PLACE CELLS, NAVIGATIONAL ACCURACY, AND THE HUMAN HIPPOCAMPUS

The hippocampal formation in both rats and humans is involved in spati al navigation. In the rat, cells coding for places, directions, and sp eed of movement have been recorded from the hippocampus proper and/or the neighbouring subicular complex. Place fields of a group of the hip pocampal pyramidal cells cover the surface of an environment but do no t appear to do so in any systematic fashion. That is, there is no topo graphical relation between the anatomical location of the cells within the hippocampus and the place fields of these cells in an environment . Recent work shows chat place cells are responding to the summation o f two or more Gaussian curves, each of which is fixed at a given dista nce to two or more walls in the environment. The walls themselves are probably identified by their allocentric direction relative to the rat and this information may be provided by the head direction cells. The right human hippocampus retains its role in spatial mapping as demons trated by its activation during accurate navigation in imagined and vi rtual reality environments. In addition, it may have taken on wider me mory functions, perhaps by the incorporation of a linear time tag whic h allows for the storage of the times of visits to particular location s. This extended system would serve as the basis for a spatio-temporal event or episodic memory system.