Increasing human leg motor cortex excitability by transcranial high frequency random noise stimulation

Purpose: Transcranial random noise stimulation (tRNS) can increase the excitability of hand area of the primary motor cortex (M1). The aim of this study was to compare the efficacy of tRNS and transcranial direct current stimulation (tDCS) on the leg motor cortex. Method: Ten healthy subjects received anodal, cathodal tDCS, tRNS and sham stimulation for 10 min using 2 mA intensity during separate experimental sessions. Single pulse transcranial magnetic stimulation (TMS) induced motor evoked potential (MEP) measurements were used to assess motor cortical excitability changes after the stimulation. Results: Similar to the hand area, we found that both tRNS and anodal tDCS induced an increase of the amplitude of the MEPs. Anodal tDCS induced a constant gradual increase of corticospinal excitability until 60 min post-stimulation, whereas the effect of tRNS was immediate with a duration of 40 min following stimulation. The cathodal tDCS induced decrease in MEP amplitude did not reach statistical significance. Conclusion: Our results suggest that although the leg area has a deeper position in the cortex compared to the hand area, it can be reached by weak transcranial currents. Both anodal tDCS and tRNS had comparable effect on cortical excitability.