Transcranial alternating current stimulation, or tACS is a non-invasive tool to modulate brain waves, which can change behavioral performance. How does it work?

Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation method used in neuroscience research. Rather than increasing brain activity, such as is done by transcranial magnetic stimulation (TMS) or transcranial direct current stimulation (tDCS), the goal of tACS is to modulate neural oscillations, also known as brain waves.

A weak alternating current is applied to the head. Although only a small amount of the already weak current reaches the brain, it is enough to affect underlying neurons. tACS is not strong enough to induce neuronal spikes, but it can bias underlying brain cells to fire more rhythmically, in the applied frequency. This is referred to as entrainment.

Indeed, computational studies as well as animal research has shown that tACS can affect neural oscillations. In humans evidence it is more difficult to provide direct evidence. On top of that, results on actual changes in behavioral performance are mixed. Some evidence suggests that tACS can increase motor, perceptual and cognitive performance. But also a lot of null-results are reported. This is likely due to the large inter-individual variability that is related to tACS.

References/resources:
Herrmann, C. S., Rach, S., Neuling, T., & Strüber, D. (2013). Transcranial alternating current stimulation: a review of the underlying mechanisms and modulation of cognitive processes. Frontiers in human neuroscience, 7, 279. https://doi.org/10.3389/fnhum.2013.00279

Johnson, L., Alekseichuk, I., Krieg, J., Doyle, A., Yu, Y., Vitek, J., Johnson, M., & Opitz, A. (2020). Dose-dependent effects of transcranial alternating current stimulation on spike timing in awake nonhuman primates. Science advances, 6(36), eaaz2747. https://doi.org/10.1126/sciadv.aaz2747

Klink, K., Paßmann, S., Kasten, F. H., & Peter, J. (2020). The Modulation of Cognitive Performance with Transcranial Alternating Current Stimulation: A Systematic Review of Frequency-Specific Effects. Brain sciences, 10(12), 932. https://doi.org/10.3390/brainsci10120932

Opitz, A., Paulus, W., Will, S., Antunes, A., & Thielscher, A. (2015). Determinants of the electric field during transcranial direct current stimulation. NeuroImage, 109, 140–150. https://doi.org/10.1016/j.neuroimage....

Schutter D. J. (2014). Syncing your brain: electric currents to enhance cognition. Trends in cognitive sciences, 18(7), 331–333. https://doi.org/10.1016/j.tics.2014.0...

Suen, P., Doll, S., Batistuzzo, M. C., Busatto, G., Razza, L. B., Padberg, F., Mezger, E., Bulubas, L., Keeser, D., Deng, Z. D., & Brunoni, A. R. (2021). Association between tDCS computational modeling and clinical outcomes in depression: data from the ELECT-TDCS trial. European archives of psychiatry and clinical neuroscience, 271(1), 101–110. https://doi.org/10.1007/s00406-020-01...

Narrated by: Miles Wischnewski

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