Transcranial direct current stimulation over the left inferior frontal gyrus improves sentence comprehension

Abstract

Background. The left inferior frontal gyrus (IFG) has been shown to be involved in sentence comprehension in many studies through its involvement in both semantic and syntactic computations. However, causal evidence for its involvement in sentence comprehension is scarce. We used transcranial direct current stimulation (tDCS) to test the causal involvement of the left IFG in sentence comprehension in a group of individuals with primary progressive aphasia (PPA). These individuals participated in a tDCS study targeting lexical retrieval only, not sentence comprehension, therefore in the present study we report on far-transfer effects of tDCS in sentence comprehension. Objective. We sought to determine whether tDCS over the left inferior frontal gyrus (IFG) coupled with lexical retrieval treatment may improve sentence comprehension in PPA. Method. Within a sham-controlled, double-blind design, we tested whether 15 daily sessions of anodal tDCS over the left IFG may improve sentence comprehension in 27 people with PPA, and whether the tDCS effects were sustained up to two months post-treatment. Results. We found that immediately post-treatment, and up to 2-months post-treatment, there was significantly larger improvement of sentence comprehension in the tDCS condition compared to sham. There were, however, differential effects of tDCS in each PPA variant and sentence-type. Importantly, participants with the epicenter of atrophy over the stimulated area (non-fluent PPA) benefited most from tDCS. Conclusion. TDCS over the left IFG induces far-transfer effects and may improve sentence comprehension in PPA. We provide causal evidence that left IFG is a critical area for sentence comprehension.