How does the brain process memory during sleep? New study offers clues


While it’s long been suspected that our brains consolidate each day’s events as we sleep, a new study is the first to show that process in action.

Researchers were able to reinforce memories as study volunteers slept by stimulating specific parts of their brains in synchrony.

The participants all had severe epilepsy and agreed to be part of the study while under observation for their seizures.

“There has been a dominant theory of how the brain consolidates memories during sleep that has been assumed in the field, but hasn’t been tested in a causal way. This is a direct test of that theory,” said Anna Schapiro, an assistant professor of psychology at the University of Pennsylvania, who was not involved in the study but has called for similar research. “I am thrilled to see this.”

As events occur during the day, they are recorded in a brain area called the hippocampus. Then, during sleep, the brain consolidates and communicates those memories to a different region called the neocortex, where they are stored long-term. Once stored, the memories are more stable and able to be retrieved later.

“This (study) helps clarify how that process might happen,” she said.

How the study worked

Although we all do it every day, sleep remains somewhat mysterious. “A lot of things are happening during sleep,” said the paper’s senior author, Itzhak Fried, a neurosurgeon at the University of California, Los Angeles.

In treating patients with epilepsy, Fried often puts electrodes in their brains and monitors them for several days to identify the focal point of their seizures. While they’re bored waiting for seizures, he asks them to participate in his research into the workings of memory.

Typically, he’s studied them awake, looking for how the brain transforms information as someone tries to commit it to memory. “Patients are able to tell us when they remember something, which is a great opportunity,” he said.

For this study, published in Nature Neuroscience, he and collaborators, including Maya Geva-Sagiv and Yuval Nir, tracked the brain activity of these patients during the deepest part of sleep, when memories are believed to be consolidated and stored. Because the brain sends waves of different frequencies during this time, Fried decided to stimulate the relevant areas in a synchronized fashion ‒ which he likened to a conductor leading an orchestra.

Before they went to sleep, volunteers were shown paired pictures of well known celebrities and animals. When shown images the next morning, those who received the synchronized signals during sleep were better able to remember which images they had seen the night before and which were new images that they hadn’t seen.

The participants feel nothing from the stimulation.

The brain replays events during sleep, but not in exactly the same way as they occurred that day. “It has to be more efficient than that,” Fried said.

Why it matters

Sleep deprivation causes brain cells to fire more slowly, he said, explaining why daytime function falls off with lack of sleep.

People with memory-related diseases such as Alzheimer’s and Parkinson’s often have disrupted sleep, Fried said, “playing a vicious cycle,” which makes the disease worse.

Fried hopes by better understanding the relationship between sleep and memory, scientists could one day help people with memory disorders.

He compared the effect of the stimulation to a hearing aid that amplifies sound to help people hear. “In principle, it would be interesting if you could amplify memory,” he said.

Theoretically, a similar approach might also be used to identify the circuit in the brain that encodes a terrible memory ‒ such as from war or abuse ‒ and use stimulation to erase or recode that memory.

Fried recently received a $7 million NIH grant to study whether artificial intelligence can help pinpoint and strengthen specific memories in the brain.

Right now, the brain stimulation technology requires complex surgery to implant electrodes deep inside the brain, he said.

“If this kind of technology will be made more minimalistic and then you combine it with a tool of deep learning and artificial intelligence, you may create a very powerful system that can assist people,” Fried said.

More information:
Maya Geva-Sagiv et al, Augmenting hippocampal–prefrontal neuronal synchrony during sleep enhances memory consolidation in humans, Nature Neuroscience (2023). DOI: 10.1038/s41593-023-01324-5

Journal information:
Nature Neuroscience

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