Sparks fade with distance: The effect of electric field distribution on global motion perception using different tES techniques

Previous evidence has shown that high frequency transcranial random noise stimulation (hf-tRNS) decreases motion coherence thresholds when a cephalic montage (i.e., return over Cz) is used. Extracephalic montages have also been employed to modulate behavioral performance, eliminating stimulation of regions under the return electrode. In this study, we examined the effects of different transcranial electrical stimulation (tES) protocols on visual motion discrimination, placing the return electrode on the ipsilateral arm. We assessed the impact of electrode localization using hf-tRNS, anodal, cathodal transcranial direct current stimulation (tDCS), and Sham stimulation over hMT+, a brain region involved in global motion perception. Motion direction discrimination was measured using random dot kinematograms (RDKs). Due to the increased distance between the stimulation and return electrodes in this montage, we expected a smaller reduction in motion discrimination thresholds compared to our previous study. The results suggest that increased interelectrode distance mitigates the efficacy of hf-tRNS. Additionally, no significant effects were observed with the other tES protocols tested. Our findings imply that the positioning of the two electrodes affects current flow characteristics, leading to reduced neuromodulation. These results underscore the importance of stimulation configuration, particularly the effect of interelectrode distance on performance. Given the widespread application of brain stimulation techniques in clinical and cognitive research, our results can guide future studies in carefully considering this further aspect of stimulation montage configurations.