The so-called ‘refreshing sleep’ is not an empty definition, but has a scientific basis. For example, it is known that sleep plays a crucial role in strengthening memory, but scientists are still trying to decode how this process takes place in the brain during the night. New research led by scientists at UCLA (University of California Los Angeles) Health and Tel Aviv University provides the first physiological evidence from inside the human brain to support the mainstream scientific theory of how memory consolidation occurs. But there’s more: Scientists also found that targeted deep brain stimulation during a crucial phase of the sleep cycle appears to enhance this process.
The study, published in ‘Nature Neuroscience’, could therefore offer new clues on the possible role of a Deep-brain stimulation: this, applied during sleep, could one day help patients with memory disorders such as Alzheimer’s disease, suggests the co-author of the work, Professor of Neurosurgery Itzhak Fried. This was achieved with a novel ‘closed-loop’ system that fired electrical impulses into one region of the brain precisely synchronized with recorded brain activity from another region.
The brain, according to the prevailing theory, converts new information into long-term memories during ‘shuteye’, when you sleep. There is a nocturnal dialogue between the hippocampus – the brain’s memory hub – and the cerebral cortex, which is associated with higher brain functions such as reasoning and planning. And this dialogue occurs during a phase of deep sleep, when brain waves are particularly slow and neurons in regions of the brain alternate between firing rapidly in sync and silence. The team’s work provides the “first major evidence down to the level of single neurons that there really is this mechanism of interaction between the memory center and the entire cortex,” says Fried, director of epilepsy surgery at UCLA Health. ‘It has scientific value both in terms of understanding how memory works in humans and using that knowledge to really boost memory.’
The memory consolidation theory was tested by the study authors at UCLA Health, using electrodes in the brains of 18 patients with epilepsy. The electrodes had been implanted on the patients to help identify the source of their seizures during hospital stays that typically last around 10 days. This analysis was also carried out on that occasion. The study was conducted over two nights and two mornings. Just before bedtime, participants were shown photo pairs of animals and 25 celebrities, including easily identifiable stars like Marilyn Monroe and Jack Nicholson. Patients were tested immediately for their ability to remember which celebrity matched which animal, and the same test was repeated in the morning after a night of undisturbed sleep.
On another night 25 new animal and celebrity pairings were shown before bedtime. This time, however, the patients received targeted electrical stimulation at night and their ability to remember mates was tested in the morning. To deliver this electrical stimulation, the researchers created a real-time closed-loop system: the system ‘listened to’ electrical signals from the brain and, when patients fell into the deep sleep period associated with memory consolidation, it emitted gentle electrical impulses. instructing rapidly firing neurons to ‘play’ in synchrony. Like a conductor, is the image used by Fried. Result: Every person tested performed better on memory tests after a night of sleep with electrical stimulation, compared to a night of undisturbed sleep. Key electrophysiological markers also indicated that information flowed between the hippocampus and throughout the cortex, providing physical evidence supporting memory consolidation.
“We have found that we have substantially improved this highway through which information flows to more permanent storage locations in the brain,” said Fried, who in 2021, with a study published in the ‘New England Journal of Medicine’, had demonstrated for the first time electrical stimulation can strengthen memory. The scientist recently received 7 million dollars from the National Institutes of Health (NIH) to study whether artificial intelligence can help in certain activities. While this study has shown the ability to improve memory in general, “the next challenge is to find out whether we have the ability to modulate specific memories.”