The Effect of Auditory Input on the Rate of Speech Production by Cochlear Implant Users
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Children with cochlear implants (CIs) present with slower speech than their typical hearing (TH) peers. The slowed speaking rate of children with CIs has received recent attention due to the relation of speaking rate with working memory and with speech intelligibility. However, the underlying causes for the slowed speech of CI recipients are not fully understood. In this thesis, three theoretical explanations are introduced to understand how degraded auditory input affects the timing of speech. First, children with CIs may have slowed speech due to an immature internal model caused by degraded auditory feedback and the lack of auditory input. Second, the speech of children with CIs may become slower because either all or a part of speech processing stages are slowed. Third, the slowed speech of children with CIs may be influenced by paralinguistic factors, such as the intention to speak more clearly. To explore the reasons for the slowed speech of CI recipients, three experiments were performed. A first experiment explored the effects of chronological factors (such as age of implant and duration of CI experience) and performance factors (such as language ability and speech perception) on the speaking rates of 75 children with CIs and 54 children with TH at four to eight years after implantation. Results showed that speaking rate was significantly slower in the CI group than the TH group, confirming previous reports. Maturation and the amount of the auditory input were important to speaking rate, supporting the immature internal model hypothesis. Out of performance factors, language ability was the best predictor of the speaking rate of children with CIs, suggesting that the speed and accuracy of linguistic processing may be a key to the slowed speech of children with CIs. A second experiment explored whether the slowed speaking rate of children with CIs results from impairments at particular linguistic levels (lexical, syntactic, phonetic/articulatory) of speech processing and whether the speaking rates at these levels are related to working memory. Results indicated that the speaking rates of participants with CIs showed the greatest reduction compared to their TH counterparts when engaged in syntactic level processing. Also, the speaking rates for the syntactic task showed the strongest relationship with children’s memory span, suggesting syntactic processing could be an important factor in the relation between speaking rate and memory. A third experiment investigated whether paralinguistic factors are involved in the slowed speech of children with CIs. Twelve teenagers with CIs and twelve counterparts with TH were asked to repeat short sentences “as fast as possible,” at their habitual speed, and “slowly and clearly.” Results showed that participants with CIs were able to increase their rate but their rate changes for the slow/clear condition was not statistically significant, suggesting that the slowed speech of participants with CIs may be due to strategic choice, rather than cognitive or physical limitations. Overall, the results of these three experiments suggest that the slowed speech of CI recipients are influenced by multiple factors, including maturation, CI experience, and linguistic and paralinguistic components. Implications for clinical practice are discussed.