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.

Étant donné que le bilinguisme et le multilinguisme sont la norme plutôt que l’exception, plusieurs enfants, y inclus plusieurs enfants ayant un trouble du spectre de l’autisme (TSA) sont élevés dans des contextes bilingues. Cependant, il y a actuellement peu de recherche examinant le développement langagier d’enfants ayant été exposés à deux langues, de la petite enfance à l’âge scolaire. Cette thèse a pour objectif d’examiner l’influence du bilinguisme sur le développement d’habiletés langagières d’enfants bilingues ayant un développement neurotypique à l’âge scolaire, ainsi que chez leurs pairs ayant un TSA. Cet objectif est atteint par l’entremise de trois manuscrits. Le premier manuscrit est une revue de la littérature dans le cadre de laquelle nous avons examiné le développement langagier d’enfants bilingues ayant un DN d’enfants ayant d’autres troubles de développement et finalement d’enfants ayant un TSA. Cette revue révéla que, tout comme leurs pairs ayant un DN, les enfants ayant un TSA peuvent devenir des locuteurs bilingues, et que le bilinguisme n’est pas néfaste à leur développement langagier. Cependant, certaines limites dans la littérature existante ont aussi été révélées. Dans un premier temps, la majorité des études publiées ont examiné les habiletés langagières d’enfants bilingues d’âge préscolaire. Toutefois, peu d’études ont examiné leur développement langagier une fois qu’ils atteignent l’âge scolaire, et ce, tant chez les enfants ayant un DN que chez les enfants ayant un TSA. Par ailleurs, peu d’études ont comparé les capacités langagières d’enfants ayant un TSA à celles de leurs pairs bilingues ayant un DN et aucune étude n’a comparé leurs capacités langagières à celles de leurs pairs unilingues ayant un DN. Les résultats qui découlent de cette revue ont menés à deux études expérimentales examinant l’influence du bilinguisme sur le développement langagier chez les enfants d’âge scolaire ayant un DN, ainsi que chez leurs pairs ayant un TSA. Notre revue de littérature suggère que l’exposition langagière pourrait jouer un rôle déterminant vis-à-vis les capacités langagières d’enfants bilingues. Le second manuscrit a donc comme objectif d’examiner la relation entre le développement langagier d’enfants étant des bilingues simultanés (exposés à leurs deux langues avant l’âge de trois ans) d’âge scolaire, et la quantité d’exposition langagière reçue dans chacune de leurs langues (le français et l’anglais). Globalement, nos résultats ont démontré une interaction entre la quantité d’exposition à l’anglais et la performance sur certaines mesures de langage expressif, mais pas avec les mesures de langage réceptif en anglais. En français, aucune interaction entre la quantité d’exposition au français et les performances sur les différentes mesures langagières n’a été trouvée. Ces différences sont potentiellement dues à des différences dans les montants d’exposition au français et à l’anglais, mais d’autres explications sont aussi discutées. De plus, les résultats suggèrent qu’en ayant approximativement 20% de leur exposition à une langue, ces enfants bilingues peuvent obtenir des scores dans la tranche moyenne sur toutes les mesures langagières. Par ailleurs, les enfants qui se servent de leurs deux langues tous les jours semblent nécessiter un moins grand pourcentage d’exposition pour développer des capacités langagières dans la moyenne. Ces résultats pourraient être expliqués par le transfert positif et une amélioration des capacités métalinguistiques plus généralement. Ce type d’amélioration pourrait permettre aux enfants qui se servent de leurs deux langues tous les jours de développer de meilleures capacités d’acquisition langagière. Les résultats de la revue de littérature ont aussi démontré que peu est connu en ce qui a trait aux capacités langagières d’enfants bilingues ayant un TSA lorsqu’ils atteignent l’âge scolaire. Conséquemment, pour le troisième manuscrit j’ai complété une étude pilote ayant comme objectif d’examiner si trois enfants bilingues simultanés d’âge scolaire et ayant un TSA (sans trouble de langage ou de déficiences intellectuelles) pouvaient atteindre des niveaux d’habiletés langagières semblables à celles de leurs pairs unilingues ayant un TSA. J’ai aussi cherché à savoir si ces trois enfants bilingues ayant un TSA pouvaient atteindre des seuils langagiers semblables à ceux de 19 pairs bilingues et 12 pairs unilingues ayant tous un DN. Les résultats ont démontré que ces bilingues simultanés ayant un TSA ont eu des performances semblables à celles de leurs pairs unilingues ayant un TSA sur des mesures langagières normées. Ils ont aussi eu des performances semblables à celles de leurs pairs bilingues et unilingues ayant un DN. L’ensemble de ces résultats démontrent que les enfants bilingues simultanés d’âge scolaire avec et sans TSA peuvent atteindre des niveaux d’habiletés langagières semblables à ceux de leurs pairs unilingues, du moins, dans une de leurs langues, sinon dans les deux. Bien que les bilingues ne sont pas deux unilingues en une seule personne, les résultats de nos recherches concordent