Replay in Humans—First Evidence and Open Questions

Memory consolidation serves to stabilize initially fragile memory traces. Rodent studies suggest that consolidation relies on replay of previously acquired stimulus-specific activity patterns. This replay is coupled to hippocampal sharp wave-ripple (SWR) events and sleep spindles. More recently, the application of multivariate analysis methods has allowed identifying stimulus-specific “engram patterns” in humans as well. These analyses have been applied to various modalities including functional magnetic resonance imaging (fMRI) and intracranial EEG (iEEG). A few initial studies suggest that engram patterns are indeed replayed after learning in humans, during awake resting state, tasks, and sleep. Here, we review these studies and point to open questions. It has been repeatedly shown that the extent of engram pattern replay predicts later memory performance, and that replay occurs during both awake resting state and sleep. On the other hand, cuing of specific memories improves memory consolidation selectively during sleep. Brain stimulation may disrupt consolidation on a behavioral level, but its effect on replay of engram patterns has not been shown yet. Finally, replay has been indirectly linked to sleep spindles, while its relationship to SWRs remains to be investigated. To summarize, the investigation of engram pattern replay in the human brain is an emerging field with still many open questions.