Memories: Molecules and Circuits

Abstract

The ability to find one�_�s way depends on the brain�_�s ability to integrate information about location, direction and distance. The algorithms responsible for this integration are implemented in a large brain network that includes both hippocampal and parahip-pocampal cortices, as indicated by the existence of place cells in the hippocampus and head-direction cells in the dorsal presubiculum and a number of other regions. Recent results have pointed to the medial entorhinal cortex (MEC) as a possible site for the dynamic representation of position in animals that move through a two-dimensional environment. Layer II of the MEC contains position-sensitive neurons - grid cells - whose firing fields form a periodic triangular pattern that tiles the entire environment covered by the animal during exploration of an open surface. Grid cells are observed in all principal layers of MEC, but intermingle with head direction cells in layers III, V and VI. The two cell types form a continuous population in which a subset of the neurons, predominantly in layers III and V, have conjunctive grid and head-direction properties. The majority of the cells are modulated by velocity. These observations suggest that, despite the differential hippocampal and neocortical connections of different layers of the MEC, the area operates as an integrated unit, with significant interlaminar interaction between cells with different functional properties. As the animal moves across a surface, activity may be translated across the superficial sheet of grid cells by convergence of position, direction and velocity information in a neural subpopulation with conjunctive firing properties