Electric brain stimulation during sleep does boost memory

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For the first time, in a small study, University of North Carolina  scientists report using trans-cranial alternating current stimulation (tACS), to target a specific kind of brain activity during sleep and strengthen memory in healthy people.

The findings offer a non-invasive method to potentially help millions of people with conditions such as autism, Alzheimer’s disease, schizophrenia, and major depressive disorder.

The findings offer a non-invasive method to potentially help millions of people with conditions such as autism, Alzheimer’s disease, schizophrenia, and major depressive disorder.

When you sleep, your brain is busy storing and consolidating things you learned that day, stuff you’ll need in your memory toolkit tomorrow, next week, or next year. For many people, especially those with neurological conditions, memory impairment can be a debilitating symptom that affects every-day life in profound ways.

For years, researchers have recorded electrical brain activity that oscillates or alternates during sleep; they present as waves on an electroencephalogram (EEG). These waves are called sleep spindles, and scientists have suspected their involvement in cataloguing and storing memories as we sleep.

“But we didn’t know if sleep spindles enable or even cause memories to be stored and consolidated,” said senior author Dr Flavio Frohlich, assistant professor of psychiatry and member of the UNC Neuroscience Centre. “They could’ve been merely by-products of other brain processes that enabled what we learn to be stored as a memory. But our study shows that, indeed, the spindles are crucial for the process of creating memories we need for every-day life. And we can target them to enhance memory.”

This marks the first time a research group has reported selectively targeting sleep spindles without also increasing other natural electrical brain activity during sleep. This has never been accomplished with tDCS – transcranial direct current stimulation – the much more popular cousin of tACS in which a constant stream of weak electrical current is applied to the scalp.

During Frohlich’s study, 16 male participants underwent a screening night of sleep before completing two nights of sleep for the study. Before going to sleep each night, all participants performed two common memory exercises – associative word-pairing tests and motor sequence tapping tasks, which involved repeatedly finger-tapping a specific sequence. During both study nights, each participant had electrodes placed at specific spots on their scalps. During sleep one of the nights, each person received tACS – an alternating current of weak electricity synchronized with the brain’s natural sleep spindles. During sleep the other night, each person received sham stimulation as placebo.

Each morning, researchers had participants perform the same standard memory tests. Frohlich’s team found no improvement in test scores for associative word-pairing but a significant improvement in the motor tasks when comparing the results between the stimulation and placebo night. “This demonstrated a direct causal link between the electric activity pattern of sleep spindles and the process of motor memory consolidation.” Frohlich said.

Dr Caroline Lustenberger, first author and post-doctoral fellow in the Frohlich lab, said, “We’re excited about this because we know sleep spindles, along with memory formation, are impaired in a number of disorders, such as schizophrenia and Alzheimer’s. We hope that targeting these sleep spindles could be a new type of treatment for memory impairment and cognitive deficits.”

Frohlich said, “The next step is to try the same intervention, the same type of non-invasive brain stimulation, in patients that have known deficits in these spindle activity patterns.”

Frohlich’s team previously used tACS to target the brain’s natural alpha oscillations to boost creativity. This was a proof of concept. It showed it was possible to target these particular brain waves, which are prominent as we create ideas, daydream, or meditate. These waves are impaired in people with neurological and psychiatric illnesses, including depression.

Abstract
Transient episodes of brain oscillations are a common feature of both the waking and the sleeping brain. Sleep spindles represent a prominent example of a poorly understood transient brain oscillation that is impaired in disorders such as Alzheimer’s disease and schizophrenia. However, the causal role of these bouts of thalamo-cortical oscillations remains unknown. Demonstrating a functional role of sleep spindles in cognitive processes has, so far, been hindered by the lack of a tool to target transient brain oscillations in real time. Here, we show, for the first time, selective enhancement of sleep spindles with non-invasive brain stimulation in humans. We developed a system that detects sleep spindles in real time and applies oscillatory stimulation. Our stimulation selectively enhanced spindle activity as determined by increased sigma activity after transcranial alternating current stimulation (tACS) application. This targeted modulation caused significant enhancement of motor memory consolidation that correlated with the stimulation-induced change in fast spindle activity. Strikingly, we found a similar correlation between motor memory and spindle characteristics during the sham night for the same spindle frequencies and electrode locations. Therefore, our results directly demonstrate a functional relationship between oscillatory spindle activity and cognition.

Authors
Caroline Lustenberger, Michael R Boyle, Sankaraleengam Alagapan, Juliann M Mellin, Bradley V Vaughn, Flavio Fröhlich

University of North Carolina School of Medicine material
Current Biology abstract


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