Effects of Device Limitations on Acquisition of the /t/-/k/ Contrast in Children With Cochlear Implants

The present study investigated how development of the /t/-/k/ contrast is affected by the unique perceptual constraints imposed on young children using cochlear implants (CIs). We hypothesized that children with CIs would demonstrate unique patterns of speech acquisition due to device limitations, rather than straightforward delays due to a lack of auditory input in the first year of life before implantation. This study focused on the contrast between /t/ and /k/ because it is acquired early in the sequence of development, requires less advanced motor control than later-acquired place contrasts, is differentiated by spectral cues (which are particularly degraded when processed by CIs), and is not easily differentiated by visual cues alone. Furthermore, perceptual confusability between /t/ and /k/ may be exacerbated in front-vowel contexts, where the spectral energy for /k/ is shifted to higher frequencies, creating more spectral overlap with /t/.

Children with CIs (n = 26; ages 31 to 66 mo) who received implants around their first birthdays were matched to peers with normal hearing (NH). Children participated in a picture-prompted auditory word-repetition task that included over 30 tokens of word-initial /t/ and /k/ consonants. Tokens were balanced across front-vowel and back-vowel contexts to assess the effects of coarticulation. Productions were transcribed and coded for accuracy as well as the types of errors produced (manner of articulation, voicing, or place of articulation errors). Centroid frequency was also calculated for /t/ and /k/ tokens that were produced correctly. Mixed-effects models were used to compare accuracy, types of errors, and centroid frequencies across groups, target consonants, and vowel contexts.

Children with CIs produced /t/ and /k/ less accurately than their peers in both front- and back-vowel contexts. Children with CIs produced /t/ and /k/ with equal accuracy, and /k/ was produced less accurately in front-vowel contexts than in back-vowel contexts. When they produced errors, children with CIs were more likely to produce manner errors and less likely to produce voicing errors than children with NH. Centroid frequencies for /t/ and /k/ were similar across groups, except for /k/ in front-vowel contexts: children with NH produced /k/ in front-vowel contexts with higher centroid frequency than children with CIs, and they produced /k/ and /t/ with equal centroid frequencies in front-vowel contexts.

Children with CIs not only produced /t/ and /k/ less accurately than peers with NH, they also demonstrated idiosyncratic patterns of acquisition, likely resulting from receiving degraded and distorted spectral information critical for differentiating /t/ and /k/. Speech-language pathologists should consider perceptual confusability of consonants (and their allophonic variations) during their assessment and treatment of this unique population of children.