DescriptionThere is considerable evidence for clinical and behavioral efficacy of transcranial electrical stimulation (tES). The effects range from suppressing Parkinsonian tremors to augmenting human learning and memory. Despite widespread use, the neurobiological mechanisms of action of tES on the intact human brain are unclear. In the work presented in this thesis, I have taken a multi-methodological approach to probe tES mechanisms. First, I studied the electric field spread induced by the application of tES, behaviorally. Second, I examined the behavioral effects of tES on human motion perception. I observed that tES (10 Hz, 0.5 mA) applied over area hMT+ (a brain area specialized in processing visual motion) attenuates motion adaptation. This result led to the hypothesis that tES-induced membrane voltage modulations reduce adaptation in motion-selective neurons. Finally, I tested this hypothesis by directly measuring tES-induced neural activity changes in a homologous area in the macaque brain (area MT). Tuning curve estimates of macaque MT neurons showed that tES attenuated the effects of visual motion adaptation on tuning amplitude and width. In addition to single cell measures, tES also significantly modified the evoked local field potentials. The results provide novel insights into how tES interacts with neural activity and establishes the awake, behaving macaque as an in-vivo animal model to study tES mechanisms.