Early Phonetic Development of Twins Differ in Auditory Function: A Longitudinal Analysis of Infants from 9 to 36 Months

Abstract

Consonantal development and syllable complexity were analyzed in one pair of twins from the age of 9 to 36 months: one had a cochlear implant while the other had typical hearing. The hearing-impaired child (Child A) was diagnosed with profound hearing loss and received cochlear implantation in his left ear at the age of 14 months. After implantation, he engaged in regular language therapy every week. His younger twin sister (Child B) was recruited for the age-matched comparison. Bi-weekly recordings were made of natural interactions between the mother and the twins. The first 50 identifiable syllables from each session were transcribed and analyzed. Major findings with reference to Moeller et al. (2007) are as follows. First, Child B's volubility was consistently higher than Child A, except during the period from 21 to 27 months. Although Child A's volubility was higher than Child B, his production was less accurate than Child B. Second, canonical babbling was first found in Child A at 18 months and became stable from 27 months on, while Child B started canonical babbling as early as 9 months of age. Third, Child A's consonantal diversity at 33 months was similar to that of Child B at 21 months of age. Finally, Child A shifted from mainly utterances of a single vowel or voiced consonants to utterances of repetitive CV, VC, or CVC with single consonant type at 27 months, seldom using two or more consonant types in a single utterance; Child B mainly used utterances of repetitive CV, VC, or CVC with single consonant type in all sessions, moving toward utterances of two or more consonant types. The results indicate that the hearing-normal child showed remarkable progress in consonant inventory and syllable complexity from 18 months of age; a similar progress was not found in the hearing-impaired child until 27 months of age. In addition, after implantation for one year, the hearing-impaired child demonstrated a delayed but similar pattern in phonetic development as the normal-hearing child. Moreover, the gap between these two children narrowed after reaching 27 months of age.