Subcortical restructuring as a function of multilingualism: Insights from monolinguals, bilinguals, trilinguals and quadrilinguals

Acquiring a language is like learning a skill, and the brain adapts structurally as language experience increases. Studies have implicated several subcortical structures in bilingual and multilingual control and processing, and highlighted their corresponding structural adaptations. However, the patterns of adaptations across different subcortical structures remain unclear. Previous findings from bilinguals and multilinguals highlight characteristics of renormalisation in a few subcortical structures, lending support to the Dynamic Restructuring Model (Pliatsikas 2020). These patterns comprise of increasing volumes during the initial stages of acquiring a new language, and subsequent volumetric reductions to baseline volumes as greater experience in the skill is accrued. T1-weighted images from 14 English monolinguals from the UK, 14 bilinguals and 14 trilinguals from Malaysia, and 14 quadrilinguals from the Czech Republic were obtained. The volumes of five subcortical regions that are often implicated in language control and processing were compared amongst the groups. The findings showed differences between groups for every structure – caudate nucleus, nucleus accumbens, putamen, globus pallidus and thalamus. Complex patterns were unveiled for each structure of interest which suggest expansions and renormalisations that differ in terms of their trajectory for each for the groups. These findings not only shed light on the dynamic progression of subcortical adaptations, but also agree with the notion of structural renormalisation as language experience grows.