Distinct encoding and post-encoding representational formats contribute to episodic sequence memory formation

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 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.