SignificanceThis study sheds new light on the neuroanatomical adaptations resulting from bilingual language exposure and use, providing crucial insights into untangling the variability of findings in the existing literature. Our results demonstrate that differences in bilingual language experiences confer a range of systematic outcomes in terms of brain/mind adaptations. In doing so, our findings strongly support a shift away from traditional designs with bilingual vs. monolingual comparisons and toward an approach of modeling the experiences within bilingualism that give rise to neurocognitive adaptations. Crucially, we maintain that experience-based factors should be accounted for in all future studies investigating the effects of bilingualism on the brain and cognition.

Bilingualism has been shown to affect the structure of the brain, including cortical regions related to language. Less is known about subcortical structures, such as the basal ganglia, which underlie speech monitoring and language selection, processes that are crucial for bilinguals, as well as other linguistic functions, such as grammatical and phonological acquisition and processing. Simultaneous bilinguals have demonstrated significant reshaping of the basal ganglia and the thalamus compared to monolinguals. However, it is not clear whether these effects are due to learning of the second language (L2) at a very young age or simply due to continuous usage of two languages. Here, we show that bilingualism-induced subcortical effects are directly related to the amount of continuous L2 usage, or L2 immersion. We found significant subcortical reshaping in non-simultaneous (or sequential) bilinguals with extensive immersion in a bilingual environment, closely mirroring the recent findings in simultaneous bilinguals. Importantly, some of these effects were positively correlated to the amount of L2 immersion. Conversely, sequential bilinguals with comparable proficiency and age of acquisition (AoA) but limited immersion did not show similar effects. Our results provide structural evidence to suggestions that L2 acquisition continuously occurs in an immersive environment, and is expressed as dynamic reshaping of the core of the brain. These findings propose that second language learning in the brain is a dynamic procedure which depends on active and continuous L2 usage.

A growing body of research shows that the brain adapts functionally and structurally to specific bilingual experiences. These brain adaptations seem related to modulations in cognitive processes (specifically the executive functions). However, the trajectory of these adaptations is varied and seems at least partially dependent on different aspects of language exposure and use. Here we provide a review of the existing theoretical models covering bilingualism-induced neuroplasticity. Moreover, we propose a unifying framework (Unifying the Bilingual Experience Trajectories, UBET) to more comprehensively map the relationship between the various neurocognitive adaptations and different aspects of bilingual experience trajectories, focusing on intensity and diversity of language use, language switching, relative proficiency, and duration of bilingual experience. Crucially, we also outline predictions regarding both relationships between different bilingual experience factors and relationships between the measurable neurocognitive adaptations. Our framework offers a theoretical backdrop and clear testable predictions for future largescale empirical studies on individual differences in bilingual trajectories and their effects on neurocognitive adaptations.

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