Dynamic effects of bilingualism on brain structure map onto general principles of experience-based neuroplasticity

Abstract

Bilingualism has been linked to structural adaptations of subcortical brain regions that are important for controlling multiple languages. However, research on the location and extent of these adaptations has yielded variable patterns, especially as far as the subcortical regions are concerned. Existing literature on bilingualism-induced brain restructuring has so far largely overseen evidence from other domains showing that experience-based structural neuroplasticity often triggers non-linear adaptations which follow expansion-renormalisation trajectories. Here we use generalised additive mixed models to investigate the non-linear effects of quantified bilingual experiences on the basal ganglia and the thalamus in a sample of bilinguals with a wide range of bilingual experiences. Our results revealed that volumes of the bilateral caudate nucleus and nucleus accumbens were significantly related to bilingual experiences. Importantly, these followed a non-linear pattern, with increases followed by plateauing in the most experienced bilinguals, suggesting that experience-based volumetric increases are only necessary up to a certain level of bilingual experience. Moreover, the volumes of putamen and thalamus were positively predicted by bilingual experiences. The results offer the first direct evidence that bilingualism, similarly to other cognitively demanding skills, leads to dynamic subcortical structural adaptations which can be nonlinear, in line with expansion-renormalisation models of experience-dependent neuroplasticity.

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

“…The caudate nucleus may thus be the fastest subcortical structure to optimise and renormalise in volume due to its integral role in language control. This coincides with recent studies showing a pattern of expansion followed by contraction of the caudate nucleus with increasing L2 immersion (DeLuca et al, 2019a;Korenar et al, 2023;Marin-Marin et al, 2022). The authors of these studies suggested that greater automation has occurred in language monitoring and selection, allowing the caudate nucleus to return to a baseline volume.…”

“…Subcortical effects in the form of larger volumes in the caudate nucleus, putamen, globus pallidus, and thalamus have been observed in bilinguals when compared with monolinguals (Burgaleta et al, 2016;Pliatsikas et al, 2017). When compared with monolinguals or bilinguals with limited language experiences, bilinguals with higher language immersion have presented with a lack of cortical effects, and/or with contractions in the caudate nucleus, nucleus accumbens and thalamus (Costumero et al, 2020;DeLuca et al, 2019a;Korenar et al, 2023;Pliatsikas et al, 2017). In conjunction with subcortical effects, there has been corroborating evidence from studies on white matter effects in bilingual adults and children (for a review see Li et al, 2014).…”

“…This prediction is based on its strong connections with frontal executive regions to deliver cognitive control and mediate the selection of appropriate behaviours (Meyer & Bucci, 2016) which are imperative in switching between languages. Furthermore, recent research has shown a non-linear relationship between the volume of the nucleus accumbens and the extent of bilingual experiences (DeLuca et al, 2019a;Korenar et al, 2023).…”

“…For the thalamus, we predicted that monolinguals would have significantly smaller volumes than bilinguals, trilinguals and quadrilinguals. The involvement of the thalamus in language switching has been described in the ACH, and past studies have shown an increase in thalamic volumes with high language immersion and more bilingual experiences (DeLuca et al, 2019a;Korenar et al, 2023). The thalamus is said to manage the ongoing selection of lexical-semantic representations (Abutalebi & Green, 2016;Wahl et al, 2008), which may be required to a greater degree when there are more languages available.…”

Unveiling the neuroplastic capacity of the bilingual brain: Insights from healthy and pathological individuals

“…interhemispheric information ow and cortico-subcortical connectivity (DeLuca et al, 2019, 2020; Morgan-Short et al, 2022; Morgan-Short & Ullman, 2022; Pliatsikas et al, 2017; Ullman, 2020).In line with this theory, (Korenar, Treffers-Daller, & Pliatsikas, 2021) demonstrated how bilingual experiences induce dynamic structural changes in the caudate and thalamus (see alsoKorenar et al, 2023). These areas, which have been extensively studied in both healthy and clinical populations, are known to play a crucial role in language control processes (see alsoDeLuca et al, 2019DeLuca et al, , 2020Pliatsikas et al, 2017).…”