An optimal oscillatory phase for pattern reactivation during memory retrieval

Computational models and in vivo studies in rodents suggest that the hippocampal system oscillates between states optimal for encoding and states optimal for retrieval. We here show that in humans, neural signatures of memory reactivation are modulated by the phase of a theta oscillation. EEG was recorded while participants were cued to recall previously learned word-object associations, and time-resolved pattern classifiers were trained to detect neural reactivation of the target objects. Classifier fidelity rhythmically fluctuated at 7-8Hz, and was modulated by theta phase across the entire recall period. The phase of optimal classification was shifted approximately 180° between encoding and retrieval. Inspired by animal work, we then computed “classifier-locked averages” to analyse how ongoing theta oscillations behaved around the time points at which the classifier indicated memory retrieval. We found strong theta (7-8Hz) phase consistency approximately 300ms before the time points of maximal neural memory reactivation. Our findings provide important evidence that the neural signatures of memory retrieval fluctuate and are time-locked to the phase of an ongoing theta oscillation.