The authors draw together the results of a series of detailed computational studies and show how they are contributing to the development of a theory of hippocampal function. A new part of the theory introduced here is a quantitative analysis of how backprojections from the hippocampus to the neocortex could lead to the recall of recent memories. The theory is then compared with other theories of hippocampal function. First, what is computed by the hippocampus is considered. The hypothesis the authors advocate, on the basis of the effects of damage to the hippocampus and neuronal activity recorded in it, is that it is involved in the formation of new memories by acting as an intermediate-term buffer store for information about episodes, particularly for spatial, but probably also for some nonspatial, information. The authors analyze how the hippocampus could perform this function, by producing a computational theory of how it operates, based on neuroanatomical and neurophysiological information about the different neuronal systems con-tained within the hippocampus. Key hypotheses are that the CA3 pyramidal cells operate as a single autoassociation network to store new episodic information as it arrives via a number of specialized preprocessing stages from many association areas of the cerebral cortex, and that the dentate

nment, spatial selectivity decreased considerably in aged memory-impaired rats compared with that of young rats and aged rats with intact memory performance. Key words: aging; hippocampus; place field; spatial learning; electrophysiology; rat The hippocampus plays a critical role in spatial learning in rats (Morris et al., 1982). Correspondingly, the activity of hippocampal complex spike cells (Ranck, 1973) correlates strongly with the location of the rat in the testing environment during exploratory behavior such that the firing of so-called "place cells" is often restricted to one or a few "place fields" within the environment (O'Keefe and Conway, 1978). Studies aimed to elucidate the nature of cognitive decline in aging have recently exploited these findings to develop animal models of age-related cognitive decline. Aged rats are impaired in spatial learning, although the severity of the deficit is highly variable (Barnes, 1988, 1994; DeToledo-Morrell et al., 1988;

We describe here hippocampal cells that respond during whole-body motion when a monkey is moved on a remote-controlled robot-mounted platform in a cue-controlled test chamber (2 x 2 x 2 m). Some of these cells responded to linear motion, and others to axial rotation. Some of these cells responded when the same motion occurred without a view of the visual field. Such cells appeared to be driven by vestibular inputs. Other cells required a view of the visual field for their response, and these cells appeared to be driv-en by the visual motion relative to the monkey of the test chamber. Further evidence that this was the case was that some of the cells responded to rotation and linear motion of the test chamber while the monkey remained stationary. Oth-er cells responded to combinations of whole-body motion and a view of the environment.

Advanced age in rats is associated with a decline in spatial memory capacities dependent on hippocampal processing. As yet, however, little is known about the nature of age-related alterations in the information encoded by the hippocampus. Young rats and aged rats identified as intact or impaired in spatial learning capacity were trained on a radial arm maze task, and then multiple parameters of the environmental cues were manipulated to characterize the changes in firing patterns of hippocampal neurons corresponding to the presence of particular cues or the spatial relationships among them. The scope of information encoded by the hippocampus was reduced in memory-impaired aged subjects, even though the Diminished memory capacity is a well known concomitant of