Hippocampal representational organization and spatial context

The hippocampus appears to undergo continual representational reorganization as animals navigate their environments. This reorganization is postulated to be reflected spatially in terms of changes in the ensemble of place cells activated, as well as changes in place field specificity and reliability for cells recorded in both hilar/CA3 and CA1 regions. The specific contribution of the hilar/CA3 region is suggested to be to compare the expected spatial context with that currently being experienced, then relay discrepancies to CA1. The properties of CA1 place fields in part reflect the spatial comparisons made in the hilar/CA3 area. In addition, CA1 organizes the input received from the hilar/CA3 place cells according to different temporal algorithms that are unique to different tasks. In this way, hippocampus helps to distinguish temporally one spatial context from another, thereby contributing to episodic memories. (C) 1999 Wiley-Liss, Inc.