Abstract/Summary

Bilinguals need effective cognitive control mechanisms to handle the concurrent activation of two languages. One of the consequences is that bilingualism has been shown to structurally impact regions subserving cognitive control and language processing to provide a newly wired neural architecture that can handle these mechanisms efficiently. However, the location, extent, and time course of these adaptations vary across studies. The inconsistency has often been attributed to differences in language use and exposure. This thesis extends these proposals and tests a hypothesis, informed by neurobiological principles of experience-dependent neuroplasticity, that the bilingualism-induced brain adaptations are non-linear. Using appropriate statistical methods, three studies are run to investigate the effects of quantified bilingual experiences on grey matter volumes of subcortical regions known to be affected by bilingualism. Structural MRI and behavioural data from 115 bilinguals with a wide variety of bilingual experiences were collected. The first study examines adaptations induced by bilingual experiences in the basal ganglia and thalamus across the whole sample. The second study separately examines bilingualism-induced changes of the caudate and the putamen in interpreters, translators, and non-professional bilinguals. The results from these studies reveal that engagement in bilingual language use can trigger region-specific grey matter increases, either sustained or followed by volumetric decreases, depending on the quantity and quality of bilingual experiences. The third study examines brain adaptations induced by different habitual code-switching practices, which impose different cognitive demands. The results suggest qualitatively diverse volumetric trajectories in caudate and thalamus for the different code-switching types. Overall, this thesis highlights that the various constituent elements of bilingual experiences may impact the brain in dynamic yet systematic ways. Crucially, the systematicity of the experience-dependent neuroplasticity can be unravelled if bilingualism is treated as a dynamic set of demanding experiences which trigger non-linear structural adaptations.