Research looks into the brain during sleep to show how memory is stored

New research looks deep into the brain, where previous learning is restored during sleep, leading to improved memory.

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

Five patients who volunteered to participate in the study had electrodes implanted into their brains to study possible treatments for their seizure disorders.

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

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

Although the number of patients studied was necessarily small, strong conclusions were possible as all five patients showed similar patterns of improvement in memory and electrical activity.

“We’re investigating how people manage to remember what they’ve learned rather than forget it,” said Ken Pauler, director of Northwestern University’s Cognitive Neuroscience Program and senior researcher on the study. “In our view, sleep contributes to this ability.”

Pauler is the James Padilla Professor of Psychology and the James Padilla Chair in Arts and Sciences at Northwestern’s Weinberg College of Arts and Sciences.

The study’s collaborators include neurology and neurosurgery researchers from the University of Chicago, as well as psychology researchers from Northwestern University, the University of Michigan and Middlebury College (Vermont).

The paper, “Electrophysiological markers of memory consolidation in the human brain when memories are reactivated during sleep,” will be published Oct. 24 in Proceedings of the National Academy of Sciences (PNAS).

How the research was conducted

One night, while each patient slept in a hospital room, the team recorded electrophysiological responses to 10 to 20 repetitive sounds. All sounds were played very softly to avoid arousal. Half of the sounds were associated with objects and their exact spatial location patients found out before going to bed with a portable computerfor example, the jingle of car keys to help remember their location.

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

New data from implanted brain electrodes have shown that the sounds of objects during sleep cause increased oscillatory activity, including increases in the theta, sigma, and gamma bands of the EEG.

The presence of electrophysiological activity in the hippocampus and adjacent medial temporal cortex when sounds were presented during sleep reflected the reactivation and strengthening of relevant spatial memories.

Gamma responses were consistently associated with the degree of improvement in spatial memory found after sleep. These electrophysiological data led the researchers to conclude that the enhancement of sleep memory conservation occurs in these areas of the brain.

“It used to be thought that these kinds of sounds would be blocked out when people were sleeping,” Paller said. “Instead, these sounds allowed us to demonstrate it the brain structures such as the hippocampus respond when memories are retrieved, helping us to retain knowledge we acquire while awake.

“Sometimes remembering and forgetting seem random. We can remember unimportant details while forgetting what we most want to remember. A new answer to this long-standing mystery revealed by this study is that memories come back when we to sleepeven when we wake up not knowing it happened,” Pauler said.