The acoustic-phonetic characteristics of infant-directed speech in Mandarin Chinese and their relation to infant speech perception in the first year of life

Current views of language acquisition suggest that there is an interplay between innate knowledge of linguistic structure and the ambient language environment. The characteristics of speech addressed to infants may play an important role in shaping infants' linguistic development. To assess the role of speech input on speech perception learning, this study investigated the acoustic characteristics of phonetic units of infant-directed speech, including vowels and lexical tones in Mandarin Chinese, and examined the links between speech input and infant speech perception performance in the first year of life.; In the first study, an instrumental analysis was employed to examine the critical acoustic characteristics of vowels and lexical tones of infant-directed speech in Mandarin and to compare these acoustic parameters in two speech styles, infant-directed vs. adult-directed speech. The results show that the speech directed to infants contains acoustically more exaggerated phonetic units when compared to adult-directed speech. For vowel segments, the acoustic distance was stretched between vowel categories, resulting in an expanded acoustic space area, and the within-vowel formant variation was also increased, generating greater acoustic variation of instances representing each vowel. Significant increases in fundamental frequency and vowel duration were also shown in infant-directed speech. For lexical tones, the pitch range of individual tones was exaggerated but the critical pitch-contour temporal shape was well preserved so that correct word meaning was preserved in infant-directed speech. The results suggest that Mandarin mothers modified their speech characteristics at the segmental level to amplify important acoustic cues for identifying phonetic units when addressed their infants.; In the second study, the exaggerated acoustic-phonetic characteristics in a particular mother's infant-directed speech were demonstrated to be significantly correlated with her infant's speech perception sensitivity in the first year of life. Specifically, the exaggerated vowel space area of mothers' infant-directed speech was strongly and consistently correlated with infant speech-discrimination ability at 6--8 and 10--12 months of age. In addition, the lexical tone expansion of mothers' infant-directed speech was correlated with infant speech-discrimination ability at 10--12 months of age. The results support the view that hyperarticulation of phonetic units in infant-directed speech facilitates infants' early speech-perception learning.