In episodic encoding, an unfolding experience is rapidly transformed into a memory representation that binds separate episodic elements into a memory form to be later recollected. Here, we sought to investigate the representational format that accounts for successful memory formation of episodic sequences. We combined RSA and multivariate decoding analysis on scalp EEG recordings while healthy female (N = 17) and male (N = 13) human participants encoded trial-unique combinations of face-object-scene picture triplets that were subsequently recalled in a test. Our findings revealed an ongoing process of image category integration throughout sequence encoding, that this process accumulated as a function of picture order in the sequence and that the degree of picture category integration throughout sequence encoding was not associated with episodic retrieval. We also found that episodic sequences that were later remembered were associated with an increased trial-specific neural similarity between EEG patterns elicited during picture sequence encoding and in the offset period, thereby indicating that a rapid memory reinstatement at the episodic offset supported the rapid transformation of an episodic sequence into a bound memory representation. The present findings demonstrate the time course and the functional relevance of the different representational formats that take place during online encoding and immediate offline periods that support the formation of memories for episodic sequences.

Prior animal and human work have shown that post-encoding reinstatement plays an important role in organizing the temporal sequence of unfolding episodes in memory. Here, we investigated whether post-encoding reinstatement serves to promote the encoding of “one-shot” episodic learning beyond the temporal structure in humans. In Experiment 1, participants encoded sequences of pictures depicting unique and meaningful episodic-like events. We used representational similarity analysis on scalp EEG recordings during encoding and found evidence of rapid picture-elicited EEG patterns reinstatement at episodic offset (around 500 msec post-episode). Memory reinstatement was not observed between successive elements within an episode, and the degree of memory reinstatement at episodic offset predicted later recall for that episode. In Experiment 2, participants encoded a shuffled version of the picture sequences from Experiment 1, rendering each episode meaningless to the participant but temporally structured as in Experiment 1, and we found no evidence of memory reinstatement at episodic offset. These results suggest that post-encoding memory reinstatement is akin to the rapid formation of unique and meaningful episodes that unfold over time.

Much work in rodents and in humans has provided evidence that post-encoding reinstatement plays an important role in stabilizing memory beyond initial learning processes. However, it remains unclear whether memory reinstatement is important for the rapid - one-shot - learning of an unfolding episode. Here, we asked whether the reinstatement of an episode may occur preferentially post-encoding, when an individual perceives a meaningful event to be concluded. We asked human participants (male and female) to encode sequences of pictures depicting unique episodic-like events. We used representational similarity analysis of scalp electroencephalography recordings during encoding and found evidence for memory reactivation of the just encoded sequence of elements at the offset of the episode. Importantly, memory reinstatement was not observed between successive elements within an episode, indicating memory reactivation was specifically induced once participants perceived the unfolding episode to be completed. We also found that memory reinstatement predicted memory recollection of an encoded episode and that offset memory reinstatement was not present when participants encoded sequences of pictures that were not perceived as meaningful episodes. These results indicate that memory reinstatement at episode offsets is a mechanism selectively engaged to support rapid memory formation of single events.

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