Modulation of verbal fluency networks by transcranial direct current stimulation (tDCS) in Parkinson's disease.

BACKGROUND Verbal fluency relies on the coordinated activity between left frontal and temporal areas. Patients with Parkinson's disease (PD) present phonemic and semantic fluency deficits. Recent studies suggest that transcranial direct current stimulation (tDCS) enhances adaptative patterns of brain activity between functionally connected areas. OBJECTIVE The aim of this study was to assess the differences in the effects induced by tDCS applied to frontal and temporo-parietal areas on phonemic and semantic fluency functional networks in patients with PD. METHOD Sixteen patients were randomized to receive tDCS to left dorsolateral prefrontal cortex (DLPFC) and left temporo-parietal cortex (TPC) in a counterbalanced order. Immediately following stimulation, patients underwent a verbal fluency paradigm inside a fMRI scanner. Changes induced by tDCS in activation and deactivation task-related pattern networks were studied using free-model independent component analyses (ICA). RESULTS Functional connectivity in verbal fluency and deactivation task-related networks was significantly more enhanced by tDCS to DLPFC than to TPC. In addition, DLPFC tDCS increased performance on the phonemic fluency task, after adjusting for baseline phonemic performance. CONCLUSIONS These findings provide evidence that tDCS to specific brain regions induces changes in large scale functional networks that underlay behavioural effects, and suggest that tDCS might be useful to enhance phonemic fluency in PD.