Effects of anodal tDCS of the lower limb M1 on ankle reaction time in young adults.

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that modulates cortical excitability and influences motor behavior. There is limited information available regarding the effects of anodal tDCS on lower limb reaction time. In this study, we aimed to investigate the effects of anodal tDCS on lower limb simple reaction time (SRT) and choice reaction time (CRT). We probed this question further by examining the effects of anodal tDCS of the lower limb M1 on an upper limb RT task and a cognitive measure. Fourteen healthy young adults received anodal tDCS and sham tDCS to the lower limb M1 on two separate testing days in a counterbalanced order. After stimulation, we assessed the effects of tDCS on ankle dorsiflexion SRT and CRT, ankle plantarflexion SRT and CRT, wrist extension SRT and CRT and the symbol digit modality test (SDMT). Anodal tDCS significantly improved response times from baseline for ankle CRT but not for ankle SRT or wrist SRT or CRT. A significant decrement (i.e., longer response time) was noted for the sham tDCS conditions. There was a significant difference between anodal and sham conditions for all RT tasks, suggesting that anodal tDCS improved RT compared to sham. No change in SDMT scores was observed for both conditions. Anodal tDCS appeared to differentially modulate ankle SRT and CRT, suggesting an influence of anodal tDCS on complex motor processes and/or the supplementary motor area. Absence of effects on wrist CRT or SDMT suggests a spatial specificity of the influence of tDCS. Anodal tDCS also appears to potentially negate the effects of fatigue or task switching that was detrimental to RT in the sham condition.