Language proficiency modulates the recruitment of non-classical language areas in bilinguals

Abstract

Bilingualism provides a unique opportunity for understanding the relative roles of proficiency and order of acquisition in determining how the brain represents language. In a previous study, we combined magnetoencephalography (MEG) and magnetic resonance imaging (MRI) to examine the spatiotemporal dynamics of word processing in a group of Spanish-English bilinguals who were more proficient in their native language. We found that from the earliest stages of lexical processing, words in the second language evoke greater activity in bilateral posterior visual regions, while activity to the native language is largely confined to classical left hemisphere fronto-temporal areas. In the present study, we sought to examine whether these effects relate to language proficiency or order of language acquisition by testing Spanish-English bilingual subjects who had become dominant in their second language. Additionally, we wanted to determine whether activity in bilateral visual regions was related to the presentation of written words in our previous study, so we presented subjects with both written and auditory words. We found greater activity for the less proficient native language in bilateral posterior visual regions for both the visual and auditory modalities, which started during the earliest word encoding stages and continued through lexico-semantic processing. In classical left fronto-temporal regions, the two languages evoked similar activity. Therefore, it is the lack of proficiency rather than secondary acquisition order that determines the recruitment of non-classical areas for word processing.

Figure 1. Mean reaction times for 16 subjects.

Subjects responded faster to old words in both languages and modalities, and responded faster to English words in the visual modality. Error bars represent the standard error of the mean.

Figure 2. Group dSPM of the mean activity during early visual (top) and auditory (bottom) word encoding.

In both modalities, activity appears to be strongly lateralized for English but largely bilateral for Spanish. For visual words, left VOT showed a marginal repetition effect (green arrows), and right VOT showed a trending Spanish>English language effect (blue arrows). For auditory words, right planum temporale, bilateral anterior STS, and right posterior fusiform showed Spanish>English effects (blue arrows). Left planum temporale showed a trending Spanish>English effect. See Supplementary Figure S2 for ROI locations and names. Color bars represent square root of F values, which are a measure of signal-to-noise.

Figure 3. Average time courses for selected ROIs to new visual words.

Several regions show significant (denoted by *) or marginal (denoted by #) Spanish (thick lines) > English (thin lines) effects during the early (150–190 ms) and late (350–400 ms) time windows (gray bars). Responses appear generally greater over an extended time period for Spanish than for English, especially in right hemisphere and posterior areas.

Figure 4. Average time courses for selected ROIs to new auditory words.

Overall responses appear greater over an extended time period for Spanish (thick lines) than for English (thin lines), especially in right hemisphere and posterior areas. Planum temporale and anterior STS show significant (denoted by *) or marginal (denoted by #) Spanish>English effects during the early encoding stage (90–110 ms), while other areas show language effects during the late lexico-semantic stage (400–450 ms). Of particular interest is the activity evoked by auditory words in right inferior LOT and lateral VOT, which are typically associated with visual object processing. The right inferior LOT area also has a significant new>old effect in Spanish, but not in English (denoted by &).

Figure 5. Group dSPM images of the mean activity evoked by visual words from 350–400 ms.

Several regions in bilateral posterior and right anterior temporal cortex showed significant Spanish>English effects (blue arrows). The activity associated with the N400m in classical left temporal and frontal language areas was not significantly different between Spanish and English. Stimulus repetition effects (green arrows) were also significant in many regions, including two regions with old>new effects (magenta arrows). See Supplementary Figure S2 for ROI locations and names. Color bars represent square root of F values, which are a measure of signal-to-noise.

Figure 6. Group dSPM images of the mean activity evoked by auditory words from 400–450 ms.

Several regions in both hemispheres showed Spanish>English effects (blue arrows). Note that the activity associated with the N400m in the regions of Wernicke's and Broca's areas was not significantly different between Spanish and English. Stimulus repetition effects (green arrows) were significant in many regions. One area, right inferior LOT, showed a new>old N400 effect in Spanish but not in English (purple arrow). See Supplementary Figure S2 for ROI locations and names. Color bars represent square root of F values, which are a measure of signal-to-noise.