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.