Transcranial alternating current stimulation (tACS) can enhance memory during sleep, laying the groundwork for new treatments. A non-invasive method to potentially help people with conditions such as autism, Alzheimer’s disease, schizophrenia, and depression.
For years, researchers have recorded electrical brain activity that oscillates or alternates during sleep on an electroencephalogram (EEG) as waves called sleep spindles. And scientists have suspected their involvement in cataloging 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 UNC neuroscientist Flavio Frohlich, PhD, assistant professor of psychiatry and member of the UNC Neuroscience Center. “They could’ve been merely byproducts 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 everyday life. And we can target them to enhance memory.”
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.
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 (see Neuroscience researchers caution public about hidden risks of self-administered brain stimulation*).
“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,” said Caroline Lustenberger, PhD, first author and postdoctoral fellow in the Frohlich lab. “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 type of non-invasive brain stimulation in patients that have known deficits in these spindle activity patterns. Based on the Current Biology paper, it’s clear the team is just getting started, with a lot of interesting possibilities to explore. For example, they note that “it is still unclear which specific cortical regions might be involved in sleep-dependent memory consolidation.”
It’s a sort of jungle in there, one with unknown species. The researchers say they may target posterior brain regions using faster frequencies (e.g., 15 Hz tACS) to optimally benefit motor memory consolidation, for example, and that maybe they should try synchronization of frontal oscillatory activity. And they want to find out if spindles synchronized across cortical regions are “essential for memory consolidation to occur” or are only spindles localized to brain regions necessary for performing the task?
Future studies will also be needed to investigate more complex ‘‘real-life’’ motor tasks that benefit from sleep and to relate those findings to sleep spindles, they add. “Future studies are also needed to further find optimized stimulation parameters by means of ideal stimulation location (centro-parietal instead of frontal) and (spindle) frequency applied.”