recognizable regions, and the axonal fiber tracts connecting these regions are relatively simple and well defined, as illustrated in Fig. 1. In the dentate gyrus the principal cell type is the granule cell, and the majority of granule cell input comes from a fiber tract called the perforant path. The perforant path contains axons that originate in the entorhinal cortex and provide a major sensory input to the hippocampus. The perforant path splits into two anatomically distinct pathways, called the …