2023-05-25 17:36:11
Brain electrical stimulation would be a promising non-invasive alternative to treat a wide range of neurological diseases and psychiatric disorders. However, the great diversity of data requires cross-checking, in order to assess the true benefits. It is in this vision that a meta-analysis of more than 100 studies came to the conclusion that transcranial stimulation by rhythmic current considerably improves cognitive functions.
To ensure optimal cognitive performance, brain waves are governed by a complex arrangement of rhythms at both unique and synchronized frequencies. This deep organization helps improve attention within high-stakes environments, or retain short- and long-term memory. Thus, the synchronization of neural activity is particularly important for computationally demanding cognitive processes, such as working memory.
Finding ways to improve cognitive performance is a major clinical challenge, especially for people with neuropsychiatric disorders or neurodegenerative diseases. Many studies have suggested that brain stimulation might help improve these performances, by acting directly on the synchronization of neuronal activity.
Conflicting data
Different forms of brain stimulation have been suggested and have shown promise in reducing the symptoms of various pathologies, such as Alzheimer’s and Parkinson’s. These alternatives would be new non-invasive and inexpensive solutions, potentially supporting existing therapeutic strategies. The meta-analysis, carried out by researchers at Boston University, specifically concerns transcranial alternating current stimulation (tACS), with the aim of improving cognitive functions in sick and healthy people.
This strategy would have the advantage of acting in such a way as to improve the synchronization of the rhythm of activity (or action potential) between the neurons. The summary, published in the journal Science
Translational Medicine, supports the hypothesis that neurons communicate more effectively with each other when they coordinate the rhythm of their activities. This coordination scheme would also be profoundly altered in people suffering from neuropsychiatric disorders.
However, the idea being recent, its real benefits on the brain are not yet well enough defined. Furthermore, “the possibility of enhancing or reviving brain rhythms to alter mental function has been the subject of considerable debate in the field of brain stimulation”, underline Shrey Grover, a psychology and neurology researcher at Boston University and lead author of the study.
According to the expert, some research highlights evidence of real positive changes in brain activity following tACS. Others believe, however, that the intensity of the stimulation is not high enough to induce convincing effects. These conflicting data underscore the need for meta-analyses, quantifying the consistency of evidence for the effectiveness of this form of therapy.
One of them, dating from 2016, revealed strong evidence for the potential of tACS for the modification of cognitive functions. However, the number of new studies has more than doubled since then, not to mention that the technologies supporting the therapy are always more sophisticated – which introduces a significant heterogeneity in the stimulation protocols. The Boston meta-analysis, involving more than 100 different studies, would be the most comprehensive and extensive to date.
Two-way stimulation
The tACS procedure consists of attaching electrodes delivering weak electrical currents to the scalp of the patient to be treated. The diffused current oscillates at specific frequencies and targets predetermined brain regions. Unlike transcranial direct current stimulation (tDCS), the protocol allows the administration of a sinusoidal electric current, adjusted to the internal brain frequency specific to each individual. The frequencies introduced are thus intended to induce the neurons to activate rhythmically, especially if the synchronization of their action potential is altered by an underlying pathology.
The new study compiles no less than 300 measurements on more than 2800 volunteers. The results showed a marked and consistent improvement in mental function following tACS. Looking at specific cognitive functions such as memory and attention, the researchers observed the greatest improvements in executive function, or the ability to adapt to new information (especially when suddenly surprised by a news). The same result was observed for short-term and long-term memory.
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To examine the effects of tACS in a pathological context, researchers looked at studies that included older people and people with neuropsychiatric conditions. Again, reliable evidence indicating an improvement in cognitive performance was found.
In addition, a tACS protocol involving the simultaneous stimulation of two different brain regions particularly appealed to the researchers. This technique aims to control how the two regions communicate with each other, and can both reduce or improve cognitive function. This double effect would be particularly promising for controlling certain psychiatric conditions such as depression, during which the region processing the reward is very inactive; or even bipolar disorder, where this region is activated in an exacerbated way.
« Current statistical evidence in the literature suggests that tACS holds promise and that improving its design might help it produce stronger and lasting changes in mental function. concludes Grover.
Source : Science Translational Medicine
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