Study-Phase Reinstatement: Encoding Spontaneous Thoughts as Memories

Can the brain improve the retrievability of an experience after it has occurred? Systems consolidation theory proposes that cortical reactivation during long post-encoding rest periods facilitates the formation of stable memory representations, a prediction supported by neural evidence in humans and animals. But such reactivation may also occur on short time scales as spontaneous thoughts come to mind during encoding, offering a potential account of classic list memory phenomena but lacking in support from neural data. Leveraging the high-temporal specificity of intracranial electroencephalography (iEEG), we investigate spontaneous reactivation of previously experienced items during brief periods between individual encoding events. Across two large-scale free recall experiments, we show that reactivation, as measured by spectral iEEG similarity, during these periods predicts subsequent recall. In a third experiment, we show that the same methodology can identify post-encoding reactivation that correlates with subsequent memory, consistent with previous results. Thus, spontaneous study-phase reinstatement reliably predicts memory behavior, linking psychological accounts to neural mechanisms and providing the first evidence for sub-second consolidation processes during encoding. ### Competing Interest Statement The authors have declared no competing interest.