Zapping the brain during sleep helps memories form

"We basically enhanced this highway by which information flows."

Brain stimulation during sleep appears to help with the formation of long-term memories — suggesting a new way to treat people with Alzheimer’s and other diseases affecting memory.

The challenge: While you sleep, your brain turns your latest new experiences into lasting memories through a process called “memory consolidation,” but researchers are still trying to piece together exactly how this happens.

What they know so far is that when you experience something new, the neurons in your brain fire in a unique pattern and that those same neurons fire in the same pattern when you’re sleeping. 

Researchers also suspect that the part of the brain where memories are initially stored (the hippocampus) communicates with another part (the prefrontal cortex) during sleep as a part of memory consolidation, but they haven’t been able to prove it directly.

“This provides the first major evidence … that there is indeed this mechanism of interaction.”

Itzhak Fried

What’s new? Now, researchers at UCLA and Tel Aviv University have not only confirmed the communication between these two parts of the brain during memory consolidation, but also demonstrated a way to strengthen the connection to improve people’s memories. 

“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,” said study co-author Itzhak Fried.

“It has both scientific value in terms of understanding how memory works in humans and using that knowledge to really boost memory,” he continued.

How it works: For their study, the researchers enlisted the help of 18 patients who were staying at UCLA Health for about 10 days. During this time, the patients had electrodes implanted in their brains to help doctors identify the source of their epileptic seizures.

Twelve of the patients were shown 25 pairs of images just before bedtime, with one image featuring a celebrity and the other an animal.

They were then tested on their ability to remember which animal was paired with a particular celebrity. This testing happened several minutes after they saw the images for the first time and then again the following morning.

a diagram showing the image pairs and how they were used during learning and testing
Credit: Geva-Sagiv, M., et al. Nature Neuroscience (2023)

The next night, the participants saw a new set of 25 image pairs. Afterwards, the researchers used the implanted electrodes to stimulate the patients’ brains with gentle pulses while they slept.

The remaining six patients weren’t included in this testing — the researchers simply analyzed their brains during the study.

The results: Six of the participants received brain stimulation in the prefrontal cortex during a phase of deep sleep associated with memory consolidation. The following morning they all performed better on the memory test than they had after a night with no stimulation.

“We found we basically enhanced this highway by which information flows to more permanent storage places in the brain,” said Fried.

Brain stimulation might be most effective at strengthening memory if delivered while people are snoozing.

Three participants received stimulation in other parts of the brain, and they demonstrated mixed results the next day. The final three received stimulation during a mix of sleep phases. Those participants tended to perform worse on the memory test the following morning.

The researchers’ analysis of the participants’ brain activity revealed markers showing that information was flowing between the hippocampus and multiple parts of the prefrontal cortex during memory consolidation, providing concrete evidence to support the theory.

Looking ahead: This isn’t the first study to show that brain stimulation can be used to improve memory, but the fact that it works during sleep is new.

Given that we already know sleep plays an important role in memory consolidation, it’s possible that brain stimulation might be most effective at strengthening memory if delivered while people are snoozing.

“Our next challenge is whether we have the ability to modulate specific memories.”

Itzhak Fried

More research is needed to find out for sure. A sample size of 12 patients — only half of whom received the prefrontal cortex stimulation in deep sleep — is very small. But if it pans out in larger studies, doctors might be able to significantly improve the memories of people with Alzheimer’s, dementia, or other diseases that affect memory.

Fried’s team is also exploring whether AI can help identify the brain activity linked to specific memories, so that brain stimulation can be used to strengthen them during sleep.

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

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