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When we remember an event, the content of that memory is represented across the brain. Detailed memory retrieval is thought to involve the reinstatement of those representations. Functional MRI combined with representational similarity analyses (RSA) of spatial patterns of brain activity has revealed reinstatement of recently-experienced events throughout a core memory retrieval network. In the present study, participants were scanned while they watched, immediately retrieved and then retrieved after a week, 24 short videos. Following the delayed retrieval, they freely recalled all videos outside of the scanner. We observed widespread within- and between-subject reinstatement effects within a posterior midline core memory retrieval network during all phases of the experiment. Within precuneus, bilateral middle temporal gyrus and the left hippocampus, reinstatement effects between the retrieval phases correlated with memory performance. These findings extend previous studies that have only employed short retention periods or highly rehearsed materials, demonstrating that memory representations for unique events are reliably reinstated over longer timeframes that are meaningful in the context of real-world episodic memory.

Understanding what is happening during an event can be helped by prior knowledge of the specific context. However, the effect of contextual knowledge on neural representations of events, and relatedly, how shared knowledge impacts on the similarity of neural processing of events across individuals is poorly understood. To investigate this, we manipulated the availability of knowledge about the narrative content of events while keeping all other aspects equally familiar. The presence of narrative knowledge boosted subjective ratings of coherency and objective measures of memory performance. Narrative knowledge had two effects on fMRI markers of neural processing: (1) it strengthened temporal inter-subject correlations in ventromedial prefrontal cortex as well as left angular and inferior frontal gyri, and (2) it also increased spatial inter-subject pattern similarity in the bilateral anterior temporal lobes. We propose that shared narrative knowledge constrains participants’ interpretation of the videos and thereby results in greater alignment of neural processing of the events. We propose a division of labour between semantic control brain regions (VMPFC, IFG and AG), which process the moment-by-moment content of the narrative, and the ATL which represents the overarching narrative gist of an event.

The hippocampus has been implicated in integrating information across separate events in support of mnemonic generalizations. These generalizations may be underpinned by processes at both encoding (linking similar information across events) and retrieval (“on-the-fly” generalization). However, the relative contribution of the hippocampus to encoding- and retrieval-based generalizations is poorly understood. Using fMRI in humans, we investigated the hippocampal role in gradually learning a set of spatial discriminations and subsequently generalizing them in an acquired equivalence task. We found a highly significant correlation between individuals’ performance on a generalization test and hippocampal activity during the test, providing evidence that hippocampal processes support on-the-fly generalizations at retrieval. Within the same hippocampal region there was also a correlation between activity during the final stage of learning (when all associations had been learnt but no generalization was required) and subsequent generalization performance. We suggest that the hippocampus spontaneously retrieves prior events that share overlapping features with the current event. This process may also support the creation of generalized representations during encoding. These findings are supportive of the view that the hippocampus contributes to both encoding- and retrieval-based generalization via the same basic mechanism; retrieval of similar events sharing common features.