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. Significance Statement Growing evidence suggests episodic recall can occur without conscious awareness, raising a fundamental question: how do memories enter awareness? Here, we investigate whether alpha oscillatory activity relates to this process. Using magnetoencephalography, we show that while episodic memories can be reinstated in the neocortex without overt recall occurring, rhythmic reactivation within the alpha band distinguishes memories that will be recalled from those that will not, suggesting reactivation must exceed a threshold for the memory contents to enter awareness. These findings challenge the idea that memory reactivation inherently results in conscious awareness and highlight a new target for brain-based interventions aimed at improving memory in both healthy individuals and clinical populations.
Benjamin J. Griffiths (Wed,) studied this question.