Researchers from Northwestern University and the University of Chicago Epilepsy Center collaborated to examine the electrical activity in the brains of five patients at the center in response to sounds administered by the research team as part of a learning exercise.
Five patients who volunteered to participate in the study had electrode probes placed in their brains to see possible treatments. This study is the first to record such electrical activity from inside the brain, whereas previous studies have measured memory processing during sleep using EEG recordings obtained by electrodes on the head.
According to the study, participants improved significantly on a recall test the next morning. By providing visual data identifying areas of the brain involved in the process of overnight memory storage, the mapped brain activity allowed researchers to make significant advances in the understanding of memory storage functions.
Strong conclusions can 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 conducted Electrophysiological responses to 10–20 sounds that were presented repeatedly were recorded one night while each patient slept in the hospital room. Each sound was played very slowly to prevent sexual arousal.
For example, the tinkling of car keys was one of the sounds patients learned to associate with objects and their precise spatial locations before falling asleep using laptop computers.
The researchers found that results from earlier studies using EEG scalp recordings showed systematic improvements in spatial recall after sleep. On laptop screens, patients gave more accurate indications of remembered locations. According to the most recent information obtained from brain electrodes implanted in patients, object sounds played while they were asleep led to increased oscillatory activity, including increases in theta, sigma and gamma EEG bands.
When sounds were played while people were asleep, there was electrophysiological activity in the medial temporal region near the hippocampus and cerebral cortex, which indicated that associated spatial memories had become activated and strengthened. Gamma responses were consistently linked to how well sleep had an effect on how well people could remember things spatially.
The researchers came to the conclusion that this electrophysiological evidence resulted in a sleep-based enhancement of memory storage in these brain regions. “There used to be a conservative belief that such sounds are blocked when people are sleeping,” Paler said.
“Instead, these sounds allowed us to demonstrate that brain structures such as the hippocampus are reactive when memories are reactivated, helping us to retain the knowledge gained while awake.”
Sometimes remembering and forgetting seems random. We can remember irrelevant details while forgetting what we want to remember. The new answer to this long-standing mystery, uncovered by this research, is that memories recur when we sleep, even when we are awake, not knowing that it happened,” Paler said.