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Retrieval Attempts Transiently Interfere with Concurrent Encoding of Episodic Memories But Not Vice Versa

In the rodent hippocampus, different phases of each theta activity cycle may be devoted to encoding and retrieval processes. These cycles of ∼3–8 Hz would allow equal processing time for each state and also provide temporal segregation to minimize their mutual interference. We show here that, by controlling the presentation asynchrony between verbal encoding and retrieval cues, theta-resolution (<100 ms) interference-free shifts between functional states are not expressed in hippocampally dependent, human “episodic” memory. Instead, retrieval attempts selectively and transiently interfere, for ∼450 ms, with the encoding of ongoing experiences. Analyses of scalp event-related potentials confirmed that the functional state of the brain during retrieval is largely unperturbed by concurrent encoding and also suggested that encoding impairments may last until a neocortical phase of retrieval can begin. The findings reveal the dynamic properties of interdependent encoding and retrieval functions that contribute to episodic memory in vivo and, moreover, show that, in humans, this form of memory does not operate with either the equality, or the rapidity, intrinsic to the theta model of rodent hippocampal function.