In speech, vowels and consonants are two the basic sounds that combined result in lexically meaningful items in all languages. In tone languages, changes in pitch, tone differences also make meaningful lexical distinctions in spoken words. Young infants appear to have no trouble perceiving speech sounds and their production of sounds peculiar to their particular language environment proceeds relatively smoothly and rapidly compared with adults’ acquisition of foreign languages. One way of looking at how infants come to acquire speech sounds of their first language is by examining the speech input they receive. The term infant-directed speech (IDS) has been coined to describe the special way adults and even older children speak to infants. IDS is different to adult-directed speech in various acoustic/phonetic modifications, such as exaggerated prosody, increased pitch and vowel hyperarticulation (Burnham, Kitamura, and Vollmer-Conna, 2002; Kuhl et al., 1997). The exaggerated prosody and increased pitch appear to be related to the expression of affect and gaining infants’ attention (Burnham, Kitamura, and Vollmer-Conna, 2002), whereas vowel hyperarticulation appears to be related to infants’ speech development for a number of reasons. Firstly, investigating how adults speak to foreigners, Uther, Knoll, and Burnham (2007) found that vowels are also hyperarticulated in foreigner-directed speech as in IDS, while other acoustic modifications such as exaggerated prosody and increased pitch, related to affective and attentional factors, are not present in foreigner directed speech. Secondly, Liu, Kuhl, and Tsao (2003) found a positive correlation between vowel hyperarticulation and infants’ native speech perception; mothers who hyperarticulated their vowels more had infants who were better able to discriminate native consonant contrasts.\ While vowel hyperarticulation in IDS to 6-month-olds has been investigated in both tone languages such as Mandarin (Liu et al., 2003), and non-tone languages such as Russian, Swedish, American English (Kuhl et al., 1997) and Australian English (Burnham et al, 2002), no parallel studies have been conducted on the possibility of tone hyperarticulation in tone language IDS. If vowel hyperarticulation is related to infants’ language development then tones in tone hyperarticulated. The possibility of tone as well as vowel hyperarticulation in IDS of the tone language Cantonese, and the development of hyperarticulation across the first 12 months of infancy were investigated here using a longitudinal sequential cohort design. Two groups of native Cantonese mothers were recorded speaking to their infants, the first group at 3, 6, and 9 months, and the second at 6, 9, and 12 months. The study had four main aims (1) to investigate whether tone hyperarticulation occurs in IDS in a tone language Cantonese (2) to investigate whether vowel hyperarticulation occurs in IDS in Cantonese (IDS in this languages had not yet been investigated) and if 1 and 2 are the cases (3) to compare tone and vowel hyperarticulation, and (4) to chart the development of tone and vowel hyperarticulation across the infant’s first 12 months. Contrary to previous findings of vowel hyperarticulation in English Russian, Swedish, and Mandarin IDS to 6-month-olds (Burnham et al., 2002; Kuhl et al., 1997); vowel hyperarticulation was not found for Cantonese IDS. More detailed acoustic analysis examining different dimensions of the vowel space suggest that after the infant is 3 months old, mothers’ vowels begin to be hypoarticulated in IDS compared to ADS on dimensions of back versus front, and high versus low. This pattern of results is consistent with vowel perception studies which suggest that infants have already tuned into the native vowel categories by 4 to 6 months (Polka and Werker, 1994). Tone hyperarticulation, on the other hand, was indeed present at 3 months and increased to peak at 6 to 9 months before declining at 12 months. This pattern of tone hyperarticulation across the first year of infancy is consistent with infant language development – in which attenuation of perception of non-native tones had been found between 6 to 9 months (Mattock and Burnham, 2006). Moreover, detailed phonetic analysis revealed that while the level tones are more hyperarticulated than the contour tones, tones with similar onsets and offsets (i.e., the two rising tones) are actually hypoarticulated in IDS at 9 and 12 months, a time when infants have already tuned into native tones. Finally, results from a preliminary native speech discrimination study using the same infants provide some initial indication that mothers who hyperarticulate tones more also had infants who are better able to discriminate native Cantonese consonants. Together these results suggest that in Cantonese IDS vowels are underspecified whereas tones are consistently over-specified particularly at 6 months when infants are tuning into native tones. Moreover, during this initial period of tone acquisition, only level tones are over specified while tones with similar onsets and offsets are underspecified. It seems likely that for Cantonese language environment infants, during the early stages of language acquisition, pitch information specified by level tones is sufficient for initial acquisition of information about the Cantonese tone space and that information about vowels is not so essential at this time. These studies show that there is indeed tone hyperarticulation in IDS in tone languages, and that in order to make sense of the vowel hyperarticulation data in tone languages, it is important to investigate both vowels and tones in tone languages with complex tone systems such as Cantonese, instead of simply applying Anglocentric notions of vowel hypoarticulation. / Doctor of Philosophy (PhD)

Thesis (Ph.D.) -- University of Western Sydney, 2008. / A thesis submitted to the University of Western Sydney, College of Arts, MARCS Auditory Laboratories, in fulfilment of the requirements for the degree of Doctor of Philosophy. Includes bibliographical references.