Assessing the effect of tDCS on inhibitory capacity: A pre-registered comparison of high definition and conventional montages on the stop signal task

Transcranial direct current stimulation (tDCS) is a promising tool for experimental and rehabilitation purposes, but its effects appear highly variable due to methodological and physiological factors. To overcome this issue, tDCS protocols combining computational modeling of electric fields and high-definition (HD)-tDCS are increasingly used to decrease the heterogeneity of tDCS effects in the motor as well as cognitive domains. However, the effects of HD-tDCS compared to conventional bipolar tDCS on behavioral variables and cortico-spinal excitability remain controversial. In this pre-registered study, we aimed to compare the differential effects of anodal conventional bipolar montage and anodal HD-tDCS montage, built upon an optimized computational model of electric field distribution, on cognitive performance in 48 participants. The Stop Signal Task (SST) was used to measure response inhibition, based on previous meta-analytic evidence showing a significant modulation of SST inhibitory performance when applying tDCS to the right inferior frontal gyrus (rIFG). Bayesian paired sample t-test comparing the effect of HD-versus sham tDCS on Stop Signal Reaction Times (SSRTs) showed moderate evidence in favor of the null hypothesis (i.e., no difference between conditions). Moreover, Bayesian analyses indicated that the data favored the null hypothesis when comparing SST performance between the HD-montage versus both conventional and sham montages, as well as between the conventional versus sham session. This study highlights the issue of the variability in the effects of tDCS, both for HD- and conventional montages, and further supports the need for replication studies even in the case of known effects in the literature.