Identification of speech by cochlear implant recipients with the multipeak (MPEAK) and spectral peak (SPEAK) speech coding strategies II. Consonants.

Abstract

The major objective of this study was to evaluate differences in consonant recognition with the Multipeak (MPEAK) and the Spectral Peak (SPEAK) speech coding strategies of the Nucleus-22 Cochlear Implant System. This objective was addressed by comparison of acoustic and electrode activation analyses of consonants with cochlear implant recipients' responses to these same consonant tokens when they used the two speech coding strategies. Nine subjects identified 14 English consonants with the MPEAK and SPEAK speech coding strategies. These strategies were compared with an ABAB design. Evaluation occurred during two weekly sessions after subjects used each strategy for at least 3 wk in everyday life. Group medial consonant [aCa] identification scores with the SPEAK strategy were significantly higher than with the MPEAK strategy (76.2% versus 67.5%; p < 0.001). This improvement was largely due to the significant increase in information transmitted for the place feature (p < 0.001) through accurate tracking of second formant transitions and spectrally specific stimulation patterns to differentiate [s] from [symbol see text] and [n] from [m], and the stop consonant bursts. For this reason, more nasal consonants were correctly identified with SPEAK, but there also were more non-nasal error responses when the nasal murmur was of unusually low amplitude. Consequently, significantly less information was transmitted for the nasality feature with SPEAK than MPEAK (p < 0.001). Electrical stimulation with the SPEAK strategy provided better spectral representation of the stop consonant bursts, tracking formant transitions into the following vowel, frication in the consonant [symbol see text], and the formants for the nasals [m] and [n] than with the MPEAK strategy. The marked improvement in recognition of the velar consonants, [g] and [k], which cannot be seen during speechreading, should allow greater ease and accuracy of communication with SPEAK than MPEAK.