The Effects of High-Definition Transcranial Alternating Current Stimulation (HD-tACS) on Dual-Task Performance and Upper-Limb Multi-Joint Motor Control

This study investigated the effects of high-definition transcranial alternating current stimulation (HD-tACS) on cognitive–motor dual-task performance and upper-limb multi-joint motor control. Sixteen right-handed healthy male participants completed a double-blind, randomized crossover experiment involving active tACS and sham tACS conditions separated by a one-week washout interval. A 4 × 1 HD-tACS montage was applied over the left dorsolateral prefrontal cortex at 5 Hz for 20 min. Participants performed a synchronous cognitive–motor dual-task paradigm that combined an N-back working memory task with an upper-limb throwing task under different task difficulty levels. Task performance outcomes included accuracy, reaction time, release velocity, peak velocity, movement duration, and dual-task cost. Upper-limb motor control outcomes included elbow and wrist joint range of motion, peak angular velocity, and peak joint flexion and extension moments. Accuracy showed a significant Stimulation × Time interaction. Under active stimulation, adjusted accuracy increased by 5.4 percentage points in N1 and 10.1 percentage points in N2, whereas the corresponding changes under sham stimulation were not significant. Reaction time, release velocity, peak velocity, and movement duration showed no significant stimulation-related changes. Release-velocity DTC also showed a significant Stimulation × Time interaction across N1 and N2. Under active stimulation, the signed DTC shifted upward, indicating a smaller release-velocity decrement relative to N0. The task-specific follow-up was significant for N2 but not N1; however, the Stimulation × Time × Task interaction was not significant. Most upper-limb kinematic and kinetic outcomes showed no significant stimulation-related changes. Active HD-tACS was associated with higher accuracy and a favorable shift in release-velocity DTC, while most motor, kinematic, and kinetic outcomes were unchanged. The effects were limited to selected behavioral outcomes and did not differ significantly between N1 and N2.