Bilateral M1 tDCS selectively modulates dynamic large-scale brain states associated with sensorimotor control relative to sham stimulation

Transcranial direct current stimulation of the primary motor cortex is widely used to influence sensorimotor performance, yet its large-scale neural mechanisms remain unclear. This study examined how unilateral and bilateral stimulation of the primary motor cortex affect sensorimotor behavior and whole-brain dynamics. Twenty healthy adults participated in a double-blind crossover study that combined robotic motor tasks performed during stimulation (on-task) with resting-state functional magnetic resonance imaging acquired during stimulation (off-task). We found that both unilateral and bilateral stimulation improved bimanual coordination compared to sham, with no difference between montages. Bilateral stimulation additionally increased proprioceptive variability in the left hand. On the neural level, only bilateral stimulation, as compared with sham, altered large-scale dynamics, increasing the occurrence rate of a frontoparietal and dorsal attention state and reducing transitions from a default mode-frontoparietal state to a thalamic-subcortical state. These brain dynamics were associated with sensorimotor performance in an exploratory analysis, thereby potentially linking stimulation-sensitive CAPs to motor performance.