Alpha oscillations track the projection of reactivated memories into conscious awareness

By definition, episodic memory is a conscious phenomenon. Memory traces reactivated by the hippocampus and reinstated in the sensory cortices need to enter conscious awareness for them to be re-experienced and overtly recalled. However, it remains unclear whether such reactivation in-and-of-itself ensures that memories will be overtly recalled. To investigate this, magnetoencephalography (MEG) recordings were analysed from thirty-one participants (18 female, 13 male) completing a video-word pair associates memory task. When combining linear classifiers and spectral analyses, sensory cortical reactivation could be observed without overt recall occurring, suggesting reactivation does not guarantee overt recall. Instead, overt recall was additively predicted by (i) an increase in reactivated representations rhythmically fluctuating within the alpha band, and (ii) a decrease in total sensory neocortical alpha power. These results are consistent with accounts which propose that reactivation benefits from desynchronising the network to provide representational space for stimulus-specific information, and/or amplifying stimulus-specific information above residual noise. Altogether, these results suggest that representational reactivation can occur without overt recall, and suggest a role for alpha oscillations in projecting internally-generated representations into conscious awareness.