Enhancing verb naming with anodal tDCS over the left inferior frontal gyrus: Insights from lesion mapping and structural connectivity estimation

Transcranial direct current stimulation (tDCS) holds promise for enhancing language recovery in post-stroke aphasia, yet its effect on verb retrieval remains less investigated. Moreover, the extent to which neurostructural damage mediates tDCS treatment efficacy is still poorly understood. We conducted a double-blind, randomized, sham-controlled crossover study to test whether anodal tDCS over the left inferior frontal gyrus (IFG), combined with verb naming therapy, improves verb retrieval in thirty-two individuals with chronic post-stroke aphasia. Each participant underwent 10 daily sessions of anodal and sham tDCS, with verb naming performance evaluated at baseline, immediately post-intervention, and at one-month follow-up. Results showed that anodal tDCS significantly enhanced verb naming relative to sham, yielding large effect sizes and sustained benefits at follow-up. Notably, all participants showed a greater training-related improvement during the anodal tDCS condition compared to sham. Moreover, voxel-based lesion-symptom mapping showed that preserved frontal regions, especially the frontal pole, were associated with better responses to anodal tDCS, while damage to posterior areas predicted poorer outcomes. This suggests that, even in chronic aphasia with extensive posterior lesions, the integrity of anterior regions may be crucial for tDCS-driven improvements in verb retrieval. A probabilistic atlas to assess which white matter pathways might be involved in tDCS-induced verb recovery further revealed that anterior tracts appeared relatively preserved, which may help maintain effective connectivity between the IFG and the frontal pole. These findings confirm the potential of tDCS as an adjunct to speech therapy and reinforce the importance of a network-based approach in post-stroke language rehabilitation.