A study involving epilepsy patients who had electrodes implanted into their brain has revealed that memory consolidation during sleep doesn’t simply involve reactivation of the new memories.
Participants were given pictures to memorize, before taking an afternoon nap. Surprisingly, brainwave activity showed that both the pictures participants later remembered and those they later forgot, were reactivated during sleep. What was crucial was not the reactivation of the picture-specific gamma band activity, but its conjunction with “ripples” (extremely rapid fluctuations in activity) in the hippocampus. Only when the reactivation occurred at the same time as the ripples in the hippocampus did participants remember the picture.
What determined whether this happened? The evidence suggests that longer (and thus deeper) processing of the picture is needed, not simply a quick superficial look.
This phenomenon only occurred during nonREM sleep, not during wakefulness (the circumstances of sleep meant little time was spent in REM sleep).
The findings confirm earlier research with rodents.
Paper available at https://www.nature.com/articles/s41467-018-06553-y
(2018). Electrophysiological mechanisms of human memory consolidation.
Nature Communications. 9(1), 4103.