Gamma tACS over M1 and cerebellar hemisphere improves motor performance in a phase-specific manner

Transcranial alternating current stimulation (tACS) modulates neural connectivity in the cortical area under the electrodes. Although gamma tACS over the M1 and the cerebellar hemisphere has been shown to improve motor performance, the details of this effect remain unclear. This study aimed to clarify whether the effect of gamma tACS over the M1 and the cerebellar hemisphere is a phase-specific. We applied tACS at 70 Hz over the left M1 and the right cerebellar hemisphere to 20 healthy adults while they performed a visuomotor control task using their right index finger for 30 s. Three stimulation conditions were applied: (1) pseudo-stimulation (sham condition), (2) 180° phase difference in the two cortical target areas (anti-phase condition) and (3) 0° phase difference (in-phase condition). The anti-phase condition decreased task error compared to the sham condition (P =  0.021), but did not differ from the in-phase condition. Our study demonstrated that the effect of gamma tACS over the M1 and the cerebellar hemisphere has phase specificity. This result suggests that intercortical functional synchronization is involved in the improvement of motor performance during gamma tACS over the M1 and the cerebellar hemisphere.