Comparative study of motor cortical excitability changes following anodal tDCS or high‐frequency tRNS in relation to stimulation duration

Background In this study, we investigate the capacity of two different non‐invasive brain stimulation (NIBS) techniques (anodal transcranial direct current stimulation (anodal tDCS) and high‐frequency transcranial random noise stimulation (hf‐tRNS)) regarding the relationship between stimulation duration and their efficacy in inducing long‐lasting changes in motor cortical excitability. Methods Fifteen healthy subjects attended six experimental sessions (90 experiments in total) and underwent both anodal tDCS of 7, 13, and 20 min duration, as well as high‐frequency 1mA‐tRNS of 7, 13, and 20 min stimulation duration. Sessions were performed in a randomized order and subjects were blinded to the applied methods. Results For anodal tDCS, no significant stable increases of motor cortical excitability were observed for either stimulation duration. In contrast, for hf ‐tRNS a stimulation duration of 7 min resulted in a significant increase of motor cortical excitability lasting from 20 to 60 min poststimulation. While an intermediate duration of 13 min hf‐tRNS failed to induce lasting changes in motor cortical excitability, a longer stimulation duration of 20 min hf‐tRNS led only to significant increases at 50 min poststimulation which did not outlast until 60 min poststimulation. Conclusion Hf‐tRNS for a duration of 7 min induced robust increases of motor cortical excitability, suggesting an indirect proportional relationship between stimulation duration and efficacy. While hf‐tRNS appeared superior to anodal tDCS in this study, further systematic and randomized experiments are necessary to evaluate the generalizability of our observations and to address current intensity as a further modifiable contributor to the variability of transcranial brain stimulation