Episodic Memory Retrieval Functionally Relies on Very Rapid Reactivation of Sensory Information

Episodic memory retrieval is assumed to rely on the rapid reactivation of sensory information that was present during encoding, a process termed "ecphory." We investigated the functional relevance of this scarcely understood process in two experiments in human participants. Wepresented stimuli to the left or right of fixation at encoding, followed by an episodicmemorytest with centrally presented retrieval cues. This allowed us to track the reactivation of lateralized sensorymemorytraces during retrieval. Successful episodic retrieval led to a very early (similar to 100-200 ms) reactivation of lateralized alpha/beta (10-25 Hz) electroencephalographic (EEG) power decreases in the visual cortex contralateral to the visual field at encoding. Applying rhythmic transcranial magnetic stimulation to interfere with early retrieval processing in the visual cortex led to decreased episodic memory performance specifically for items encoded in the visual field contralateral to the site of stimulation. These results demonstrate, for the first time, that episodic memory functionally relies on very rapid reactivation of sensory information.