Study looks inside the brain during sleep to show how memory is stored: Reactivating memories triggers electrical activity signifying improvement in storage

A new study looks deep into the brain, where earlier learning was reactivated during sleep, resulting in improved memory.

Neuroscientists from Northwestern University teamed up with clinicians from the University of Chicago’s Epilepsy Center to study electrical brain activity in five of the center’s patients in response to sounds administered by the research team as part of a learning exercise.

The five patients who volunteered to participate in the study had electrode probes implanted in the brain to explore potential treatments for their seizure disorders.

While previous studies have used EEG recordings captured by electrodes on the head to measure memory processing during sleep, this is the first study to record such electrical activity from the brain.

The study found that participants significantly improved their performance the next morning in a recall test. The mapped brain activity allowed the researchers to take a major step forward in understanding how memory storage works by providing visual data that identifies the areas of the brain involved in the process of memory storage during the night.

Although the number of patients studied was necessarily small, strong conclusions were possible because all five patients showed the same patterns of memory enhancement and electrical activity.

“We’re looking at how people manage to remember the things they’ve learned, rather than forget them,” said Ken Paller, director of the Cognitive Neuroscience Program at Northwestern and senior investigator on the study. “Our view is that sleep contributes to that ability.”

Paller is a professor of psychology and the James Padilla Chair in Arts and Science at the Weinberg College of Arts and Sciences at Northwestern.

Research collaborators include neurology and neurological surgery researchers at the University of Chicago, and psychology researchers at Northwestern, the University of Michigan, and Middlebury College (Vermont).

The article, “Electrophysiological Markers of Memory Consolidation in the Human Brain When Memories Reactivated during Sleep”, will be published on October 24 in the Proceedings of the National Academy of Sciences (PNAS).

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How the research was conducted

One night, while each patient was sleeping in a hospital room, the team recorded electrophysiological responses to 10-20 sounds presented repeatedly. All sounds were played very quietly to avoid excitement. Half of the sounds were associated with objects and their precise spatial locations that patients learned before bedtime using a laptop, such as the ringing sound of car keys, to remember their location.

After sleep, the researchers found systematic improvements in spatial memory, replicating results from previous studies using EEG recordings of the scalp. Patients more accurately indicated the locations they remembered on the laptop screen.

The new data from the implanted brain electrodes showed that object sounds presented during sleep elicited increased oscillatory activity, including increases in theta, sigma and gamma EEG bands.

The presence of electrophysiological activity in the hippocampus and the adjacent medial temporal region of the cerebral cortex, when the sounds were presented during sleep, reflected the reactivation and reinforcement of corresponding spatial memories.

Gamma responses were consistently associated with the degree of improvement in spatial memory after sleep. This electrophysiological evidence led the researchers to conclude that sleep-based enhancement of memory storage occurs in these brain regions.

“It used to be the orthodox assumption that such sounds would be blocked when people sleep,” Paller said. “Instead, with these sounds, we were able to show that brain structures like the hippocampus respond when memories are reactivated, which helps us retain the knowledge we gain when we’re awake.

“Sometimes remembering and forgetting seems arbitrary. 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.

Story source:

materials supplied by Northwestern University. Originally written by Stephanie Kulke. Note: Content is editable for style and length.

Jacky

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