Brain Stimulation During Deep Sleep May Boost Recall Memory

Researchers have long suspected that a good night’s sleep can improve our memory. The mechanism behind this link has remained unclear. A new study published in the journal Nature Neuroscience by researchers at the University of California Los Angeles Health (UCLA Health) and Tel Aviv University has provided records from inside the brain that support one of the leading theories of memory consolidation during sleep. Electrical stimulation in targeted brain regions may also further improve memory.

How does the brain boost memory during sleep?

A long-held theory suggests that the brain converts new data it has picked up over the day into long-term memories during sleep. At the heart of this ability, suggest sleep scientists, is a crosstalk between the brain’s memory center – the hippocampus – and the seat of higher cognition – the cerebral cortex – while we snooze. This process is suggested to occur during a chunk of our brain’s deep sleep, where our brain waves slow down, and neurons begin a stop-start firing pattern that switches between coordinated electrical fusillades and signal silence.

Itzhak Fried, a professor of neurosurgery, psychiatry and biobehavioral sciences at the David Geffen School of Medicine at UCLA, director of epilepsy surgery at UCLA Health and the study’s co-author, said, “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. It has both scientific value in terms of understanding how memory works in humans and using that knowledge to really boost memory.” 

Proving this theory has been a slow struggle for science. Recording from inside the human brain is difficult at the best of times and to do so over the course of a night’s sleep requires a dizzy maze of experimental design quandaries to be completed. Luckily, Fried and colleagues were given a unique opportunity.

A syncing feeling

Their study cohort consisted of 18 epilepsy inpatients at UCLA Health. They had been implanted with electrodes that would allow their physicians to identify the source of their seizures. Over two nights and mornings, Fried’s team were allowed to conduct memory experiments with the patients. Just before sleep, they were shown 25 card pairs consisting of animals matched to iconic celebrities like Marilyn Monroe. They were then tested on their ability to recall the pairings. After a night’s sleep, their memory was assessed again.

On the experiment’s second night, the patients were shown 25 new pairings and tested on recall again. But while they slept, targeted electrical signals were delivered to their brains using a closed-loop system. This technology was primed to wait until the brain signals characteristic of deep sleep were recognized and then deliver gentle pulses of stimulation to help sync up the rapidly firing neurons. Their memory was then tested once again in the morning.

The signals were targeted at different brain regions in different patient groups. In six patients who received the syncing stimulation to white matter – brain tissue mainly consisting of neural connections called axons – in the prefrontal region of the cerebral cortex, stimulation led to better morning memory compared to an undisturbed night’s sleep. Stimulation targeting other brain regions had more mixed outcomes.

The information highway

The team says that their data support the idea that hippocampal–cerebral cortex connections were an important factor in this memory consolidation. “We found we basically enhanced this highway by which information flows to more permanent storage places in the brain,” Fried said. His next goal is to target specific memories, rather than just general recall. “In our new study, we showed we can enhance memory in general,” Fried continued. “Our next challenge is whether we have the ability to modulate specific memories.”

Reference: Geva-Sagiv M, Mankin EA, Eliashiv D, et al. Augmenting hippocampal–prefrontal neuronal synchrony during sleep enhances memory consolidation in humans. Nat Neurosci. 2023:1-11. doi: 10.1038/s41593-023-01324-5 

This article is a rework of a press release issued by UCLA Health. Material has been edited for length and content.