Awake reactivation predicts memory in humans.

How are new experiences transformed into memories? Recent findings have shown that activation in brain regions involved in the initial task performance reemerges during postlearning rest, suggesting that "offline activity" might be important for this transformation. It is unclear, however, whether such offline activity indeed reflects reactivation of individual learning experiences, whether the amount of event-specific reactivation is directly related to later memory performance, and what brain regions support such event-specific reactivation. Here, we used functional magnetic resonance imaging to assess whether event-specific reactivation occurs spontaneously during an active, postlearning delay period in the human brain. Applying representational similarity analysis, we found that successful recall of individual study events was predicted by the degree of their endogenous reactivation during the delay period. Within the medial temporal lobe, this reactivation was observed in the entorhinal cortex. Beyond the medial temporal lobe, event-specific reactivation was found in the retrosplenial cortex. Controlling for the levels of blood oxygen level-dependent activation and the serial position during encoding, the data suggest that offline reactivation might be a key mechanism for bolstering episodic memory beyond initial study processes. These results open a unique avenue for the systematic investigation of reactivation and consolidation of episodic memories in humans.