Online transcranial magnetic stimulation reveals dynamic interactions between language control and processing in bilingual language production.

Abstract

Language production in bilinguals relies on the collaborative interaction between two neural systems: the language control system (e.g. the right inferior frontal gyrus) and the language processing system (e.g. the left superior temporal gyrus). However, the causal mapping between these brain systems and their cognitive functions, as well as the temporal dynamics between these two systems during bilingual language production, remain unclear. To investigate this, our study applied online transcranial magnetic stimulation to the right inferior frontal gyrus and left superior temporal gyrus in Chinese-English bilinguals performing a language-switching task. Transcranial magnetic stimulation was administered within seven 100 ms time windows (TW1 to TW7) following the picture onset. We observed time window-specific enhancements of language control, as shown by a significant reduction in switching costs upon the stimulations on both sites relative to the sham stimulation: specifically, the right inferior frontal gyrus at TW1, TW2, TW4, and TW5 and the left superior temporal gyrus at TW2 and TW5. These findings indicate a double-strike top-down control mechanism underpinned by the pathway from the right inferior frontal gyrus to the right inferior frontal gyrus, potentially during both stages during production: the language task schema and the lemma selection. Our study provides the first evidence of the dynamic interactions between language control and processing systems causally underpinning bilingual language production.