The Effect of Bilingualism on Brain Development

Post by Cody Walters

What’s the science?

While it is well-known that bilingual brains are structurally different from monolingual brains, it is less clear how these structural changes unfold over time. This week in Brain Structure and Function, Pliatsikas et al. explored how the brains of over 700 monolingual and bilingual individuals differentially developed through childhood and adolescence. 

How did they do it?

The authors analyzed an existing neuroimaging dataset (from the Pediatric Imaging, Neurocognition, and Genetics project) that consists of monolingual and bilingual individuals aged 3-21. The dataset includes structural MRI and DTI data which the authors used to quantify cortical thickness, surface area, and volume as well as white matter fiber tract integrity (as measured by fractional anisotropy and mean diffusivity). In order to examine the relationship between age and brain development, the authors used generalized additive models (GAMs). GAMs use a basis set of smooth functions, where each function has an associated coefficient that can be adjusted to fit the data. The coefficients were estimated using penalized regression in order to prevent overfitting.

What did they find?

The authors found that monolinguals (as compared to bilinguals) had increased cortical thickness and volume across several parieto-frontal structures during early childhood. However, bilinguals had increased cortical thickness and volume across several regions in late childhood and early adolescence, with this increase converging back to monolingual levels in many cortical areas. Interestingly, the precuneus was the only region that showed a difference in cortical surface area between monolinguals and bilinguals, with monolinguals having greater precuneus surface area in early childhood. Altogether, bilinguals generally had lower cortical metrics (e.g., thickness and volume) than monolinguals in early childhood, but higher metrics in late childhood and early adolescence that either persisted in some regions or converged back to monolingual levels in others. No differences were observed between monolinguals and bilinguals in subcortical structures or the cerebellum. Only one white matter tract (connecting the striatum to the inferior frontal cortex) showed differences between monolinguals and bilinguals, and this difference followed the same trend as the cortical data, with the fiber tract density being lower in bilinguals during early childhood but higher in late adolescence. 

What’s the impact?

This study provides new insight into how the bilingual brain develops over a nearly 20 year period. These data largely highlighted differences in striato-frontal connectivity as well as frontal and parietal structures. These findings suggest that, during development, bilingualism affects brain structures underlying the parieto-frontal dorsal language stream (which is involved in mapping speech sounds to articulatory representations); basal ganglia-based procedural memory (which underlies the implicit learning of sequences and plays important roles in syntax, morphology, phonology, and articulation); and executive function (which allows bilinguals to appropriately alternate between their two languages).

The effect of bilingualism on brain development from early childhood to young adulthood, (2020). Access the publication here.