Doctoral Dissertation: Language Interaction in Child Bilingual Speech: an Acoustic Study of Diphthongs

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In the course of acquiring their first language, children develop a set of phonemes specific to that language. Learning two languages simultaneously requires the acquisition of two sets of phonemes. Do bilingual children keep these two sets of phonemes separate from each other, or do the phonemes of the two languages interfere with one another during any stages of language development? If there is interaction between the two systems, what is the nature of that interaction and how does it differ from that found in adult bilinguals? To address these questions, the current project examines how the two phonetic systems of bilingual children interact by comparing French-English bilingual children to age-matched monolingual children of both languages and adult French-English bilinguals. The study focuses on the acquisition of diphthongs, which are complex vowel segments that have not received much attention in the literature. Previous studies on the production of monophthong vowels have shown effects of language interaction in bilingual children, as evidenced by overlapping phonetic properties, which were significantly different from what was found for monolingual speakers. The results of this study will indicate whether the previously established findings on the acquisition of monophthong vowels can be generalized to that of the diphthongs. Additionally, by examining the acquisition of diphthongs, this project also addresses significant methodological issues on how to best capture the dynamic properties of complex segments by using the Smoothing Spline ANOVA, which is a relatively novel method for comparing differences in vowel production. This project will determine whether or not this technique is adequate for capturing diphthongs' dynamicity and whether or not it will produce more precise results than previous techniques. The results of the current project will be relevant to parents of bilingual children, and to various associations working with bilingual children that may be curious about the progression of children's dual language acquisition. The results may also be relevant to clinical applications such as speech-language pathology. Given the phonetic complexity of diphthongs, the current study may reveal that they are generally more difficult to acquire than monophthong vowels. Thus, children may show speech delays involving the production of diphthongs. The results of the current study may become useful to clinicians for the construction of baseline data regarding the production of diphthongs. Speech therapists may then refer to this baseline, particularly when trying to differentiate between real clinical speech problems and potential byproducts of bilingualism, which may otherwise be interpreted as speech impairment.