Speech Information Extraction and Integration in Cochlear Implant Subjects with Contralateral Acoustic Hearing

Recognition of speech in quiet is relatively easy for cochlear implant subjects. However, recognition of melody in quiet or speech in noise is difficult due to insufficient encoding of spectral fine structure in contemporary cochlear implants. Introducing low-frequency acoustic fine structure by means of a hearing aid in addition to a cochlear implant has become an effective treatment option for people with moderate-to-severe hearing loss. The combined electro-acoustic stimulation can improve cochlear implant performance in recognition tasks that require resolving low-order harmonics in a stimulus. In addition, performance of the combined stimulation depends upon the efficiency of integrating information from two different modalities. The first part of the dissertation employed a unique stimulus paradigm of concurrent vowels, in which temporal glimpsing and high-level contextual cues were minimized, to measure the efficiency of information integration between acoustic and electric stimulation in actual electro-acoustic subjects. The integration efficiency in actual subjects was generally lower than that in simulated normal-hearing subjects. The actual integration efficiency was also highly variable but was significantly correlated with the amount of low-frequency residual acoustic hearing in the non-implanted ear. The second part of the dissertation used Mandarin, a tonal language, to evaluate the electro-acoustic stimulation benefit and information extraction in actual Mandarin-speaking subjects who used a cochlear implant and a hearing aid in the contralateral ear. Significant benefit of the combined stimulation was found in Mandarin speech recognition in noise and was significantly correlated with the amount of low-frequency residual hearing. The present dissertation suggests that the amount of residual acoustic hearing be considered in the future management of hearing impairment and design of auditory prostheses.