Objetivos de la evaluación del desempeño.

Figure 1 shows the mean difference between pre-implant and post-implant scores and the 95% confidence interval across all subjects and all raters for the spontaneous speech samples. Paired

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sample t-tests showed that although there was a trend towards improved production of some items (voice break, speech rate, rhythm, pauses and pleasantness), none of the items improved significantly after implantation. The data showed large inter-individual variability between different speakers.

Part 2: Evaluation of deaf speech during read text

Figure 2 shows the mean difference between pre-implant and post-implant scores and the 95% confidence interval across all subjects and all raters for the read speech samples. Although there was a trend towards improved production of several segmental aspects, a paired sample t-test showed that after a Bonferroni correction for multiple comparisons, only the consonants, /t/ (p<0.001), /g/ (p<0.001) and the item vowel space (p<0.001) improved significantly after implantation. Looking at the individual data of these items, figure 3 showed that the vast majority of the individual data points were above the black diagonal line, indicating improvements in the score. Again there was a wide range in performance and improvements were small.

Figure 1.

Mean difference between preoperative and postoperative ratings and 95% confidence interval of all items in Part 1 (spontaneous speech). Higher scores indicate more improvement.

        A   C         B Figure 3.

Pre-implant compared to post-implant rated score of each individual, pooled over 17 raters. Data is shown of each item that improved significantly after implan- tation: a) consonant /t/;b) consonant /g/ and c) vowel space. Diagonal line shows complete agreement between pre- and post-implant scores. Data points are labeled with subject number. Higher scores indicate better speech production.

Figure 2.

Mean difference between preoperative and postoperative ratings and 95% confidence interval of all items in Part 2 (read text). Higher scores indicate more improvement.

P-values are marked with *= p<0.05; **=p<0.01; ***=p<0.001.

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Speech recognition

Paired sample t-test showed that phoneme scores improved significantly from 6% before implantation with hearing aid to 29 % one year after implantation (p< 0.001). No correlations were found between the gain in speech production on each item (i.e. post- minus pre-implant score) and the gain in phoneme score (i.e. post- minus pre-implant score).

A one-tailed Pearson correlation test showed, after Bonferroni correction, significant correlations between the phoneme score one year after implantation and pre-operative rated score of the items vowel space (p<0.001), cluster reduction (p<0.001), stress use (p<0.001), intelligibility (p<0.001), articulation (p<0.001), pleasantness (p<0.001) and deaf speech (p<0.001). Figure 4 show a typical example of a segmental, a suprasegmental and a general item.

          A           C           B Figure 4.

Examples of correlations between speech production before implantation and speech perception after im- plantation. Shown are the correlations between pho- neme score and the rated score on typical examples of:

a) a segmental item “vowel space”; b) a suprasegmen- tal item “stress use” and c) a general item “intelligibility”. Individual data points are shown, representing mean score of 17 raters. Data points are labelled with subject number.

DISCUSSION

In this explorative study a panel of raters evaluated several aspects of speech production and the overall quality of speech in prelingually deaf adults. This study showed a trend towards improved speech production of these adults one year after implantation. Despite improvements in speech recognition performance one year after implantation, gain in speech perception was not related to gain in speech production. We did found correlations between pre-operative speech production and postoperative speech recognition (cf. Klop et al., 2007; van Dijkhuizen et al., 2011). Unlike previous studies, a larger group of prelingually deaf CI users was included (Evans and Deliyski, 2007; Klop et al., 2007; van Dijkhuizen et al., 2011).

With regard to speech production, the results of this study showed mainly improvements in segmental aspects of speech after implantation: the production of consonant /t/ and /g/ improved and a trend towards fewer syllable reduction, cluster reductions and improved production of the consonants /l/ was seen. A comparable improvement in consonant production after implantation has been shown in previous studies with children but not with adults (Bouchard et al., 2007; Peng et al., 2004; Tobey et al., 1991). We also found an improved vowel production, which is in line with previous studies with postlingually deaf adults, using objective and subjective measures (Hirano et al., 1997; Horga and Liker, 2006; Langereis et al., 1995, 1997).

