During hippocampal replay, neural ensembles in the hippocampus fire in patterns that recapitulate those that occurred during prior experiences, but in a time-compressed manner. Such reactivation occurs during offline states, such as sleep and pauses in behaviour, and tends to coincide with brief, distinctive bouts of high-frequency oscillatory activity known as sharp wave ripples (SWRs) in the hippocampal local field potential. Replay during sleep was discovered in 1996, and subsequent studies built up an understanding of its role as a key neural mechanism of memory consolidation.

It was not until almost a decade later that a pair of papers definitively reported the occurrence of sequential hippocampal replay during the awake state. Their discoveries were made possible by a combination of powerful state-of-the-art experimental and analytical approaches. Both groups used electrophysiological techniques capable of simultaneously recording from more single neurons during freely moving behaviour than was typically possible. Then, enabled by these large ensemble recordings, they applied sequence detection algorithms that could identify sequences of spiking during SWRs that matched behavioural timescale sequences during movement. This contrasted with more typical analyses that compared the spiking patterns of neurons in a pairwise fashion. Both papers focused on the ‘directionality’ of replay: in 2006, Foster and Wilson reported that awake replay seemed to ‘rewind’ the sequence of firing that occurred during an initial experience, a phenomenon that is now known as reverse replay. In 2007, Diba and Buzsáki added the important qualification that both forward and reverse sequences could be found during pauses in awake behaviour. By contrast, replay during sleep is generally thought to occur in the forward direction.