The neural cost of the auditory perception of language switches: an event-related functional magnetic resonance imaging study in bilinguals

Abstract

One of the most remarkable abilities of bilinguals is to produce and/or to perceive a switch from one language to the other without any apparent difficulty. However, several psycholinguistic studies indicate that producing, recognizing, and integrating a linguistic code different from the one in current use may entail a processing cost for the speaker/listener. Up to now, the underlying neural substrates of perceiving language switches are unknown. In the present study, we investigated the neural mechanisms of language switching during auditory perception in bilinguals. Event-related functional magnetic resonance imaging was performed in 12 early, highly proficient Italian/French bilinguals, who were more exposed to their second language. Subjects had to listen to narratives containing "switched passages" that could either respect (i.e., regular switches) or violate (i.e., irregular switches) the constituents of sentence structure. The results indicate that switching engages an extensive neural network, including bilateral prefrontal and temporal associative regions. Moreover, a clear dissociation is observed for the types of switches. Regular switches entail a pattern of brain activity closely related to lexical processing, whereas irregular switches engage brain structures involved in syntactic and phonological aspects of language processing. Noteworthy, when switching into the less-exposed language, we observed the selective engagement of subcortical structures and of the anterior cingulate cortex, putatively involved in cognitive and executive control. This suggests that switching into a less-exposed language requires controlled processing resources. This pattern of brain activity may constitute an important neural signature of language dominance in bilinguals.

Figure 1.

The figure illustrates the psycholinguistic background of the early bilinguals in the present study. Reaction times for the semantic and grammatical judgment tasks are reported on top whereas the combined results related to L1 and L2 are reported on the bottom (for details, see Results).

Figure 2.

The simple main effects of the four different types of switches: regular switches into L1 (top, left); regular switches into L2 (top, right); irregular switches into L1 (bottom, left); irregular switches into L2 (bottom, right). For anatomical details, see Table 2.

Figure 3.

Pattern of brain activity revealed by the conjunction analysis between all four types of switches.

Figure 4.

Direct comparisons between languages (L1 and L2) and types of switches (regular and irregular). Regular switches into the less-exposed language (L1) compared with those into L2 (top, left) activated a network of brain areas associated with cognitive control (for details, see text). This pattern was found also for irregular switches into the less-exposed language (L1) compared with those into L2 (bottom, left). The reverse comparisons, i.e., regular switches into L2 compared with those into L1 (top, right) and irregular switches into L2 compared with those into L1 (bottom, right), did not engage this network of brain areas.