avec ceux de recherches antécédentes et démontrent que le bilinguisme n’est pas néfaste pour le développement langagier oral d’enfants d’âge scolaire ayant un DN ou ayant un TSA. / Since bilingualism and multilingualism are the norm rather than the exception, many children, including children with autism spectrum disorder (ASD), live in bilingual or minority-language households. However, little research has examined the language development of school-aged children who have been exposed to two languages since early childhood, and fewer still have examined bilingual school-aged children with ASD. In this thesis, I examine the influence of bilingualism on language development in both school-aged ND children and in their peers with ASD. This objective will be achieved through the three manuscripts making up this thesis. For the first manuscript, we completed a review of the literature in which we examined the language development of ND bilingual children, of bilingual children with various developmental disorders, and of bilingual children with ASD. This review revealed that, as with neurotypically developing (ND) children, children with ASD can become bilingual speakers, and that bilingualism is not detrimental to their language development. Our review also revealed some of the limits of the existing literature. First, the majority of existing studies had examined bilingual language development in preschool-aged children with and without ASD, but had not examined the language development of these bilingual children once they reached school age. Second, few studies had compared the language abilities of bilingual children with ASD to those of their ND bilingual peers, and at time of publication, none had compared them to those of ND monolingual children. The results from this review lead to two studies examining the influence of bilingualism on language development in school-aged ND children and in children with ASD. Our review of the literature suggested that language exposure may play an important role in determining bilingual children’s language ability. However, little was known about its influence in school-aged children. Therefore, for the second manuscript, I examined whether there was a relationship between language development and the amount of language exposure that ND simultaneous bilinguals (i.e. children having been exposed to both languages before the age of three years) received in each of their languages (French and English). I found a relationship between lifetime exposure to English and performances on some English-language expressive language measure, but not on receptive language measures. In French, no relationship between the amount of exposure to French and performances on any of the French-language measures was found. These differences in findings could be due to differences in language exposure ranges across both languages but other potential explanations are also discussed. Findings also show that when receiving 20% of their exposure to a language, most children were able to obtain scores within the average range on the language measures. Moreover, children who were exposed to both of their languages on a daily basis often had stronger performances on language measures than children who did not receive daily exposure to both of their languages. I hypothesized that positive transfer and improved metalinguistic abilities more generally may have contributed to improved language learning abilities in children who used both of their languages on a daily basis. Findings from the literature review also revealed that little was known about the language abilities of bilingual children with ASD once they reached school-age. With that in mind, the third manuscript reports on a pilot study that examined whether three simultaneous bilingual school-aged children with ASD (without a comorbid language disorder or an intellectual disability) could achieve language levels similar to those of their two French-monolingual peers with ASD. I also explored whether these bilingual children with ASD could reach language proficiency levels similar to those of their 19 ND simultaneous bilingual peers, in both of their languages (French and English), and whether they had similar abilities to those of their 12 ND French-monolingual peers. I found that the simultaneous bilingual children with ASD performed similarly on standardized language measures to their two monolingual peers with ASD, in both of their languages. They also had similar performances to those of their ND bilingual peers, as well as to those of their ND monolingual peers. Overall, these findings reveal that ND school-aged simultaneous bilinguals and their simultaneous bilingual peers with ASD (without a language disorder or intellectual disability) can achieve language proficiency levels similar to those of their monolingual peers, in at least one, if not both of their languages. While bilingual children are not two monolinguals within one child, our findings are similar to previous findings and show that bilingualism does not appear to impede the language development of school-aged ND bilingual children and bilingual children with ASD.