Neurophysiological Impact of a Fronto-temporal tDCS Stimulation in Healthy Subjects: a Multimodal Imaging Approach

tDCS is a technique emerging as a prospective therapy for neurologic, psychiatric and addictive disorders. Specifically, fronto-temporal tDCS, with anodal stimulation over the left dorsolateral prefrontal cortex (DLPFC) and cathodal stimulation over the left temporo-parietal junction (TPJ), has been reported to reduce treatment-resistant auditory hallucinations (AH), negative symptoms and insight of the illness in schizophrenia. However, despite an increasing use in clinical settings, tDCS suffers from limitations, especially regarding the strength and the duration of therapeutic effects. Some imaging reports suggest that tDCS effects are not restricted to the brain areas located under the electrodes, but spread through distributed cortical networks functionally connected with the targets and reach subcortical areas. Overall, these studies suggest that tDCS modulates functional connectivity within and across resting-state networks and brain activity. However, these effects are currently described at different levels depending on the imaging technique used. Moreover, the majority of studies have focused on motor cortex stimulation, while the specific effects of fronto-temporal tDCS are scarce. Finally, effects of the stimulation applied online are rarely inspected. According to the therapeutic effects of fronto-temporal tDCS on schizophrenia and the dopaminergic pathophysiological hypothesis of schizophrenia, the effect of fronto-temporal tDCS on dopaminergic transmission is of major interest. As the cortex is densely connected with basal ganglia areas, tDCS effects are probably capable to reach subcortical dopaminergic areas. Indeed, recent fMRI studies highlighted subcortical effects of tDCS applied at the cortical level including modulations of cortico-striatal and thalamo-cortical functional connectivity. In addition, some studies suggest that cortical stimulation by other approaches, such as transcranial magnetic stimulation (rTMS) modulates dopaminergic trans