Transcranial alternating current stimulation improves ballistic motor performance in trained and untrained limbs of healthy older adults.

Transcranial alternating current stimulation (tACS) using a combined theta-gamma waveform can improve unilateral ballistic motor performance in the trained limb of older adults. The purpose of this study was to examine the effect of individual (theta) and combined (theta-gamma) tACS waveforms on ballistic motor performance in the trained and untrained contralateral limb (i.e. cross-limb transfer) of older adults. Sixty right-handed healthy older adults (68.9 ± 5.2 years) received either high-definition theta-gamma (6 Hz theta, 75 Hz gamma), theta (6 Hz), or sham tACS over right primary motor cortex (M1) during ballistic left-thumb abduction training for 20 min. Behavioural changes were quantified as changes in trained (left) and untrained (right) thumb acceleration. Transcranial magnetic stimulation (TMS) was used to assess changes in left and right M1 excitability, via motor-evoked potentials (MEPs) and paired-pulse short-interval intracortical inhibition (SICI). While training drove greater motor performance in both hands (all P < 0.001), theta tACS was associated with the largest improvements (P < 0.02, compared with theta-gamma and sham tACS) and was the only condition that demonstrated a post-training potentiation of MEP amplitude (P < 0.02). However, a positive relationship between performance improvement of trained and untrained hands was limited to the theta-gamma tACS condition (R2 = 0.501, P < 0.001). These results suggest that specific tACS waveforms may have unique effects on different facets of motor learning; while theta tACS can augment performance gain, theta-gamma tACS may influence cross-limb transfer of performance.