Brain volumes as predictors of tDCS effects in primary progressive aphasia

•tDCS over the left IFG improves written naming/spelling more than sham in PPA.•Left AG and left PCC volumes are associated with tDCS effects for trained words.•Left MFG, left SMG, and right PCC volumes predict tDCS effect in untrained words.•Lexical/semantic processing in trained items is associated with left AG.•Rule-based generalization in untrained items is associated with left MFG and SMG. The current study aims to determine the brain areas critical for response to anodal transcranial direct current stimulation (tDCS) in PPA. Anodal tDCS and sham were administered over the left inferior frontal gyrus (IFG), combined with written naming/spelling therapy. Thirty people with PPA were included in this study, and assessed immediately, 2 weeks, and 2 months post-therapy. We identified anatomical areas whose volumes significantly predicted the additional tDCS effects. For trained words, the volumes of the left Angular Gyrus and left Posterior Cingulate Cortex predicted the additional tDCS gain. For untrained words, the volumes of the left Middle Frontal Gyrus, left Supramarginal Gyrus, and right Posterior Cingulate Cortex predicted the additional tDCS gain. These findings show that areas involved in language, attention and working memory contribute to the maintenance and generalization of stimulation effects. The findings highlight that tDCS possibly affects areas anatomically or functionally connected to stimulation targets.