Abstract
Greater excitability in primary motor cortex (M1) is associated with increased preparatory activity in simple reaction time (RT) tasks. While evidence suggests that up-regulating cortical excitability using tDCS can facilitate simple RT, it remains unclear if similar RT facilitation can occur in choice RT as limited preparation occurs when the response is unknown in advance. The current experiment investigated whether increasing M1 excitability would lead to a decrease in choice RT for the limb contralateral to the stimulation. Additionally, it was hypothesized that if lateralized M1 excitability is related to response selection probability, the limb contralateral to the stimulation would be chosen more often during a free-choice task. Participants performed a choice RT task requiring either a 20? right or left wrist extension upon illumination of an associated stimulus (right or left box, respectively). In 20% of trials the central fixation illuminated, corresponding to a free-choice of either movement. Participants completed pre- and post-tDCS RT blocks of 100 trials. Between blocks, tDCS (1mA*10 min) was delivered over either the right or left motor representation for the wrist. Results showed that post-tDCS RTs were faster for only the contralateral hand in both forced-choice and free-choice trials (p’s<.05). Moreover, no change was observed in the proportion of contralateral hand responses on free-choice trials following tDCS. Together, these results suggest that M1 excitability is related to response initiation speed even when a choice is required. Furthermore, a larger facilitation of free-choice RT may also indicate that response selection processing was affected by tDCS.Acknowledgments: Supported by NSERC