No significant differences were found in suprasegmental aspects of speech. Concerning the voice pitch and intonation, it is not surprising that these items did not improve because CIs are largely unable to encode voice pitch information (Straatman et al., 2010). Part of lack of improvement in these items might, therefore, be a result of the speaker’s inability to detect the fluctuations in the fundamental frequency. This was in contrast to the study of Evans et al., who found a decrease in F0 in all subjects after implantation (Evans and Deliyski, 2007). In addition, unlike Evans et al. who used the nasometer (Evans and Deliyski, 2007), no changes were found in nasalisation in the present study. These differences might be a result of differences in measurement method (objective vs. subjective) or the small size of the study group of Evans et al. (Evans and Deliyski, 2007).Surprisingly, no changes were found in loudness control.

It should be noted that despite the improvements in some segmental aspects of speech, deviant speech still persisted one year after implantation. It is unclear whether prelingually deaf adults reach their plateau of speech production after one year. Based on our clinical experience, we believe that speech production might continue to improve even after the one-year evaluation period. This belief follows the reasoning that first speech perception performance should increase before changes in speech recognition occur. It is suggested that optimal performance levels in speech recognition can be reached one year after implantation (for review see Teoh et al., 2004a); however, also longer periods

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of time are reported (Waltzman et al., 2002). The evaluation of long-term speech production outcomes should therefore be considered in future research. In addition, no correlation was found between speech recognition and production measured with the subjective rating method. It is possible that the auditory feedback after CI in some individuals is too limited to result in adequate speech recognition, but it might be sufficient in these individuals to result in limited improvements of speech production.

Although our subjects meet the CI inclusion criteria (see Materials and Method section), there were inter-individual variations in speech production as well as speech recognition within the prelingually deaf group. This was in line with previous studies, showing large inter-individual differences in performance in late implanted prelingually deaf adults (Schramm et al., 2002; Teoh et al., 2004a; Waltzman et al., 2002). Teoh et al. concluded that these differences are particularly related to differences in subject characteristics rather than implant device properties (Teoh et al., 2004a).

Preoperative prediction of the outcome after implantation is still a clinical dilemma. The present study showed correlations between several segmental (i.e. vowel space, cluster reduction), suprasegmental (i.e. usage of stress) and general (i.e. articulation, intelligibility and deaf speech) aspects of speech. The relation between post-implant speech recognition and pre-implant speech production was in line with previous studies (Klop et al., 2007; van Dijkhuizen et al., 2011). Van Dijkhuizen et al. also found high correlations when using the pre-implant production of vowels. They hypothesised that speech production before implantation might be an indicator of the amount of effective auditory stimulation during childhood. Auditory input during this period is necessary for adequate maturation of the auditory cortical system (Kral, 2013; Kral and Sharma, 2012; Teoh et al., 2004b). It has been hypothesised that lack of auditory input during the first years of life result in cross-modal reorganization of the auditory areas by visual functions, which might hinder auditory processing after implantation (Hirano et al., 2000; Lee et al., 2001; Lee et al., 2007; Lee et al., 2005; Teoh et al., 2004b).

In contrast to the study of van Dijkhuizen et al., we did not use the pre-operative speech production as selection criteria for implantation (van Dijkhuizen et al., 2011). Our results strengthen their hypothesis that pre-operative speech production is not only a sufficient predictor in subjects with above-average speech production but also in individuals with below-average speech production. Although the method used in our study is time consuming and probably difficult to use in clinical situations, it strengthens the formerly suggested predictive value of pre-operative speech production for post-implant speech recognition performance in prelingually deaf adults.

CONCLUSION

This study showed that some perceptual segmental aspects of speech production improve after cochlear implantation in prelingually deaf oral-oriented adults. Although deviated speech still persisted one year after implantation, improvements were found in the production of consonant /t/ and /g/ and in the vowel space. This study also illustrates that gain in speech production was not related to gain in speech recognition in prelingually deaf adults. Correlations were found between pre-operative speech production and postoperative speech recognition, indicating that speech production can be used as a predictor for speech recognition performance after implantation.