Researchers looked deep inside the brain to see how memory is stored
How do people remember the things they've learned?
To get to the bottom of the mystery, scientists undertook a study that looked deep inside the brain, where "previous learning was reactivated during sleep," resulting in a refined memory.
Neuroscientists from Northwestern University and clinicians from the University of Chicago Epilepsy Center examined 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.
Previous research studied memory processing during sleep using EEG (Electroencephalography) recordings obtained by electrodes on the head. This study wanted to record electrical activity from inside the brain - electrode probes were implanted in the patient's brains.
The study revealed that a good night's sleep did wonders. The participants significantly improved their performance in a recall test the next morning. According to a press release published by the institution, "the mapped brain activity allowed the researchers to take a big step forward in understanding how memory storage works by providing visual data identifying the areas of the brain engaged in the process of overnight memory storage."
A significant improvement in spatial recall was detected
"We are investigating how people manage to remember the things they've learned, rather than forgetting them," Ken Paller, director of the Cognitive Neuroscience Program at Northwestern and senior researcher on the study, said in a release. "Our view is that sleep contributes to that ability."
For the study, the team recorded electrophysiological responses to 10-20 sounds. As each patient slept in a hospital room, the sounds were played repeatedly but quietly to avoid the patients waking up.
Most of these sounds were associated with objects and their spatial locations that the participants learned before sleep using a laptop.
Post sleep, the researchers found a significant improvement in spatial recall - replicating results from previous studies using EEG recordings from the scalp. In comparison, patients could more accurately pinpoint the remembered locations on the laptop screen.
The sounds played during sleep helped in increasing oscillatory activity
The study revealed that the presence of electrophysiological activity in the hippocampus and the adjacent medial temporal area of the cerebral cortex, when the sounds were presented during sleep, "reflected the reactivation and strengthening of corresponding spatial memories."
"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."
The data from the implanted electrodes showed that the sounds played during sleep helped in increasing oscillatory activity, including increases in theta, sigma, and gamma EEG bands. This electrophysiological evidence helped researchers conclude that sleep-based enhancement of memory storage takes place in these brain regions.
"At times, remembering and forgetting seem 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.
Their study paper was published in the Proceedings of the National Academy of Sciences.
Human accomplishments depend on learning, and effective learning depends on consolidation. Consolidation is the process whereby new memories are gradually stored in an enduring way in the brain so that they can be available when needed. For factual or event knowledge, consolidation is thought to progress during sleep as well as during waking states and to be mediated by interactions between hippocampal and neocortical networks. However, consolidation is difficult to observe directly but rather is inferred through behavioral observations. Here, we investigated overnight memory change by measuring electrical activity in and near the hippocampus. Electroencephalographic (EEG) recordings were made in five patients from electrodes implanted to determine whether a surgical treatment could relieve their seizure disorders. One night, while each patient slept in a hospital monitoring room, we recorded electrophysiological responses to 10 to 20 specific sounds that were presented very quietly, to avoid arousal. Half of the sounds had been associated with objects and their precise spatial locations that patients learned before sleep. After sleep, we found systematic improvements in spatial recall, replicating prior results. We assume that when the sounds were presented during sleep, they reactivated and strengthened corresponding spatial memories. Notably, the sounds also elicited oscillatory intracranial EEG activity, including increases in theta, sigma, and gamma EEG bands. Gamma responses, in particular, were consistently associated with the degree of improvement in spatial memory exhibited after sleep. We thus conclude that this electrophysiological activity in the hippocampus and adjacent medial temporal cortex reflects sleep-based enhancement of memory storage.