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) reduces motion coherence thresholds when applied with a cephalic montage (i.e., return electrode over Cz). Extracephalic montages, which avoid stimulating regions under the return electrode, have also been used to modulate behavioral performance. In this study, we investigated 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 positioning 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). Given 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. Our results suggest that increasing interelectrode distance alters current flow characteristics - such as current distribution and focality - within the cortical areas under the target electrode, producing different effects. Additionally, no significant effects were observed with the other tES protocols tested. Our findings suggest that change in the interelectrode distance influences current flow characteristics, such as current distribution and focality, within the cortical areas under the target electrode, resulting in differential neuromodulatory effects. These results highlight the importance of stimulation configuration on performance, particularly a potential electric field shift due to the change in the interelectrode distance. Given the widespread application of brain stimulation techniques in clinical and cognitive research, our results can guide future studies carefully considering this further aspect of stimulation montage configurations.