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During the first years of life, children rapidly learn to process speech from a continuous acoustic signal, and soon become able to understand and produce the sounds, words and structure of their native language. Children growing up in a bilingual environment face an additional challenge: they must simultaneously discover and separate their bilingual input into individual (yet potentially overlapping) systems, with independent sound units, vocabularies and grammars, without knowing a priori how many languages are spoken in their environment. In spite of this, language acquisition in young bilinguals follows, to an extent, a similar time-line as in monolinguals. Understanding how children come to discover the presence of two languages in their input, and to what extent they are able to keep them apart, are to this day crucial questions to the field of childhood bilingualism. In this thesis we focus on these two questions by exploring how perceptual and environmental properties of the input can help or hinder the discovery and lexical development of two languages, and whether the phonological representations formed by young bilinguals are language-specific. In order to investigate these questions, we take a multidisciplinary approach, using both empirical and computational techniques, which can provide different insights on the task of early language separation.
In the first part of this dissertation we examine the problem of discovering two languages in the input from an acoustic perspective. Based on a large body of research on language discrimination abilities in newborns and infants, and inspired by previous modelling work, we aim to provide a computational account of infant perception of multilingual speech. Borrowing a state-of-the-art system from speech technologies, we conducted a series of computational experiments that can help us understand what kind of representations young infants form when hearing different languages, and how different factors may shape their perception of language distance.
In the second part, we investigate several environmental aspects of bilingual exposure. Previous research on quantitative and qualitative properties of bilingual input had shown strong influences of each language's relative amount of exposure on infants' lexical development, but diverging results were reported regarding the impact of the separation of the two languages in their environment. We used a home diary method to investigate the co-existence of two languages in young bilinguals' input, and explore how this and other environmental factors may influence their vocabulary acquisition.
Finally, in the last part of this dissertation, we consider bilingual preschoolers' perception of language-specific phonological rules. Unlike other properties of young bilinguals' phonological systems, their acquisition and separation of phonological rules has barely been explored, with the only prior evidence coming from production studies. We conducted a behavioral experiment using a touchpad videogame to test French-English bilinguals' cross-linguistic perception of phonological assimilations.
Overall, this thesis contributes new insights to the question of language separation and acquisition in early bilingualism, with multiple perspectives for future research on this topic.