Reverse replay of behavioural sequences in hippocampal place cells during the awake state

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.