J. van Pelt, M.A. Corner H.B.M. Uylings and F.H. Lopes da Silva (Eds.) Pmgress in Bmin Research, Vol 102 Q 1994 Elsevier Science BV. All rights reserved. CHAPTER 22 335 Neural networks in the brain involved in memory and recall Edmund T. Rolls and Alessandro Treves* Department of Experimental Psychology, Uniwrsity of Oxford South Parks Road, OxjorrCi, OX1 3UD, U.K. Introduction Damage to the hippocampus and related struc- tures leads to anterograde amnesia, i.e. an inabil- ity to form many types of memory. Old memories are relatively spared. Recent memories, formed within the last few weeks or months, may be impaired (Squire, 1992). The learning tasks that are impaired include spatial and some non-spatial tasks in which information about particular episodes, such as where a particular object was seen, must be remembered. Further, some hip- pocampal neurons in the monkey respond to combinations of visual stimuli and places where they are seen (Rolls, 1990a,b, 1991; Rolls and O’Mara, 1993). It is also known that inputs con- verge into the hippocampus, via the adjacent parahippocampal gyrus and entorhinal cortex, from virtually all association areas in the neocor- tex, including areas in the parietal cortex concerned with spatial function, temporal areas concerned with vision and hearing, and the frontal lobes (Fig. 1). An extensively divergent system of output projections enables the hippocampus to feed back into most of the cortical areas from *Present address: S.I.S.S.A. - Biophysics, via Beirut 2-4,34013 Trieste, Italy. which it receives inputs. On the basis of these and related findings the hypothesis is suggested that the importance of the hippocampus in spatial and other memories is that it can rapidly form ‘epi- sodic’ representations of information originating from many areas of the cerebral cortex, and act as an intermediate term buffer store. In this paper, analyses of how the architecture of the hippocampus could be used as such a buffer store are considered, and then a hypothesis on how recent memories could be recalled from this store back into the cerebral cortical association areas, to be used as needed in the formation of long-term memories, is presented (see Marr, 1971; Rolls, 1989a,b, 1991; Treves and Rolls, 1994). The hippocampus Hippocampal 013 circuitry (see Fig. 1) Projections from the entorhinal cortex reach the granule cells (of which there are lo6 in the rat) in the dentate gyrus (DG) via the perforant path (pp). The granule cells project to CA3 cells via the mossy fibres (MF), which provide a sparse but possibly powerful connection to the 3 X lo5 CA3 pyramidal cells in the rat. Each CA3 cell receives approximately 50 mossy fibre inputs, so that the sparseness of this connectivity is thus 0.005%. By contrast, there are many more-pos-