Transcranial direct current stimulation (tDCS) facilitates verb learning by altering effective connectivity in the healthy brain

Recent studies have shown that the left inferior frontal gyrus (IFG) plays a key role in language learning. Facilitatory stimulation over this region by means of anodal transcranial direct current stimulation (tDCS) can modulate linguistic abilities in healthy individuals and improve language performance in patients with post-stroke aphasia. Neuroimaging studies in healthy participants have suggested that anodal tDCS decreases task-related activity at the stimulated site when applied during different language tasks, and changes resting-state connectivity in a larger network of areas associated with language processing. However, to date, the neural correlates of the potential beneficial effects of tDCS on verb learning remain unclear. The current study investigated how anodal tDCS during verb learning modulates task-related activity and effective connectivity in the healthy language network. To this end, we combined a verb learning paradigm during functional neuroimaging with simultaneous tDCS over the left IFG in healthy human volunteers. We found that, relative to sham stimulation, anodal tDCS significantly decreased task-related activity at the stimulated left IFG and in the right homologue. Effective connectivity analysis showed that anodal tDCS significantly decreased task-related functional coupling between the left IFG and the right insula. Importantly, the individual decrease in connectivity was significantly correlated with the individual behavioural improvement during anodal tDCS. These results demonstrate, for the first time, that the behavioural improvements induced by anodal tDCS might be related to an overall decrease in processing effort both with respect to task-related activity and effective connectivity within a large language network.