Effects of beta-tACS on corticospinal excitability: A meta-analysis

Over the past decade several studies have shown that transcranial alternating current stimulation (tACS) delivered at the beta (15-25 Hz) frequency range can increase corticospinal excitability of the primary motor cortex (M1). The aim of this study was to systematically quantify the effect size of beta-tACS on corticospinal excitability in healthy volunteers, as well as to identify significant outcome predictors. A meta-analysis was performed on the results of 47 experiments reported in 21 studies. Random effects modelling of the effect sizes showed that beta-tACS significantly increases M1 excitability ((E) over bar = 0.287, 95% CI = 0.133-0.440). Further analysis showed that tACS intensities above 1 mA peak-to-peak yield a robust increase in M1 excitability, whereas intensities of 1 mA peak-to-peak and below do not induce a reliable change. Additionally, results showed an impact of tACS montages on these effects. No difference in effect size for online compared to offline application of tACS was found. In conclusion, these findings indicate that beta-tACS can increase cortical excitability if stimulation intensity is above 1 mA, yet more research is needed to titrate the stimulation parameters that yield optimal results. (C) 2019 The Authors. Published by Elsevier Inc.