A dual-site HD-tDCS of the left inferior frontal gyrus and inferior parietal lobe modulates picture-word naming performance in healthy individuals

The selection of appropriate lexical entries constitutes a crucial stage in the process of speech production. The theories of lexical choice competition and the hypothesis of response exclusion both propose that inhibitory control might play a significant role in handling cognitive conflicts arising from lexical selection. Previous studies have utilized transcranial direct current stimulation (tDCS) to enhance speech function in individuals by primarily targeting stimulation on the left inferior frontal gyrus (IFG). However, the effects of this intervention have been inconsistent in healthy individuals. In this study, we apply tDCS successively to both the left inferior frontal gyrus (IFG) and the left inferior parietal lobe (IPL) of healthy individuals, along with picture naming training. We aim to investigate the potential enhancement of word production performance in healthy individuals through a combined intervention targeting verbal functioning and inhibitory control. The experimental findings indicated that speech training in isolation did not have a significant impact on picture-word interference performance. Furthermore, it was observed that speech training in healthy individuals only showed an effect when combined with stimulation of two specific brain regions. However, this effect varied depending on the type of interfering word. The findings of this study support the lexical selection and generation require the involvement of inhibitory control, and provide an analysis of the phenomenon of unstable effects of tDCS applied to left IFG in healthy individuals. Additionally, new insights are provided for the clinical application of tDCS in treating speech dysfunction.