The hippocampus has long been known to be involved in spatial navigational learning in rodents^{1, 2}, and in memory for events in rodents^{3, 4}, primates⁵ and humans⁶. A unifying property of both navigation and event memory is a requirement for dealing with temporally sequenced information. Reactivation of temporally sequenced memories for previous behavioural experiences has been reported in sleep in rats^{7, 8}. Here we report that sequential replay occurs in the rat hippocampus during awake periods immediately after spatial experience. This replay has a unique form, in which recent episodes of spatial experience are replayed in a temporally reversed order. This replay is suggestive of a role in the evaluation of event sequences in the manner of reinforcement learning models. We propose that such replay might constitute a general mechanism of learning and memory.

1. Picower Institute for Learning and Memory, Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 46-5223, Cambridge, Massachusetts 02139, USA

Correspondence to: David J. Foster¹ Correspondence and requests for materials should be addressed to D.J.F. (Email: djfoster@mit.edu).

Received 13 December 2005 | Accepted 20 January 2006 |

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