Bilingual continuum: mutual effects of language and cognition
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One of the main findings of research on bilingualism in the last twenty years is the fact that both languages are always active, to some extent, and interact with each other. This interaction gives rise to a computationally complex feature of the bilingual mind, namely that the two languages compete with each other. Many studies have addressed the linguistic consequences of this competition (e.g. differences in linguistic attainment), while others have instead addressed the cognitive consequences (e.g. training effects on cognitive control). These two strands of research, when brought together, can shed light on the dynamics of language processing and of its relationship with other cognitive abilities; however, they do not often converge. The first aim of this thesis is to seam them together. The second aim of this thesis is to understand the effects of specific aspects of language experience on linguistic and non linguistic abilities. A critical assumption I make is that bilingualism is not a dichotomous variable, but rather a continuum, characterised by several aspects such as linguistic proficiency, age of acquisition, and daily exposure. All of these factors interact with each other to give rise to potentially infinite types of bilingual experiences, and arguably modulate how bilinguals deal with competing languages. However, the effects of these factors on linguistic and non linguistic abilities are poorly understood. Hence, in this thesis I examine if the bilingual experience affects other cognitive abilities (study 1), how the ability to handle this competition is modulated by experience (study 2), and how it affects language processing (study 3). To examine how specific dimensions of the bilingual continuum affect these abilities, I compare four populations of bilinguals, whose linguistic experience ranges from late bilinguals who are immersed in their native language and are passive users of their second language, to early highly proficient bilinguals who use both languages actively. My first study examines cognitive control performance and shows that high active proficiency and early age of acquisition, together, represent beneficial circumstances for the ability to modulate cognitive control; however, their effects are not strong enough to override individual variability. The second study investigates how the bilingual experience modulates the ability to access the two languages separately, overcoming the competition between them at different levels. This could be at a local level, i.e. the level of the individual linguistic representation (e.g. naming time of a specific word), or at a global or whole language level (e.g. overall naming latencies across languages). The results show that proficiency affects local competition, and age of acquisition affects global competition, whereas daily language exposure regulates competition at both the local and the global levels. My third study examines the processing of pronouns, which are particularly demanding linguistic structures. It shows that active proficiency and age of acquisition, together, define circumstances in which pronoun processing may vary between individuals, independently of structural differences between their languages. This suggests that bilinguals with long-term exposure to more than one language and high active proficiency may use some linguistic structures in the same way as individuals with different linguistic backgrounds, i.e. explicitly interpret them in similar ways, but process them in marginally different ways. Through these studies, this thesis brings together research on linguistic and cognitive aspects of bilingualism by identifying three dimensions of the bilingual experience – proficiency, exposure and age of acquisition – and their effects on language processing, language control and cognitive control.