The acquisition of voicing contrasts and /s/-clusters in children with hearing loss
Hearing aids (HAs) and/or cochlear implants (CIs) help children with hearing loss (HL) achieve functional spoken language. Nonetheless, clinicians often report difficulties with phonological contrasts, and speech errors seem to vary with device type. Voicing contrasts are often reported to be especially challenging for children with HL, with difficulties lasting sometimes until 12 years. However, voicing contrasts are critical for differentiating word meanings in both onset (e.g., beach vs. peach) and coda positions (e.g., dog vs. dock); difficulties with producing these contrasts can lead to communication breakdown and academic challenges. Word-initial /s/-stop clusters also present a challenge, with the /s/ often dropped, especially for children with HAs (e.g., stick à tick), raising questions about the phonological representations for such words. The aim of this thesis was therefore to better understand the acoustic realization of durational cues to voicing contrasts by both children with HAs and children with CIs, at all places of articulation. This in turn should have implications for intervention.
This thesis consisted of five studies. Studies 1 and 2 examined how acoustic cues to onset and coda voicing are realised in Australian English-speaking 4-5-year-olds and adults with normal hearing (NH). This then provided a comparative baseline for investigating the same issues in children with HL. The results showed that 4-5-year-olds with NH have acquired a productive knowledge of onset and coda voicing contrasts, using the same range of acoustic cues as adults, though acoustic values are not yet fully adultlike. Studies 3 and 4 then investigated the production of onset and coda voicing contrasts in 4-5-year-olds with HL. The results showed that children with HL only have a limited use of the acoustic cues to mark voicing in onset position, with no effect of device type, and that children with HAs make use of more acoustic cues to coda voicing contrasts than their peers with CIs. This suggests that children with HL have a productive knowledge of voicing contrasts, but that their phonological representations are not yet acoustically implemented like those of their peers with NH. The children with CIs also showed a less robust realization of coda voicing contrasts than those with HAs. Finally, Study 5 examined how word-initial /s/-stop clusters are produced in children with HL. The results showed that children with HL have very good accuracy, though the children with HAs omit stops more often than their peers with either CIs or NH. The reduction pattern of children with HL matches what has been observed in previous studies on children with NH, suggesting a still-developing phonology rather than a perceptual deficit.
Taken together, the findings of this thesis suggest that 4-5 year-olds with HL have productive knowledge of both stop onset and coda voicing contrasts, although their acoustic realizations are not always like that of their peers with NH. Children with HL may need more time to fine-tune the acoustic implementation of phonological contrasts, and this may vary as a function of device type. In addition, their production of /s/-stop clusters is very good and close to that of their peers with NH. Altogether, early intervention has helped these children reach functional language even though their acoustic implementations are not yet fully like those of their NH peers.