No effect of anodal tDCS on motor cortical excitability and no evidence for responders in a large double-blind placebo-controlled trial.

Abstract Background Transcranial direct current stimulation (tDCS) has emerged as a non-invasive brain stimulation technique. Most studies show that anodal tDCS increases cortical excitability. However, this effect has been found to be highly variable. Objective To test the effect of anodal tDCS on cortical excitability and the interaction effect of two participant-specific factors that may explain individual differences in sensitivity to anodal tDCS: the Brain Derived Neurotrophic Factor Val66Met polymorphism (BDNF genotype) and the latency difference between anterior-posterior and lateromedial TMS pulses (APLM latency). Methods In 62 healthy participants, cortical excitability over the left motor cortex was measured before and after anodal tDCS at 2mA for 20 minutes in a pre-registered, double-blind, randomized, placebo-controlled trial with repeated measures. Results We did not find a main effect of anodal tDCS, nor an interaction effect of the participant-specific predictors. Moreover, further analyses did not provide evidence for the existence of responders and non-responders. Conclusion This study indicates that anodal tDCS at 2mA for 20 minutes may not reliably affect cortical excitability.