During sleep, the brain reactivates memory traces that it can no longer remember later on. But what causes us to forget? In a study published today in Nature Communications, a team of researchers present their investigation into the activity patterns that are associated with remembering and forgetting.
The team recorded the brain activity of epilepsy patients who had electrodes implanted into their brains for the purpose of surgical planning. Test participants were given a series of pictures to memorize, after which they took an afternoon nap. When looking at a picture, the activity in the brain shows a pattern that differs somewhat from picture to picture. The researchers were able to measure these differences in high-frequency activity fluctuations called gamma-band activity. They analyzed brain activity both during the learning task and also during sleep. They then tested which images the participants could remember after sleep and which they could not.
The gamma-band activity that was typical of certain motifs occurred when looking at the images and again during sleep. The brain reactivated the activity patterns—both for images the participants later remembered and for those they later forgot. "The forgotten images do not simply disappear from the brain," says Dr. Hui Zhang of Ruhr-Universität Bochum.
The decisive factor in whether an image was forgotten or retained was not the reactivation of the image-specific gamma-band activity, but the activity in a memory-associated brain region called the hippocampus. This region exhibits extremely rapid fluctuations in activity called ripples. A picture was only recalled later on when the reactivation occurred at the same time as the ripples in the hippocampus. This phenomenon only occurred during certain sleep phases and not when the participants were awake.
A second factor in whether an image was forgotten or retained was the depth of image processing in the brain. The researchers differentiated the gamma-band activity measured when viewing the images into a superficial- and a deep-processing stage. The superficial processing took place during the first half-second after the presentation of the image, the deeper processing after that. Only when the gamma-band activity from the deep-processing phase was reactivated during the ripples did the participants later remember the image.