Manual asymmetries and cortical modulation: relationships between motor control and interhemispheric connectivity.

Manual asymmetries in goal-directed aiming have been linked to interhemispheric interactions, yet it remains unclear whether experimentally modulating hemispheric activity alters both motor behavior and interhemispheric functional coupling. We investigated how bilateral transcranial direct current stimulation (tDCS) targeting primary motor cortices modulates manual asymmetries and interhemispheric connectivity during aiming. Twelve healthy right-handed male adults completed a protocol comprising three sessions: dominant hemisphere inhibition (DHI; cathode over C3/anode over C4), non-dominant hemisphere inhibition (NDHI; cathode over C4/anode over C3), and sham stimulation. In each session, participants performed a goal-directed aiming task with both hands before and after 20 min of tDCS. Behavioral outcomes included reaction time, movement time, response time, spatial accuracy (radial error), kinematic (peak velocity; relative time to peak velocity), and online control indices (number of discontinuities and first submovement error). EEG was recorded continuously, and interhemispheric coupling between motor regions was quantified in the high-alpha band during preparation and execution phases. tDCS produced selective behavioral effects, including condition-dependent changes in reaction time and response-time asymmetry, modulation of relative time to peak velocity, and increased movement discontinuities under DHI compared with sham, while movement time and several accuracy metrics showed minimal change. Interhemispheric coupling during preparation was largely unchanged; however, during execution, a robust Condition × Hand interaction emerged, with NDHI increasing coherence changes and coherence asymmetry relative to DHI. These findings suggest that bilateral tDCS can differentially shape interhemispheric functional coupling during movement execution and modulate specific temporal/kinematic components of aiming, with limited impact on overall movement time and endpoint accuracy.