High-definition transcranial direct current stimulation (HD-tDCS), a non-invasive brain stimulation technique to alter cortical excitability, presents a powerful tool to modulate brain activation associated with response inhibition. However, previous research has yielded inconclusive results of stimulation effects on impulsivity. Hence, it is essential to better understand the underlying mechanisms in order to delineate factors that influence responsiveness to (HD-)tDCS.
In a double-blind, placebo-controlled simultaneous HD-tDCS-fMRI study, 45 male chronic tobacco users (TU) and 45 non-smoking male participants (non-TU) performed the Stop Signal Task before and after sham tDCS or 20 minutes of 1.5mA anodal HD-tDCS over the right inferior frontal gyrus during resting-state fMRI.
Behavioral data revealed significant improvements in Stop Signal Reaction Times (SSRTs) following active and sham stimulation in non-TU. TU showed improved SSRTs following sham but not active stimulation.
Results further highlighted significant variability in HD-tDCS-induced electric fields (EFs). Seed-to-voxel analysis indicated increased resting-state functional connectivity (rsFC) under the anode, particularly to the left prefrontal cortex, in active compared to sham stimulation during initial stimulation periods and post-stimulation.
Collectively, our findings suggest that the enhanced SSRTs are primarily attributed to a training effect, rather than being influenced by HD-tDCS. This contradicts many studies using conventional tDCS set-ups reporting improved response inhibition following anodal prefrontal tDCS. In TU, anodal HD-tDCS even seems to suppress the training effect. Considering the high variability of EFs, our study highlights the importance of taking individual differences into account when assessing the impact of HD-tDCS, urging further investigation in this domain.