by Figuring out how to improve one’s mental abilities has been of considerable interest to psychology and neuroscience researchers like myself for decades. From improving attention in high-risk environments like air traffic management to reviving memory in people with dementia, the ability to improve cognitive function could have far-reaching consequences. New research suggests that brain stimulation can help achieve the goal of increasing mental function.
At Boston University’s Reinhart Laboratory, my colleagues and I examined the effects of an emerging brain stimulation technology — transcranial alternating current stimulation, or tACS — on different mental functions in patients and healthy people.
During this procedure, people wear an elastic cap embedded with electrodes that deliver weak electrical currents oscillating at specific frequencies to the scalp. By applying these controlled currents to specific regions of the brain, it is possible to alter brain activity by stimulating neurons to fire rhythmically.
Why would rhythmically firing neurons be beneficial? Research suggests that brain cells communicate effectively when they coordinate the timing of their fires. Critically, these rhythmic patterns of brain activity show marked abnormalities during neuropsychiatric illnesses. The goal of tACS is to externally induce rhythmic brain activity that promotes healthy mental function, particularly when the brain may not be able to produce these rhythms on its own.
However, tACS is a relatively new technology and it is not yet clear how it works. Whether it can strengthen or revive brain rhythms to alter mental function has been a topic of considerable debate in the field of brain stimulation. While some studies find evidence of changes in brain activity and mental function with tACS, others suggest that the currents normally used in people may be too weak to have a direct effect.
When faced with conflicting data in the scientific literature, it can be helpful to conduct a type of study called a meta-analysis that quantifies the consistency of evidence across multiple studies. A previous meta-analysis conducted in 2016 found promising evidence for the use of tACS in changing mental function. However, the number of studies has more than doubled since then. The design of tACS technologies has also become increasingly sophisticated.
We set out to perform a new meta-analysis of studies using tACS to alter mental function. To our knowledge, this work is the largest and most comprehensive meta-analysis to date on this topic, consisting of over 100 published studies with a combined total of over 2,800 human participants.
After compiling over 300 measures of mental function across all studies, we observed consistent and immediate improvement in mental function with tACS. When we looked at specific cognitive functions such as memory and attention, we found that tACS produced the strongest improvements in executive function or adaptability in the face of new, surprising, or conflicting information.
We also saw improvements in the ability to pay attention and memorize information, both for short and long periods of time. Together, these results suggest that tACS could particularly improve specific types of mental function, at least in the short term.
To examine the effectiveness of tACS for those particularly vulnerable to changes in mental function, we examined data from studies that included older adults and people with neuropsychiatric conditions. In both populations, we observed reliable evidence of improvements in cognitive function with tACS.
Interestingly, we also found that a specialized type of tACS that can target two brain regions at the same time and manipulate how they communicate can improve or reduce cognitive function. This bidirectional effect on mental function can be particularly useful in the clinic. For example, some psychiatric conditions, such as depression, may involve a reduced ability to process rewards, while others, such as bipolar disorder, may involve a highly active reward processing system. If tACS can change mental function in any direction, researchers can develop flexible and targeted designs that meet specific clinical needs.
Developments in the field of tACS are bringing researchers closer to the ability to safely improve mental function in a non-invasive way that does not require medication. Current statistical evidence in the literature suggests that tACS has promise, and improving its design may help it to produce stronger, longer-lasting changes in mental function.
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