This study sheds new light on the relative impact of switching between languages and switching between cultures on Executive Functions (EFs) in bilinguals. Several studies have suggested that bilingualism has a measurable impact on executive functioning, presumably due to bilinguals' constant practice in dealing with two languages, or two cultures. Yet, the evidence on the relative contribution of culture and bilingualism to EFs is not well understood, because disentangling language, culture and immigration status is very difficult. The novelty of our approach was to keep the language pair and immigration status constant, whilst the cultural identity of participants was systematically varied, and measured at the individual level (not just at group level). Two groups of Turkish-English bilinguals, all adult immigrants to the United Kingdom, took part in the study, but one group (n = 29) originated from mainland Turkey and the other (n = 28) from Cyprus. We found that the bilinguals experienced smaller Conflict Effects on a Flanker task measuring inhibition, by comparison with monolingual British participants (n = 30). The key variable explaining EF performance variance at the individual level turned out to be bilinguals' Multicultural Identity Style. In particular those who indicated that they attempted to alternate between different British and Turkish (Cypriot) identity styles were found to have shorter RTs on incongruent trials of the Flanker task. The two multicultural identity variables, Alternating and Hybrid Identity Styles, together explained 32% in RTs over and above Education, Working Memory and Nonverbal reasoning (overall explained variance 49%). Thus, the data provide strong evidence for the impact of culture on EFs. We suggest that, as a result of their daily practice in recognizing cultural cues which highlight the need to switch to a different cultural frame, multicultural bilinguals develop a heightened context-sensitivity, and this gives them an advantage over monolinguals in a Flankers task. Our approach, which draws on models from cross-cultural psychology, bilingualism and executive functioning, illustrates the importance of theory building in which sociolinguistic and cultural variables are integrated into models of EFs.

Bilingualism has been linked to structural adaptations of subcortical brain regions that are important nodes in controlling of multiple languages. However, research on the location and extent of these adaptations has yielded variable patterns. Existing literature on bilingualism-induced brain adaptations has so far largely overseen evidence from other domains that experience-based structural neuroplasticity often triggers non-linear adaptations which follow expansion-renormalisation trajectories. Here we use generalised additive mixed models (GAMMs) to investigate the non-linear effects of quantified bilingual experiences on the basal ganglia and thalamus in a sample of bilinguals with wide range of bilingual experiences. Our results revealed that volumes of bilateral caudate nuclei and accumbens were positively related to bilingual experiences in a non-linear pattern, with increases followed by decreases, in the most experienced bilinguals, suggesting a return to baseline volume at higher levels of bilingual experience. Moreover, volumes of putamen and thalamus were positively linearly 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.

Bilingualism impacts brain structure, especially in regions involved in language control and processing. However, bilingual experiences are complex and the relation of brain structural changes to key aspects of bilingual language use are thus far poorly understood. Here we used structural MRI and non-linear correlational analysis in Czech-English bilinguals to investigate the effects of habitual code-switching practices on brain structure. We studied the effects of usage frequency of various code-switching types on the volumes of the caudate nucleus and the thalamus, two prominent subcortical structures subserving code-switching. Overall code-switching frequency was measured as well as extent of engagement in various code-switching types. These types were categorised by directionality (L1-L2 or L2-L1) and the level of separation of the two languages. Caudate volumes were positively correlated with overall code-switching frequency, irrespective of code-switching type or directionality, although stronger effects were found for L1-L2 switches. Thalamic volumes were positively correlated with engagement in forms of CS for which the two languages are kept more separate, with stronger effects observed in L2-L1 switches. This pattern of results underscores the theoretical and methodological importance of using detailed measures of bilingual experiences when investigating the sources of bilingualism-induced neuroplasticity.

Bilingualism has been associated with changes in our language-related and domain-general cognition. However, it remains controversial whether bilingualism-related cognitive effects are robust and stable. Also, it is still unclear what about being bilingual causes the plasticity of cognitive processes. This article offers a selective overview of the literature on bilingualism and cognition. We will discuss results from studies which investigated sources of cognitive plasticity in bilinguals, using prominent bilingual factors. We argue that, at least in part, the field deals with the controversies by viewing bilingualism through the perspective of usage-based (or experience-based) approaches, albeit such a link is not always made explicitly. Viewing bilingual variables as indicators of language use and engagement with both languages might offer promising ways forward while allowing for comparisons of existing studies on bilingualism with more recent ones, which build on the usage-based perspective more explicitly.

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.