Determining the corticospinal, intracortical and motor function responses to transcranial alternating current stimulation of the motor cortex in healthy adults: A systematic review and meta-analysis

Background: Transcranial Alternating Current Stimulation (tACS) employs low-intensity sinusoidal currents to influence cortical plasticity and motor function. Despite extensive research, inconsistent results require a comprehensive review of tACS efficacy. Objective: This study systematically assesses tACS effects on corticospinal and intracortical excitability, and motor function over the motor cortex (M1), focusing on alpha, beta, and gamma frequencies. Methods: Relevant studies were identified through database searches and citations were tracked until July 10, 2023. The methodological quality of the included studies (29) was evaluated by Downs and Black. Data synthesis involved meta-analysis (n = 25) and best evidence synthesis (n = 5). Results: Meta-analysis revealed that alpha and beta tACS with intensities > 1 mA and tACS with individualised alpha frequency (IAF) increased corticospinal excitability (CSE). tACS over M1 improved motor function, irrespective of stimulation frequency and intensity. Sub-analysis showed that alpha and beta tACS with an intensity ≤ 1 mA led to improved motor function, while gamma tACS at 2 mA enhanced motor function. Additionally, beta tACS at a fixed frequency of 20 Hz, as well as both low gamma (30–55) and high gamma (55–80) tACS, resulted in improved motor function. A stimulation duration of 20 min led to improvements in both CSE and motor function, and tACS with electrode sizes smaller than 35 cm² and an electrode montage over M1-supraorbital region (SOR) were found to enhance motor function. Notably, both online and offline tACS improved motor function, regardless of stimulation factors. Conclusion: tACS modulates CSE and improves motor function, with outcomes dependent on stimulation parameters and timing.