How age of bilingual exposure can change the neural systems for language in the developing brain: a functional near infrared spectroscopy investigation of syntactic processing in monolingual and bilingual children

Abstract

Is the developing bilingual brain fundamentally similar to the monolingual brain (e.g., neural resources supporting language and cognition)? Or, does early-life bilingual language experience change the brain? If so, how does age of first bilingual exposure impact neural activation for language? We compared how typically-developing bilingual and monolingual children (ages 7-10) and adults recruit brain areas during sentence processing using functional Near Infrared Spectroscopy (fNIRS) brain imaging. Bilingual participants included early-exposed (bilingual exposure from birth) and later-exposed individuals (bilingual exposure between ages 4-6). Both bilingual children and adults showed greater neural activation in left-hemisphere classic language areas, and additionally, right-hemisphere homologues (Right Superior Temporal Gyrus, Right Inferior Frontal Gyrus). However, important differences were observed between early-exposed and later-exposed bilinguals in their earliest-exposed language. Early bilingual exposure imparts fundamental changes to classic language areas instead of alterations to brain regions governing higher cognitive executive functions. However, age of first bilingual exposure does matter. Later-exposed bilinguals showed greater recruitment of the prefrontal cortex relative to early-exposed bilinguals and monolinguals. The findings provide fascinating insight into the neural resources that facilitate bilingual language use and are discussed in terms of how early-life language experiences can modify the neural systems underlying human language processing.

Keywords: Bilingualism; Language development; Neural Signature Hypothesis; Sentence processing; Syntax; fNIRS neuroimaging.

Fig. 1

(a) fNIRS placement (a) key locations in Jasper (1958) 10–20 system. The detector in the lowest row of optodes was placed over T3/T4; (b) Probe arrays were placed over left-hemisphere language areas and their right-hemisphere homologues as well as the frontal cortex. (c) Location of 46 channels.

Fig. 2

Neural activations for SO relative to OS sentence types for (a) child participants and (b) adult participants (t-statistic map from HbO, p = .05, corrected). Participants showed increased neural activity in the left hemisphere SO sentence types relative to OS sentence types. Children showed increased neural activity in the temporal lobe (MTG) and adults showed increased neural activity in the LIFG.

Fig. 3

Neural activation of early-exposed bilingual children as compared to monolingual children (t-statistic map from HbO, p = .05, corrected). Early-exposed bilingual children show more robust neural activation in (a) the left and (b) the right hemispheres (bilateral Inferior Parietal Lobule, STG), and (c) frontal lobes (DLPFC) as compared to monolingual children.

Fig. 4

Neural activation of later-exposed bilingual children as compared to early-exposed bilingual children (t-statistic map from HbO, p = .05, corrected). Later-exposed bilingual children show more robust neural activation in (a) the left and (b) the right hemispheres (bilateral STG, right Inferior Parietal Lobule) and (c) frontal lobes (DLPFC, Frontopolar area) as compared to early-exposed bilingual children.

Fig. 5

Neural activation of later-exposed bilingual children as compared to monolingual children (t-statistic map from HbO, p = .05, corrected). Later-exposed bilingual children show more robust neural activation in (a) the left and (b) the right hemispheres (bilateral STG), and (c) frontal lobes (DLPFC, Frontopolar area) as compared to monolingual children.

Fig. 6

Neural activation of early-exposed bilingual adults as compared to monolingual adults (t-statistic map from HbO, p = .05, corrected). Bilingual adults show more robust neural activation in (a) the left (IFG, STG) and (b) the right hemispheres (IFG, Inferior Parietal Lobule) as compared to monolingual adults. (c) Bilingual and monolingual adults do not differ in frontal lobe activation.

Fig. 7

HbO concentration in bilateral Broca's area (BA), Superior Temporal Gyrus (STG) and Dorsolateral Prefrontal Cortex (DLPFC) for Object-Subject (OS) and Subject-Object (SO) sentences types across monolinguals, early- and later-exposed bilinguals. Later-exposed bilinguals show greater change in HbO concentration in bilateral Broca's Area (BA) and in the Dorsolateral Prefrontal Cortex (DLPFC) for both Object-Subject (OS) and Subject-Object (SO) sentences types relative to monolinguals (p < .05). Standard errors are indicated by bars.