Episodic memory retrieval success is associated with rapid replay of episode content

Memory for everyday experience shapes our representation of the structure of the world, while retrieval of these experiences is fundamental for informing our future decisions. The fine-grained neurophysiological mechanisms that support such retrieval are largely unknown. We studied participants who first experienced, without repetition, unique multi-component episodes. One day later, they engaged in cued retrieval of these episodes whilst undergoing magnetoencephalography (MEG). By decoding individual episode elements, we found that trial-by-trial successful retrieval was supported by sequential replay of episode elements, with a temporal compression factor greater than 60. The direction of replay supporting this retrieval, either backward or forward, depended on whether a participants goal was to retrieve elements of an episode that followed or preceded a retrieval cue, respectively. This sequential replay was weaker in very high performing participants, where instead we found evidence for simultaneous clustered reactivation. Our results demonstrate that memory-mediated decisions are supported by a rapid replay mechanism that can flexibly shift in direction in response to task requirements. One Sentence SummaryRecall of extended episodes of experience is supported by compressed replay of memory elements that flexibly changes direction depending on task temporal orientation.