Combining Voxel-based Lesion-symptom Mapping (VLSM) With A-tDCS Language Treatment: Predicting Outcome of Recovery in Nonfluent Chronic Aphasia

Background: Most of transcranial direct current stimulation (tDCS) studies in aphasia have already shown that significant changes in language performance could be improved by increased monitoring of the perilesional cortex in the left lesioned hemisphere with excitatory anodal tDCS (A-tDCS). However, no report to date has investigated which areas may be less predictable or may play a major role in the recovery from language impairment after brain stimulation. Objective: We investigated the relationship between the localization of damage in the left hemisphere and the amount of language recovery after A-tDCS. Methods: We conducted a Voxel-lesion mapping-symptom (VLSM) analysis on twenty non-fluent aphasics who underwent a language treatment in concomitance with left A-tDCS delivered over the left inferior frontal gyrus (IFG) and a sham condition. Significant changes in language performance before and after the two conditions were examined in three language tasks (picture description, noun and verb naming). Results: VLSM analysis revealed that damage to distinct left hemispheric structures and, in particular, to the basal ganglia, the insula and the superior and inferior longitudinal fasciculi, resulted in lower responses to A-tDCS in all language measures. Conclusions: Beneficial effects after A-tDCS over the left IFG depend on the anatomical integrity of different left subcortical structures among which are the white matter language pathways. Future studies combining different approaches on larger samples of subjects will further elucidate our understanding of how the human brain responds to tDCS.