Slide Slam L7
Bilingual experiences induce dynamic volumetric changes of striatum and thalamus
Michal Korenar1,2, Jeanine Treffers-Daller1, Christos Pliatsikas1; 1University of Reading, 2Charles University
Bilingualism has been linked to structural adaptations of striatum and thalamus which have been implicated as important structures in controlling multiple languages (Green & Abutalebi, 2013). However, research on the location and extent of these adaptations yielded variable patterns, reporting changes in different regions and effects in different directions (e.g., Tao et al., 2021). The inconsistency in previous findings may stem from the assumption that bilingualism has linear effects on structural brain adaptations. This means that existing studies have so far overseen two important lines of research: (1) evidence showing that demanding experiences often trigger brain adaptations that are not linear, but rather following an expansion-renormalisation trajectory: the initial increases in volumes are followed by decreases once the peak efficiency in the task subserved by the relevant structures is reached (Wenger et al., 2017) (2) hypothesis proposed by the Dynamic restructuring model (DRM, Pliatsikas, 2020), which posits that bilingualism is an example of a demanding experience and as such should therefore trigger distinct dynamic brain adaptations governed by the quantity and quality of bilingual experiences. Here we used generalized additive models (GAMs) to directly investigate non-linear effects of bilingual experiences on striatum (caudate, putamen, accumbens) and the thalamus. Our sample consisted of 114 Czech-English bilinguals with a wide range of bilingual experiences; these were assessed with a well-attested bilingual composite score (LSBQ, Anderson et al., 2018), which is a continuous measure capturing the relevant bilingual experiences pertaining to language use, exposure, and proficiency. We built a series of GAMs for each of our regions of interest with the bilingual experiences as a continuous predictor of subcortical volumes, controlling for age and gender. Volumes of caudate nuclei and accumbens were positively related to bilingual experiences. Critically, in participants who reached a certain point of such experiences, the volumes started to decrease. Volumes of putamen and thalamus were positively linearly predicted by LSBQ. The results confirm that specific bilingual experiences, similarly to other demanding skills, can govern dynamic non-linear adaptations in line with expansion-renormalisation model of experience-dependent neuroplasticity. Furthermore, the observed pattern of volumetric changes of caudate and accumbens suggests that these structures undergo the bilingualism-induced adaptations first. In contrast, the results pertaining to the volumes of putamen and thalamus signify an immerit absence of evidence for renormalisation of these structures. This suggests later onset of adaptations of putamen and thalamus compared to caudate and accumbens. This constitutes a first direct support for the time course and dynamicity of subcortical changes brought about by bilingualism as proposed by the DRM. We argue that future studies on bilingualism-related brain correlates should account for the dynamicity of the effects of the bilingual experiences. The use of GAMs has the potential to become a valuable method in unravelling the effects of demanding skills on brain structure, with important implications for future research in the field of bilingualism and beyond.