Anodal tDCS over SMA decreases the probability of withholding an anticipated action

Previous research has shown that the supplementary motor area (SMA) is critical in movement inhibition. Recently it was shown that applying transcranial direct current stimulation (tDCS) over SMA affected participants' ability to inhibit their movement in a stop-signal reaction time task (Hsu et al. [11]). Of interest in the current study was whether modulating SMA excitability using tDCS would have similar effects in an anticipation-timing stop-signal task. Participants performed 2 sessions each consisting of a pre- and post-tDCS block of 160 trials in which they were instructed to extend their wrist concurrently with the arrival of a pointer to a target (i.e., a clock hand reaching a set position). In 20% of trials (stop trials) the pointer stopped 80, 110, 140, 170, or 200 ms prior to the target, and on these trials participants were instructed to inhibit their movement if possible. Anodal and cathodal tDCS (separated by at least 48 h) was applied for each participant between the pre- and post-tDCS blocks. No change in the proportion of successfully inhibited movements on stop trials was found following cathodal tDCS (p>.05). However, anodal tDCS resulted in a decreased proportion of successfully inhibited movements on stop trials (p=002), and an earlier movement onset on control trials (p<.01). This suggests that the SMA may be more involved in initiation than in inhibition of anticipatory movements. Furthermore these data suggest that differences in initiation and inhibitory processes exist between stop-signal reaction time and anticipation-timing stop-signal tasks.