A new study delves deep into the brain to see if earlier learning was reactivated during sleep, resulting in better memory.
Northwestern University and the University of Chicago Epilepsy Center researchers worked together to evaluate electrical activity in the brains of five patients at the centre in response to noises delivered by the research team as part of a learning exercise. Electrode probes were placed in the brains of the five individuals who volunteered to participate in the study to investigate potential treatments.
This is the first study to record such electrical activity directly within the brain, whereas prior studies used EEG recordings taken from electrodes on the head to examine memory processing during sleep.
According to the study, participants significantly improved on a recall test the following morning. By providing visual data identifying the regions of the brain involved in the process of overnight memory storage, the mapped brain activity allowed the researchers to make significant progress in their understanding of how memory storage functions.
Strong conclusions could be drawn despite the small number of patients studied, as all five patients displayed similar patterns of memory improvement and electrical activity.
How the study was done
Electrophysiological responses to 10-20 sounds that were repeatedly presented were recorded one night while each patient slept in a hospital room. To prevent arousal, every sound was played extremely softly. The jingling sound of car keys, for example, was one of the sounds that patients learned to associate with objects and their precise spatial locations before going to sleep using a laptop computer.
The results of earlier studies using EEG scalp recordings showed systematic improvements in spatial recall after sleep, the researchers found. On the laptop screen, patients gave more precise indications of the remembered locations.
According to the most recent information obtained from the brain electrodes implanted in patients, object sounds played while they were asleep caused increased oscillatory activity, including increases in theta, sigma, and gamma EEG bands.
When the sounds were played while people were sleeping, there was electrophysiological activity in the hippocampus and the nearby medial temporal area of the cerebral cortex, which indicated that the corresponding spatial memories had been activated and strengthened.
Gamma responses were consistently linked to how much sleep had an impact on how well people could remember things spatially. The researchers came to the conclusion that sleep-based enhancement of memory storage occurs in these brain regions as a result of this electrophysiological evidence.
“The orthodox assumption used to be that such sounds would be blocked out when people are sleeping,” Paller said. “Instead, these sounds allowed us to demonstrate that brain structures such as the hippocampus are responsive when memories are reactivated, helping us to retain the knowledge we gain when we’re awake.
“At times, remembering and forgetting seems random. We can remember irrelevant details while forgetting what we most want to remember. The new answer to this long-standing mystery, highlighted by this research, is that memories are revisited when we sleep, even though we wake up not knowing it happened,” Paller said.
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