Vowels and tones in infant directed speech : hyperarticulation for both, but different developmental patterns

A number of studies have shown that mothers hyperarticulate vowels in their Infant Directed Speech to their 6-month-old infants. Here we investigate the possibility that such hyperarticulation might also occur for lexical tone for mother-infant dyads in tone language environments, and possible changes in mothers' vowel and such tone hyperarticulation in IDS across the infant's first year. IDS from a total of 22 native Cantonese speaking mothers was recorded, 11 when their infants were 3-, 6-, and 9-month-old, and another 11 when their infants were 6-, 9-, and 12-month-old, and mothers asked to use nine target words in their speech; one for each of the three corner vowels /i/, /a/ and /u/), and another six for each of the Cantonese tones on the vowel /i/. Vowel hyperarticulation was investigated using first and second formant values, and tones using fundamental frequency onset and offset. Preliminary results for 5 mothers presented here indicate that both vowel and tone hyperarticulation occur, but that while vowel hyperarticulation emerges around 6 months and increases from 6 to 9 to 12 month, tone hyperarticulation occurs only at 6 and 9 months. The results suggest that, as for vowels, tone space is hyperarticulated in IDS, but returns to Adult Directed Speech levels earlier for tones than vowels. Possible reasons for this are discussed, as are future studies with other tone languages with smaller tonal inventories.