Transcranial alternating current stimulation (tACS) provides moderate cognitive benefits in healthy older adults as well as those with neuropsychiatric disorders, results of the largest and most comprehensive meta-analysis of tACS to date show.
"tACS has shown great promise at enhancing mental function, but whether this technology can truly fulfill its promise has been a topic of considerable debate in the field of brain stimulation," senior investigator Robert Reinhart, PhD, with the Cognitive & Clinical Neuroscience Laboratory, Boston University, told Medscape Medical News.
Given conflicting evidence on tACS for boosting cognition, Reinhart and colleagues leveraged statistical meta-analytic techniques to quantify how consistent the evidence is across several studies.
The time was right to do this, Reinhart said, as the number of tACS studies has "more than doubled since the previous meta-analysis and tACS designs have rapidly evolved, becoming increasingly sophisticated."
The study was published online May 24 in Science Translational Medicine.
Significant, Immediate Improvement
The analysis included 102 studies of tACS in healthy individuals as well as those with neurological or psychiatric conditions published between 2006 and 2021.
Together, these studies involved 2893 participants (1290 men and 1603 women) with a mean age of 30 years; 333 participants were older (mean age 67 years).
A total of 177 participants had a clinical disorder such as major depressive disorder, attention deficit hyperactivity disorder (ADHD), epilepsy, Parkinson's disease, schizophrenia, and mild cognitive impairment.
When compiling over 300 measures of mental function across all the studies, there was evidence for significant, reliable, and immediate improvement in mental function with tACS, Reinhart told Medscape Medical News.
When examining specific mental functions separately — such as memory, attention, or intelligence — tACS produced the strongest improvement in executive control, or the ability to adapt behavior in the face of new, surprising, or conflicting information, he noted.
"We also found improvements in the ability to pay attention, the ability to memorize information for both short and long periods of time, as well as in measures of intelligence. Together, these results suggest that tACS has the ability to particularly improve specific kinds of mental function, at least in the short term," Reinhart said.
To establish the effect of tACS in people who might most need it, the researchers examined how tACS impacted mental function in two subpopulations which may be particularly vulnerable to brain changes: older adults and clinical populations.
"In both subpopulations, we found reliable evidence for improvements in mental function with tACS. Interestingly, we also found that in specialized tACS, which can target two brain regions at the same time, manipulating the relationship between the two regions can both enhance or reduce mental function," Reinhart told Medscape Medical News
This bidirectional regulation of mental function could be particularly useful in the clinic, he noted.
"For example, conditions like depression may involve reduced reward-processing capabilities while others like bipolar disorder may involve a highly active reward-processing system. With the capability to change mental function in either direction, we may be able to flexibly develop targeted designs to cater to specific clinical needs," Reinhart said.
The researchers caution that while the analyses suggest immediate enhancements in cognitive function with tACS, they do not speak to the sustainability of these improvements. The durability of cognitive changes is a key question for future studies.
"Great Potential" but Questions Remain
Commenting on this research for Medscape Medical News, Shaheen Lakhan, MD, PhD, a neurologist and researcher in Boston, said tACS has shown "great potential" in modulating brain systems and circuits. "However, it is important to note that tACS is still in the early stages of development and requires further refinement."
"Before tACS can become widely applicable, several crucial aspects need to be addressed. Firstly, we need to determine the specific brain regions that should be targeted for optimal results. Additionally, we must identify which types of individuals would benefit the most from this technology," said Lakhan, who was not involved in the meta-analysis.
"Moreover, it is crucial to ascertain whether the benefits observed in controlled experiments truly translate into real-life scenarios such as driving a car, work performance, academic achievements, and improved social relationships. It is one thing to witness improvements on brain tests, but it is essential to understand the practical implications of these enhancements," Lakhan commented.
He said he envisions "a combination of drugs, devices, and applications will work together harmoniously within a closed system to modulate the brain, specifically targeting and alleviating conditions like depression, anxiety, and hyperactivity."
"However, this advancement raises profound questions about the boundaries between clinical use and neuro-enhancement, challenging our existing social constructs," said Lakhan.
"Our personalities, to a certain extent, are shaped by the intricate networks within our brains. Remarkably, our brain signatures can reveal aspects of our character, including agreeableness, openness, conscientiousness, neuroticism, and even our political affiliations," he added.
"As we venture into this exciting realm of brain unlocking and manipulation, it is imperative that we approach it with caution, ethical considerations, and a deep understanding of the potential consequences," Lakhan said.
"Only through responsible exploration and careful navigation can we fully harness the power of these technologies while respecting the complexities of our individual identities and the broader social implications they entail," he added.
This work was supported by grants from the National Institutes of Health and a gift from an individual philanthropist. Reinhart and Lakhan report no relevant financial relationships.
Sci Transl Med. Published online May 24, 2023. Abstract
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Cite this: Rapid Cognitive Improvement With Noninvasive Brain Stimulation - Medscape - May 26, 2023.