Vocal development in infants with Down syndrome and infants who are developing normally.

The development of early vocalizations was investigated with 13 infants who had Down syndrome and 27 infants who were developing normally at bimonthly intervals from 4 to 18 months of age. A perceptually based framework was used to categorize utterances according to their developmental relations with adult, well-formed, or "canonical" syllables. Over time, both groups demonstrated increased production of mature vowel and canonical consonant-vowel syllables (characterized by full vowels, consonants, and rapid, well-coordinated articulatory movements) and decreased production of less mature "quasi-vowel" and marginal syllables. Infants in both groups were also quite variable in vocal development, both within group and across time.