Hippocampal place cells: parallel input streams, subregional processing, and implications for episodic memory.
The hippocampus is thought to be involved in episodic memory in humans. Place cells of the rat hippocampus offer a potentially important model system to understand episodic memory. However, the difficulties in determining whether rats have episodic memory are profound. Progress can be made by considering the hippocampus as a computational device that presumably performs similar transformations on its inputs in both rats and in humans. Understanding the input/output transformations of rat place cells can thus inform research on the computational basis of human episodic memory. Two examples of different transformations in the CA3 and CA1 regions are presented. In one example, CA3 place fields are shown to maintain a greater degree of population coherence than CA1 place fields after a rearrangement of the salient landmarks in an environment, in agreement with computational models of CA3 as an autoassociative network. In the second example, CA3 place field appears to store information about the spatiotemporal sequences of place fields, starting with the first exposure to a cue-altered environment, whereas CA1 place fields store this information only on a temporary basis. Finally, recordings of hippocampal afferents from the lateral and medial entorhinal cortex (EC) suggest that these two regions convey fundamentally different representations to the hippocampus, with spatial information conveyed by the medial EC and nonspatial information conveyed by the lateral EC. The dentate gyrus and CA3 regions may create configural object+place (or item+context) representations that provide the spatiotemporal context of an episodic memory.