LTD-like plasticity of the human primary motor cortex can be reversed by γ-tACS

Background: Cortical oscillatory activities play a role in regulating several brain functions in humans. However, whether motor resonant oscillations (i.e. beta and gamma) modulate long-term depression (LTD)-like plasticity of the primary motor cortex (M1) is still unclear. Objective: To address this issue, we combined transcranial alternating current stimulation (tACS), a technique able to entrain cortical oscillations, with continuous theta burst stimulation (cTBS), a transcranial magnetic stimulation (TMS) protocol commonly used to induce LTD-like plasticity in M1. Methods: Motor evoked potentials (MEPs) elicited by single-pulse TMS, short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) were evaluated before and 5, 15 and 30 min after cTBS alone or cTBS delivered during beta-tACS (cTBS-beta) or gamma-tACS (cTBS-gamma). Moreover, we tested the effects of beta-tACS (alone) on short-latency afferent inhibition (SAI) and gamma-tACS on SICI in order to verify whether tACS-related interneuronal modulation contributes to the effects of tACS-cTBS co-stimulation. Results: cTBS-gamma turned the expected after-effects of cTBS from inhibition to facilitation. By contrast, responses to cTBS-beta were similar to those induced by cTBS alone. beta- and gamma-tACS did not change MEPs evoked by single-pulse TMS. beta-tACS reduced SAI and gamma-tACS reduced SICI. However, the degree of gamma-tACS-induced modulation of SICI did not correlate with the effects of cTBS-gamma. Conclusion: gamma-tACS reverses cTBS-induced plasticity of the human M1. gamma-oscillations may therefore regulate LTD-like plasticity mechanisms. (C) 2019 Elsevier Inc. All rights reserved.