Introduction

The hippocampus is anatomically and neurophysiologically one of the best known structures of the mammalian brain (for a review, see Witter, 1993). Besides, it plays a fundamental role in memory storage (for a review, see Squire, 1992), and has extensive connections with many areas of the neocortex, both incoming and outgoing, through the entorhinal cortex (Squire et al., 1989). Partly for these reasons, the hippocampus has been the object of several theoretical investigations and models. One of the most widespread functional hypotheses is that the hippocampus has the role of a fast episodic memory, and has to perform cued retrieval to release information to the neocortex, in which memory is slowly reorganized in semantic structures (see, e.g. McClelland et al., 1995). The hippocampus may be not a permanent store for episodes; it appears experimentally that there exists “hippocampal” forgetting (Zola-Morgan and Squire, 1990), even if the relation of typical forgetting times with the storage of the same information in neocortex is not known. In any case, it is likely that information is stored in the hippocampus, and a mechanism for retrieving it from the hippocampus is needed (Rolls, 1995).

Here we introduce a model of the information flow in the hippocampal system, focusing on the role of the connections between entorhinal cortex, CA3 and CA1. We note that our model is independent of the functional hypotheses just described, and of the detailed implications of behavioural theories. We base the model on neuroanatomical and physiological evidence, though with some form of approximation to allow for an analytical development.