Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that has been shown to alter cortical excitability and activity via application of weak direct currents. Beyond intracortical effects, functional imaging as well as behavioral studies are suggesting additional tDCS-driven alterations of subcortical areas, however, direct evidence for such effects is scarce. We aimed to investigate the impact of tDCS on cortico-subcortical functional networks by seed functional connectivity analysis of different striatal and thalamic regions to prove tDCS-induced alterations of the cortico-striato-thalamic circuit. fMRI resting state data sets were acquired immediately before and after 10 min of bipolar tDCS during rest, with the anode/cathode placed over the left primary motor cortex (M1) and the cathode/anode over the contralateral frontopolar cortex. To control for possible placebo effects, an additional sham stimulation session was carried out. Functional coupling between the left thalamus and the ipsilateral primary motor cortex (M1) significantly increased following anodal stimulation over M1. Additionally, functional connectivity between the left caudate nucleus and parietal association cortices was significantly strengthened. In contrast, cathodal tDCS over M1 decreased functional coupling between left M1 and contralateral putamen. In summary, in this study, we show for the first time that tDCS modulates functional connectivity of cortico-striatal and thalamo-cortical circuits. Here we highlight that anodal tDCS over M1 is capable of modulating elements of the cortico-striato-thalamo-cortical functional motor circuit.
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