Abstract
The reactivation of neural activity that was present during the encoding of an event is assumed to be essential for human episodic memory retrieval and the consolidation of memories during sleep. Pioneering animal work has already established a crucial role of memory reactivation to prepare and guide behaviour. Research in humans is now delineating the neural processes involved in memory reactivation during both wakefulness and sleep as well as their functional significance. Focusing on the electrophysiological signatures of memory reactivation in humans during both memory retrieval and sleep-related consolidation, this review provides an overview of the state of the art in the field. We outline recent advances, methodological developments and open questions and specifically highlight commonalities and differences in the neuronal signatures of memory reactivation during the states of wakefulness and sleep.This article is part of the Theo Murphy meeting issue ‘Memory reactivation: replaying events past, present and future’.
Highlights
Episodic memory, the capacity to remember and relive past experiences, represents a cornerstone of human cognition and the sense of our self [1]
These pioneering studies revealed that the sequences of neuronal firing which occurred during exploration of a novel environment in the hippocampus, are spontaneously reactivated during subsequent non-rapid eye movement (NREM) sleep [13,14]
Recollection was accompanied by content-specific memory reactivation in visual areas, which in turn was coupled to ripple emergence. These findings provide an important link of human awake memory reactivation to studies in rodents, while emphasizing the functional role of hippocampal high-frequency ripples in memory processes [57,58,60,63,64,65]
Summary
The capacity to remember and relive past experiences, represents a cornerstone of human cognition and the sense of our self [1]. Initial evidence originated from research on memory consolidation, investigating how the brain strengthens memories during offline periods such as sleep These pioneering studies revealed that the sequences of neuronal firing which occurred during exploration of a novel environment in the hippocampus, are spontaneously reactivated during subsequent non-rapid eye movement (NREM) sleep [13,14]. As sensory input has been shown to trigger and bias hippocampal and accompanying neocortical reactivation processes, it has been suggested that they might subserve memory retrieval, thereby enabling memory-guided decision making [12]. While these findings without doubt represent important breakthroughs, a pressing question is whether comparable processes guide the multi-facetted expressions of human memory.
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