Cortical Thickness Is Related to Variability in Heritage Bilingual Language Proficiency

Abstract

Research suggests that bilingual experience is associated with gray matter changes, such that initial language gains are associated with expansion and language expertise is associated with renormalization. Previous studies on language proficiency development primarily focused on between-subjects, quasiexperimental comparisons of monolinguals and bilinguals. This study proposes a new paradigm to examine language expertise and cortical thickness within heritage bilinguals (n = 215), as well as between bilinguals and monolinguals (n = 145), using data combined from eight previous magnetic resonance imaging studies. In general, results highlight variability within bilinguals, finding relationships between cortical thickness and English proficiency that are relatively consistent within monolinguals, but inconsistent within bilinguals. In all participants, higher levels of proficiency in English-monolinguals' only language and bilinguals' second but stronger language-were negatively related to cortical thickness. In bilinguals, higher proficiency in the weaker, albeit first learned, language was positively related to cortical thickness. Moreover, there was an interaction between language group and English proficiency in predicting cortical thickness, such that the relationship between proficiency and thickness was stronger in monolinguals than in bilinguals. Findings also demonstrate that the regions associated with language expertise differ between bilinguals and monolinguals. Future directions for cognitive-developmental neuroscience research in bilinguals are suggested, particularly the longitudinal examination of cortical changes in relation to bilingual experiences.

Keywords: bilingualism; cortical thickness; gray matter; language; language proficiency.

Figure 1

Cortical Thickness Correlations With English Proficiency Controlling for SES and Age in English Monolinguals Note. Blue (left bar) indicates a negative correlation. This figure only has negative correlation, with brighter shade meaning thinner cortex. The left and right images are lateral views of the relationships in the left and right hemispheres, respectively. All results presented have been Monte Carlo-corrected (n = 95). SES = socioeconomic status.

Figure 2

Cortical Thickness Correlations With Spanish Proficiency Controlling for English Proficiency, English AoA, Age, and SES in Bilinguals Note. Red/yellow (right bar) indicates a negative correlation. This figure only has positive correlation, with brighter shade meaning thicker cortex. The left and right images are lateral views of the relationships in the left and right hemispheres, respectively. All results presented have been Monte Carlo-corrected (n = 177). AoA = age of acquisition; SES = socioeconomic status.

Figure 3

Cortical Thickness Correlations With English Proficiency Controlling for Spanish Proficiency, English AoA, Age, and SES in Bilinguals Note. Blue (left bar) indicates a negative correlation. This figure only has negative correlation, with brighter shade meaning thinner cortex. The left and right images are lateral views of the relationships in the left and right hemispheres, respectively. All results presented have been Monte Carlo-corrected. AoA = age of acquisition; SES = socioeconomic status.

Figure 4

Cortical Thickness Correlations With English Proficiency in All Participants, Controlling for SES and Age Note. Blue (left bar) indicates a negative correlation. This figure only has negative correlation, with brighter shade meaning thinner cortex. The left and right images are lateral views of the relationships in the left and right hemispheres, respectively. All results presented have been Monte Carlo-corrected (n = 272). SES = socioeconomic status.

Figure 5

English Proficiency Interacts With Language Group to Predict Left-Hemispheric Cortical Thickness in All Participants, Controlling for SES and Age Note. Red/yellow (right bar) indicates a stronger relationship between English proficiency and cortical thickness in monolinguals compared to bilinguals. This figure only have positive correlation, with brighter shade meaning stronger relationship. All results presented have been Monte Carlo-corrected. SES = socioeconomic status.

Figure 6

English Proficiency and Language Group Interaction Plots in Predicting Left-Hemispheric Cortical Thickness in All Participants, Controlling for SES and Age at the Caudal Middle Frontal Gyrus Note. Proficiency was calculated as a percentage, with range from 0 to 1. SES = socioeconomic status.