Anodal tDCS over the supplementary motor area increases motor overflow during imagined aiming movement

Neural networks subserving motor imagery and action observation overlap with those involved in motor execution. The precise roles of specific cortical areas in these processes are not fully understood. One cortical area implicated in motor imagery is the supplementary motor area (SMA). To test the role of SMA in motor imagery, participants without any known neurological disorders (N = 23) completed three versions of a reciprocal manual aiming task (imagined, observed, and executed movements) before and after transcranial direct current stimulation (tDCS) was applied to the SMA. Anodal, cathodal, and sham (inactive) stimulation protocols were compared within participants. Although the speed-accuracy trade-off observed in the aiming task was not affected by tDCS, the amount of incidental movement during imagery (motor overflow) increased following anodal stimulation. Additionally, imagined movement time was associated less strongly with an implicit measure of motor imagery (hand laterality judgement) following anodal tDCS compared to the sham condition. No effects of tDCS were found for action observation or movement execution. These results suggest that anodal stimulation applied over SMA may subtly disrupt processes underlying motor imagery, either through unintended inhibitory effects or via excitatory effects on adjacent areas of the motor cortex.