Neural mechanisms of language switch

Interest has continued in the neural substrates of language switch, which allows multilingual people to select an appropriate language. Using functional magnetic resonance imaging, we investigated the neural substrates for switching between different languages (cross-language switch) in comparison with those for switching between different tasks or between different action sequences. Subjects were 20 native Japanese (L1) speakers with moderate to high proficiency in English (L2). They were asked to judge pronunciation of visually presented Arabic numerals in either L1 or L2 (phonological judgment task) or the numerical meaning of the same stimuli (numerical judgment task). The switching of the tasks was semi-randomly cued by a background color change. Several brain regions showed significantly greater activity for the forward cross-language switching (L1 to L2) than the backward cross-language switching (L2 to L1). Such cross-language switch regions included the right prefrontal cortex (PFC), left superior temporal/supramarginal gyrus (STG/SMG), anterior cingulate cortex (ACC), left inferior frontal gyrus (IFG), and caudate nucleus. Among these cross-language switch regions, the left IFG and caudate nucleus were also involved in the task switching (switching between the phonological and numerical tasks). These findings were supported by an action-sequence switch experiment examining brain activity during switching among different tapping sequences. In the right PFC, moreover, activity during the forward cross-language switching was positively correlated with the subjects' proficiency in L2. The present study suggests that the right PFC, left IFG, left STG/SMG, ACC, and caudate nucleus might subserve differential aspects of cross-language switch in late bilinguals.