Objective evaluation of vowel identification with the digisonic cochlear implant

Perceptual assessment of speech recognition performance with cochlear implants is confronted with problems raised by patients' availability and specificity. These problems can be avoided by using objective evaluation methods based on automatic classification of speech sounds. In the present report, different tuning strategies for the Digisonic cochlear implant were evaluated by discriminant analysis. The results presented here concern the correct classification of vowels. A set of 600 vocalic items, corresponding to 10 vowel categories and produced by 20 French speakers, were processed by the Digisonic DX10 with two different strategies. In the linear strategy, center frequencies of the 15 Digisonic channels were almost equally spaced in acoustic units (Hz). In the mel strategy, center frequencies were almost equally spaced in psychoacoustic units. For each strategy, energy levels of the frequency channels were quantified for each vocalic item and processed by statistical discriminant analysis for classification into 10 vowel categories. Results mainly show that the percent correct classification (PCC) is larger for the mel strategy and that the improvement is due to a higher concentration of frequency channels below 4 kHz. Implications of these results for the tuning of cochlear implants are discussed.