Hippocampal and cortical place cell plasticity: implications for episodic memory

In humans, the hippocampus is essential for storing episodic memories. These event memories require the rapid storage of novel associations, but little is known about the cellular correlates of such rapid plasticity. We studied patterns of activity and plasticity in the CA1 region of the hippocampus and in anatomically adjacent cortical regions as rats explored a novel arm of a maze to identify the neural correlates of hippocampally dependent memory formation. We found that hippocampal place fields exhibited three phenomena that may have direct relevance to the encoding of episodic memories: (1) very rapid plasticity upon exposure to the new environment, (2) instability in representations formed after short periods of exploration, and (3) a dissociation between the stability of a hippocampal representation and the apparent familiarity of a location. In contrast, cortical regions showed less dramatic changes. Taken together, these findings suggest that hippocampal activity undergoes a period of rapid reorganization during the encoding of novel information, and that even after this reorganization is complete, areas outside the hippocampus have not yet formed stable memories.