Abstract
This dissertation reports on the influence of bilingualism on infants’ sound and word acquisition in the first two years of life. It targets the question of whether mono- and bilingual infants follow the same developmental trajectory of language acquisition, it displays similarities and differences between mono- and bilingual infants, and
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discusses the advantages and disadvantages that are the consequence of receiving bilingual language input. Previous debate has focused on whether bilingual infants are delayed in their sound and word learning. Some studies present a (temporary) lag in bilingual sound (i.e., Bosch & Sebastián-Gallés, 2003a) and word (i.e., Volterra & Taeschner, 1978) acquisition. However, other studies report no evident delay in bilingual sound (i.e., Sundara & Scutellaro, 2011) and vocabulary (Pearson et al., 1993) development. In the current dissertation, mono- and bilingual infants from 5 to 18 months were tested on their perception of consonants, vowels and tone contrasts, and their associative word learning ability. Parents filled in a questionnaire on the vocabulary of their children, and another questionnaire on the degree of exposure to each language if their children were exposed to more than one language. In the sound perception experiments, no evident/clear delay was observed between mono- and bilingual infants. For consonants, an early unstable perception pattern was observed in bilingual infants in the first year of life, leaving space for future research. Moreover, bilingual infants displayed more sensitivity than monolinguals in their perception of native vowels and non-native tones. In word acquisition, bilingual infants kept the same pace as monolinguals in their associative word learning performance, as well as total conceptual receptive and expressive vocabulary size. In the study of parental report of their bilingual infants, it was found that parents’ intuition of their infants’ degree of exposure of each language counts in not only the languages directly spoken to them, but also the linguistic input from the ambient environment. Two hypotheses were formed based on the current findings as well as on those in the previous literature. The first hypothesis, heightened acoustic sensitivity hypothesis, states that bilingual infants are more sensitive to the acoustic details in the input than are monolinguals. Heightened acoustic sensitivity brings bilingual infants advantages and disadvantages along the language developmental trajectory. On the one hand, heightened acoustic sensitivity may facilitate contrast detection. On the other hand, it may lead to a prolonged category formation process. The second hypothesis, the minimum threshold hypothesis, assumes a minimum input requirement (either absolute or relative based on frequency) for infants to acquire native sounds and words. Future research on infant bilingualism should study the relationship between language exposure and acquisition patterns.
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