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Facing previous mixed findings between monolingual and bilingual infants’ phonetic development during perceptual reorganization, the current study aims at examining the perceptual development of a native vowel contrast (/I/-/i/) by Dutch monolingual and bilingual infants. We tested 390 Dutch monolingual and bilingual infants from 5 to 15 months of age through a visual habituation paradigm. Mixed-effect model analyses were conducted within 320 infants, with infants’ log10 transformed looking time as the dependent variable, age (4-level) and language background (2-level) as the fixed factors, and participant and order (2-level) as the random factors. All infants show weak initial sensitivity to the contrast regardless of language background(s), and sensitivity improves with age. By the second half of the first year, infants discriminate the contrast, indicating the emergence of the relevant vowel categories. In addition, a perceptual lead is observed in bilingual infants, probably due to: 1) a perceptual transfer from the close-category counterpart of the other native language; 2) heightened acoustic sensitivity in bilingual infants given their rich linguistic experience; and 3) a general bilingual cognitive advantage. The influences of contrast salience and bilingualism on language development are discussed. Overall, these findings constitute an extension of existing work on vowel perception and display a novel acceleration effect for the bilingual infants in phonetic perception. In addition, we propose a novel heightened acoustic sensitivity hypothesis, arguing that bilingual infants may pay more attention to acoustic details in the input than their monolingual peers. The observed progressive phonetic discrimination pattern of the native contrast contributes to our knowledge in infant language development, and specifically perceptual reorganization patterns, in the first year after birth. The observed acceleration effect, along with its explanations, provides new insights into the influence of bilingualism and potential bilingual advantages in infancy.
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