Deep-brain stimulation during sleep can strengthen memory

While it’s known that sleep plays a crucial role in strengthening memory, scientists are still trying to decode how this process plays out in the brain overnight. Now scientists from UCLA Health and Tel Aviv University have provided physiological evidence from inside the human brain supporting the dominant scientific theory on how the brain consolidates memory during sleep.

Furthermore, the researchers found that targeted deep-brain stimulation during a critical time in the sleep cycle appeared to improve memory consolidation. This was achieved by a novel ‘closed-loop’ system that delivered electrical pulses in one brain region precisely synchronised to brain activity recorded from another region, as described in the journal Nature Neuroscience.

According to the dominant theory for how the brain converts new information into long-term memories during sleep, there’s an overnight dialogue between the hippocampus — the brain’s memory hub — and the cerebral cortex, which is associated with higher brain functions like reasoning and planning. This occurs during a phase of deep sleep, when brain waves are especially slow and neurons across brain regions alternate between rapidly firing in sync and silence.

The researchers had the opportunity to test this theory of memory consolidation via electrodes in the brains of 18 epilepsy patients at UCLA Health. The electrodes had been implanted in the patients’ brains to help identify the source of their seizures during hospital stays typically lasting around 10 days.

The study was conducted across two nights and mornings. Just before bedtime, study participants were shown photo pairings of animals and 25 celebrities. They were immediately tested on their ability to recall which celebrity was paired with which animal, and they were tested again in the morning after a night of undisturbed sleep.

On another night, they were shown 25 new animal and celebrity pairings before bedtime. This time, they received targeted electrical stimulation overnight, and their ability to recall the pairings was tested in the morning. To deliver this electrical stimulation, the researchers created a real-time closed-loop system that they likened to a musical conductor: the system ‘listened’ to the brain’s electrical signals, and when patients fell into the period of deep sleep associated with memory consolidation, it delivered gentle electrical pulses instructing the rapidly firing neurons to ‘play’ in sync.

Each individual tested performed better on memory tests following a night of sleep with the electrical stimulation compared to a night of undisturbed sleep. Key electrophysiological markers also indicated that information was flowing between the hippocampus and throughout the cortex, providing physical evidence supportive of memory consolidation.

“We found we basically enhanced this highway by which information flows to more permanent storage places in the brain,” said study co-author Dr Itzhak Fried, from UCLA. He claimed the research could offer new clues for how deep-brain stimulation during sleep could one day help patients with memory disorders like Alzheimer’s disease.

“This provides the first major evidence down to the level of single neurons that there is indeed this mechanism of interaction between the memory hub and the entire cortex,” he said. “It has both scientific value in terms of understanding how memory works in humans and using that knowledge to really boost memory.”

Back in 2012, Fried authored a study that for the first time showed that electrical stimulation can strengthen memory. Since then he has continued to explore how deep-brain stimulation could improve memory, and he recently received a $7 million grant to study whether artificial intelligence can help pinpoint and strengthen specific memories in the brain.

“In our new study, we showed we can enhance memory in general,” Fried said. “Our next challenge is whether we have the ability to modulate specific memories.”

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