Brain Structure and Age of Acquisition in Bilinguals and Monolinguals

Researchers debate whether the age of second language acquisition (AoA) plays any role in determining brain structure. Whereas some studies suggest that bilinguals handle both languages via a single set of cognitive control regions regardless of AoA, others suggest that each language depends on different sets of brain regions for late (but not early/simultaneous) bilinguals. Likewise, structural neuroimaging studies have come to contrasting conclusions: either AoA does not affect structure, or later AoA relates to gray matter expansion in cognitive control and language processing regions. These differing results may occur for two main reasons: 1) low power caused by small group sizes, and 2) a lack of consistency in measures (of brain structure and of AoA) used across studies. This study aimed to address both of these gaps. MRI scans were used to measure cortical thickness, volume, and density via whole-brain analyses of 216 bilingual and 145 monolingual human adults (male and female). Results showed that the neuroanatomical correlates of AoA differ in terms of cortical thickness, volume, and density. Specifically: 1) late bilinguals had thicker cortex than monolinguals, simultaneous bilinguals, and early bilinguals in left and right frontal, temporal, parietal, and occipital regions; 2) no volume differences were found, and 3) compared to monolinguals, late bilinguals had greater density in the left middle frontal gyrus and right pars opercularis and postcentral gyrus, whereas early bilinguals had greater density in the bilateral temporal pole, right middle frontal gyrus, and right postcentral gyrus. The one similar finding across all brain structure measures was that monolinguals and simultaneous bilinguals did not differ. Results suggest that, relative to simultaneous bilinguals and monolinguals, late bilinguals handle language via structural increases in language processing and cognitive control areas. The present results demonstrate that it is important to use multiple measures of brain structure in order to clarify our understanding of bilingual language acquisition, as well as the importance of obtaining sufficient power to test hypotheses using whole-brain analyses.