The left pars triangularis lesions impair semantic interference resolution regardless of competitor modality.

When we speak, selecting the right word takes longer whenever its semantic associates become active simultaneously. Although such semantic interference (SI) activates the left inferior frontal gyrus (LIFG), its lesions do not systematically increase SI induced by written word distractors. Therefore, whether the LIFG plays a causal role in resolving SI remains an open question. We hypothesized that it does, but the influence of LIFG lesions on SI from a written distractor in a given trial is obscured by concurrent interference from participants' own spoken responses in preceding trials. To test this hypothesis, we disentangled SI from these two competitor modalities in 25 post-stroke individuals who performed a picture-word interference task. Supporting the hypothesis, larger lesions to the pars triangularis of the LIFG increased SI from participants' previous spoken responses whenever they were semantically related to the current response. With this effect accounted for, larger pars triangularis lesions also increased SI from semantically related written distractors. In contrast, lesions to all other examined neuroanatomical regions either decreased or did not influence SI. Thus, the left pars triangularis does play a causal role in resolving SI regardless of competitor modality.