LOS ANGELES — Scientists and doctors have long agreed that our brains work to consolidate and strengthen our memories while we sleep, but how that process actually plays itself out each night is still very much up for debate. Now, new collaborative research led by scientists at UCLA Health and Tel Aviv University has uncovered the first physiological evidence from inside the human brain supporting the dominant scientific theory regarding how the brain consolidates memory during sleep.
Study authors found that targeted deep-brain stimulation during a key time in the sleep cycle appears to improve memory consolidation. All in all, the research team believes this work offers important new clues when it comes to how deep-brain stimulation during sleep may one day aid patients living with memory disorders like Alzheimer’s disease.
These findings were made possible by a novel “closed-loop” system that delivered electrical pulses to one brain region precisely synchronized to brain activity recorded from another region.
According to the current dominant theory for how the brain converts new information into long-term memories during sleep, a type of “overnight dialogue” takes place between the hippocampus (the brain’s memory center) and the cerebral cortex — another brain region associated with higher brain functions like reasoning and planning. This takes place during a phase of deep sleep, in which brain waves are very slow and neurons across brain regions alternate between rapidly firing in sync and silence.
“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,” says study co-author Itzhak Fried, the director of epilepsy surgery at UCLA Health and professor of neurosurgery, psychiatry, and biobehavioral sciences at the David Geffen School of Medicine at UCLA, in a university release. “It has both scientific value in terms of understanding how memory works in humans and using that knowledge to really boost memory.”
Study authors had a unique opportunity to test this memory-sleep theory through the use of electrodes in the brains of 18 epilepsy patients at UCLA Health. Those electrodes had originally been implanted in patients’ brains to help identify the source of their seizures during hospital stays typically lasting around a week and a half.
This project was conducted over the course of two nights and mornings. Before bedtime, each participant viewed photo pairings of animals and 25 celebrities, including easily identifiable stars such as Marilyn Monroe or Jack Nicholson. The team then immediately tested the group on their capacity to remember which celebrity was paired with which animal, and then tested them again the following morning after a night of undisturbed sleep.
On the second night, participants viewed 25 new animal and celebrity pairings before going to sleep. This time around, however, they received targeted electrical stimulation overnight, before their ability to recall the pairings was tested in the morning. In order to deliver the electrical stimulation, researchers created a real-time closed-loop system that Prof. Fried likens to a musical conductor; the system “listens” to brain’s electrical signals, and when patients fall into the period of deep sleep associated with memory consolidation, it delivers gentle electrical pulses instructing the rapidly firing neurons to “play” in sync.
Each patient performed better on the memory tests after getting a night of sleep with the electrical stimulation in comparison to a night of undisturbed sleep. Key electrophysiological markers also suggest that information was flowing between the hippocampus and throughout the cortex, providing pivotal physical evidence supporting memory consolidation.
“We found we basically enhanced this highway by which information flows to more permanent storage places in the brain,” Prof. Fried adds.
In 2012, Prof. Fried authored a New England Journal of Medicine study that showed, for the first time ever, that electrical stimulation can strengthen memory. Since then, his work has continued to explore how deep brain stimulation may improve memory. He recently received a $7 million NIH grant to study if artificial intelligence can help locate and strengthen specific memories in the mind.
“In our new study, we showed we can enhance memory in general,” Prof. Fried concludes. “Our next challenge is whether we have the ability to modulate specific memories.”
The study is published in the journal Nature Neuroscience.
You might also be interested in:
- Deep brain stimulation emerges as ‘promising’ Alzheimer’s treatment
- Deep sleep can protect older adults from Alzheimer’s-related memory loss
- Lamps zapping the brain with light beams may help reverse symptoms of dementia