Slow-Oscillatory Transcranial Alternating Current Stimulation Over the Primary Motor Cortex Improves Motor Skill Acquisition.

Slow-oscillatory brain activity (< 4 Hz) is crucial in memory consolidation and motor performance. Although transcranial alternating current stimulation (tACS) can modulate cortical oscillatory activity and influence motor learning, the effects of slow oscillation modulation via tACS remain unclear. This study investigated the effects of 0.75-Hz tACS applied to the primary motor cortex (M1) on the acquisition and retention of a visuomotor tracking task. Fifty-four right-handed healthy adults (mean age: 21.2 ± 0.6 years) were assigned to one of three groups: M1-tACS, Cz-tACS, or sham. In the M1-tACS and sham groups, electrodes were placed over the right M1 and left supraorbital ridge. In the Cz-tACS group, electrodes were placed over the Cz (International 10-20 system) and left supraorbital ridge to assess site specificity. tACS was administered for 30 min at 0.75 Hz and 1.0 mA, while participants practiced the visuomotor task. A retention test was conducted the following day. Motor learning was assessed using the power approximation index from the learning curve and error rates. The M1-tACS group showed a significantly lower approximation index than the sham group (p = 0.033), indicating enhanced learning. Error rates immediately after practice and on the following day were also significantly lower in the M1-tACS group (p = 0.039 and p = 0.007, respectively). No significant differences were found in the Cz-tACS group. These results suggest that slow-oscillatory tACS targeting the M1 facilitates motor skill acquisition and retention, with effects specific to the hand area of M1.