A) Stimulated putative norm electric field modeling for tACS electrode montage over the left and right M1 (left and middle panels). A 4 × 1 electrode montage was used for each M1. Currents with peak-to-peak amplitudes of 1 mA were delivered to the stimulation sites with 0° phase differences. The current flow stimulation was conducted via SimNiBS 4.0 (Saturnino et al. 2019) on the MNI152 head model. Peak electric field strength was defined as the 99.9th percentile of the field. MNI coordinates of the human motor area template (Mayka et al. 2006) were used to estimate the mean electric field strength at left M1 (−37, −21, 58; radius 10 mm) and right M1 (37, −21, 58; radius 10 mm). Vol50 and Vol75: mesh volume with field strength ≥50% of the 99.9th percentile and ≥75% of the 99.9th percentile, respectively. B) Schematic representation of the procedural order of tasks and timeline during tACS and sham sessions. Each participant attended one tACS and one sham session. PPT = Purdue Pegboard Task, ds-TMS = dual-site transcranial magnetic stimulation, rsEEG = resting-state electroencephalography, tACS = transcranial alternating current stimulation. Note: A human figure was generated using Microsoft Copilot.