Study Questions Mechanism Behind Transcranial Alternating-Current Stimulation

By Megan Brooks

NEW YORK—Several studies have suggested that applying low-frequency transcranial alternating-current stimulation (tACS) during sleep can improve memory, perhaps by synchronizing and enhancing neural rhythms related to memory. But a new study has found no obvious effect of tACS on brain wave activity.

It was “very surprising” to find that low-frequency tACS did not affect brain waves, Dr. Anli Liu, assistant professor of neurology of NYU School of Medicine and neurologist at NYU Langone Health's Comprehensive Epilepsy Center, noted in an interview with Reuters Health.

“When we started the study, we were hoping to find the opposite because our intention was to understand the physiology so we could maximize the tACS protocol. We were surprised that we couldn't find anything, even when we increased the stimulation intensity and looked at the data every way possible to try to find even a small effect size,” Dr. Liu said.

Dr. Liu and her colleagues studied 17 patients undergoing intracranial monitoring for epilepsy surgery. They tested the same tACS protocol that has been applied during sleep to increase brain rhythms supportive of memory function but did not find any measurable effect.

“These findings should give researchers pause and encourage further study into the mechanisms behind tACS,” Dr. Liu said in a news release. The study was published online October 31 in Nature Communications.

Dr. Liu told Reuters Health that tACS has received a lot of “attention and excitement within the noninvasive neuromodulation community. Several labs have tried to use this protocol to enhance brain activity by applying during wakefulness and sleep. We applied it in the same way that has been used in several previous studies to try to enhance sleep-brain activity, and unfortunately we didn't find a physiological effect.”

Dr. Liu emphasized that her team’s findings apply only to this specific low-frequency tACS protocol. “We can't say anything about higher frequencies of tACS applied, or transcranial direct-current stimulation (tDCS), which are different protocols,” she said.

“We did not intend to discredit anyone's work,” Dr. Liu said, “and I don't think the take-home message should be that the entire field of transcranial electrical stimulation should be discredited. People involved in this field are interested in developing non-invasive therapies that may eventually help patients.”

“However, I think one of the takeaway points is that our understanding of the way this kind of stimulation works is way too simplistic, especially in relation to how complex the brain is. I think we have disproved a dominant theory as to how stimulation might work. We don't think that it enhances brain activity at least in the way that it is commonly applied,” Dr. Liu said.

“We likely need to increase the amount of current that actually reaches the brain surface to enhance brain activity. And we need to explore other potential mechanisms for how transcranial electrical stimulation might work; maybe it is affecting supportive neural tissue, or maybe it's working through peripheral sensory effects, or there might be some placebo effects involved,” said Dr. Liu. “Once we have a better understanding of the mechanism, we can move forward as a field to try to maximize these intermediate pathways, and begin to bring therapies to patients.”

SOURCE: https://go.nature.com/2z0dcMm

Nat Comm 2017.

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