RESUMO
In cochlear implants (CI), spread of neural excitation may produce channel interaction. Channel interaction disturbs the spectral resolution and, among other factors, seems to impair speech recognition, especially in noise. In this study, two tests were performed with 20 adult normal-hearing (NH) subjects under different vocoded simulations. First, there was a measurement of word recognition in noise while varying the number of selected channels (4, 8, 12 or 16 maxima out of 20) and the degree of simulated channel interaction ("Low", "Medium" and "High"). Then, there was an evaluation of spectral resolution function of the degree of simulated channel interaction, reflected by the sharpness (Q10dB) of psychophysical tuning curves (PTCs). The results showed a significant effect of the simulated channel interaction on word recognition but did not find an effect of the number of selected channels. The intelligibility decreased significantly for the highest degree of channel interaction. Similarly, the highest simulated channel interaction impaired significantly the Q10dB. Additionally, a strong intra-individual correlation between frequency selectivity and word recognition in noise was observed. Lastly, the individual changes in frequency selectivity were positively correlated with the changes in word recognition when the degree of interaction went from "Low" to "High". To conclude, the degradation seen for the highest degree of channel interaction suggests a threshold effect on frequency selectivity and word recognition. The correlation between frequency selectivity and intelligibility in noise supports the hypothesis that PTCs Q10dB can account for word recognition in certain conditions. Moreover, the individual variations of performances observed among subjects suggest that channel interaction does not have the same effect on each individual. Finally, these results highlight the importance of taking into account subjects' individuality and to evaluate channel interaction through the speech processor.
RESUMO
BACKGROUND: The microphone is the first link of the sound processing chain and if there is dirt on the membrane or in the input port, the frequency response is modified. OBJECTIVE: The effect of microphone cleaning in cochlear implants (CI) was studied by measuring speech perception scores in the noise of CI users and of normal hearing (NH) subjects with a CI simulator. MATERIAL AND METHODS: Sixty-one adult CI users and 20 NH were tested. In CI subjects, speech comprehension scores in noise were compared before and after cleaning microphone ports with one of two different cleaning procedures. NH listeners were tested using a vocoder that simulated soiled microphones. Two main coding schemes were considered, channel-picking (CP) and fixed-channel (FC). RESULTS: With NH subjects, the effect of the coding strategy and degree of soiling was statistically significant (p < 10-4); for the intermediate signal to noise ratios (SNRs), the recognition percentages were clearly affected. With CI users, cleaning the microphones significantly improved the recognition scores (p < 10-4). The two cleaning procedures considered turned out to be significantly different (p < 10-4). CONCLUSIONS: It is suggested to perform a regular microphone port cleaning to avoid deep soiling which leads to a decrease in speech understanding.
