Transcranial alternating current stimulation of the sensorimotor system: towards customized protocols

Abstract The use of Transcranial Alternating Current Stimulation (tACS) on the motor system is growing fast. However, it is as attractive as full of controversies due to the variety of the reported effects. It may now be possible to orchestrate the brain oscillatory activity by stimulating the brain trough external periodic electrical forces. However, the mere advantage of having specific frequency effects on brain functions, has been obscured by the reasonable lack of clear-cut evidence due to issues such as the amount of time to induce entrainment effects, the offline versus online effects, the phase effects and the individual differences such as age and brain state. Here we review a series of studies that break through and/ or consider such issues, revealing that tACS can be an optimal tool for investigating the sensorimotor system with great potential in clinical application. Moreover, we will show how it is possible to test online effects of tACS by a Transcranial Magnetic Stimulation (TMS) – tACS combined approach. Throughout the experiments, the navigated stimulation system is used to allow the exact repositioning of the TMS coil within and across the experimental conditions. Transcranial Alternating Stimulation of the primary motor cortex induces frequency- and state-dependent effects. Online tACS is more effective rather than offline stimulation. Therefore, We show that transcranial electrical stimulation effects are depending by several factors that have to be controlled such as electrodes montage, intensity, online and offline delivering and phase of the sinusoidal stimulation. tACS is an amazing tool to boost cortical excitability and induce enhancement of behavioral performance. The tACS research should step forward toward customized protocols that allow replicable clear effects. Acknowledgements: This work/article is an output of a research project implemented as part of the Basic Research Program at the National Research University Higher School of Economics (HSE University) Research Category and Technology and Methods Basic Research: 8. Transcranial Alternating Current Stimulation (tACS) Keywords: tACS, Motor cortex, TMS