Improving the Speech Intelligibility By Cochlear Implant Users

In this thesis, we focus on improving the intelligibility of speech for cochlear implants (CI) users. As an auditory prosthetic device, CI can restore hearing sensations for most patients with profound hearing loss in both ears in a quiet background. However, CI users still have serious problems in understanding speech in noisy and reverberant environments. Also, bandwidth limitation, missing temporal fine structures, and reduced spectral resolution due to a limited number of electrodes are other factors that raise the difficulty of hearing in noisy conditions for CI users, regardless of the type of noise. To mitigate these difficulties for CI listener, we investigate several contributing factors such as the effects of low harmonics on tone identification in natural and vocoded speech, the contribution of matched envelope dynamic range to the binaural benefits and contribution of low-frequency harmonics to tone identification in quiet and six-talker babble background. These results revealed several promising methods for improving speech intelligibility for CI patients. In addition, we investigate the benefits of voice conversion in improving speech intelligibility for CI users, which was motivated by an earlier study showing that familiarity with a talker’s voice can improve understanding of the conversation. Research has shown that when adults are familiar with someone’s voice, they can more accurately – and even more quickly – process and understand what the person is saying. This theory identified as the “familiar talker advantage” was our motivation to examine its effect on CI patients using voice conversion technique. In the present research, we propose a new method based on multi-channel voice conversion to improve the intelligibility of transformed speeches for CI patients

Multifaceted evaluation of a binaural cochlear‐ implant sound‐processing strategy inspired by the medial olivocochlear reflex

[ES]El objetivo de esta tesis es evaluar experimentalmente la audición de los usuarios de implantes cocleares con una estrategia de procesamiento binaural de sonidos inspirada en el reflejo olivococlear medial, denominada "estrategia MOC". La tesis describe cuatro estudios dirigidos a comparar la inteligibilidad del habla en ruido, la localización de fuentes sonoras y el esfuerzo auditivo con procesadores de sonido estándar y con diversos procesadores MOC diseñados para reflejar de forma más o menos realista el tiempo de activación del reflejo olivococlear medial natural y sus efectos sobre la comprensión coclear humana

Factors affecting the perception of noise-vocoded speech: stimulus properties and listener variability.

This thesis presents an investigation of two general factors affecting speech perception in normal-hearing adults. Two sets of experiments are described, in which speakers of English are presented with degraded (noise-vocoded) speech. The first set of studies investigates the importance of linguistic rhythm as a cue for perceptual adaptation to noise-vocoded sentences. Results indicate that the presence of native English rhythmic patterns benefits speech recognition and adaptation, but not when higher-level linguistic information is absent (i.e. when the sentences are in a foreign language). It is proposed that rhythm may help in the perceptual encoding of degraded speech in phonological working memory. Experiments in this strand also present evidence against a critical role for indexical characteristics of the speaker in the adaptation process. The second set of studies concerns the issue of individual differences in speech perception. A psychometric curve-fitting approach is selected as the preferred method of quantifying variability in noise-vocoded sentence recognition. Measures of working memory and verbal IQ are identified as candidate correlates of performance with noise-vocoded sentences. When the listener is exposed to noise-vocoded stimuli from different linguistic categories (consonants and vowels, isolated words, sentences), there is evidence for the interplay of two initial listening 'modes' in response to the degraded speech signal, representing 'top-down' cognitive-linguistic processing and 'bottom-up' acoustic-phonetic analysis. Detailed analysis of segment recognition presents a perceptual role for temporal information across all the linguistic categories, and suggests that performance could be improved through training regimes that direct attention to the most informative acoustic properties of the stimulus. Across several experiments, the results also demonstrate long-term aspects of perceptual learning. In sum, this thesis demonstrates that consideration of both stimulus-based and listener-based factors forms a promising approach to the characterization of speech perception processes in the healthy adult listener