Dual-site multifocal HD-tDCS modulates bilingual language control via brain activity and connectivity

Bilinguals rely on language control mechanisms to manage cross-language interference. Interactions between the pre-supplementary motor area (pre-SMA) and the left cerebellum play an important role in bilingual language control. However, our previous brain stimulation study targeting the cerebellum alone failed to detect significant behavioral changes in bilingual language control. In this study, we combined dual-site tDCS with fMRI to investigate its effects on bilingual language control and compared it with conventional single-site tDCS. Using a within-subject design, Chinese-English bilinguals underwent three sessions: pre-stimulation baseline, single-site tDCS over pre-SMA, and dual-site tDCS over pre-SMA and left cerebellum. Following dual-site stimulation, we observed reduced brain activity within the language control network, including the right cerebellar lobule VI, and strengthened thalamo-cerebellar connections during bilingual language production. In a separate behavioral experiment, language switch cost was lower after dual-site stimulation than at baseline but did not differ significantly between dual-site and single-site stimulation. Together, these findings provide preliminary evidence that dual-site tDCS modulates bilingual language control, potentially by strengthening thalamo-cerebellar connectivity and modulating cerebellar neural activity. These results also highlight the functional interaction between the pre-SMA and the cerebellum in supporting bilingual language control.