Lexical tone development, music perception and speech perception in noise with cochlear implants: The effects of spectral resolution and spectral mismatch

This work examines lexical tone development, speech, and music perception with cochlear implants, in relation to the effects of spectral resolution (i.e., number of channels) and monaural and binaural frequency-place mismatch (i.e., spectral shift and compression) in cochlear implants. The purpose of studying the effects of spectral resolution with cochlear implants was that the contemporary systems of cochlear implants provide only temporal envelope information of acoustic signals that is sufficient for speech perception in quiet but fails to provide enough spectral resolution for adequate presentation in noisy environments due to the lack of representation of the fine structure. Poor spectral resolution causes poor representation of pitch information that is critical for music appreciation and lexical tone perception in tonal languages. Chapter 2 discusses the development of novel and objective measures to the assessment of tone production in pediatric cochlear implants users. The measures included acoustic analyses of F0 distribution, cross correlation of F0 contours, and a neural network approach. Chapter 3 reports lexical tone development in a large group of Mandarin-speaking children with cochlear implants. The contributing factors to the children's tone perception and production development were separately identified in multivariable regression analyses. Cochlear implants with shallow insertion presents a unique phenomenon where the content of the speech signal is delivered to higher frequency regions of the cochlea than the more mid-range regions commonly used in speech recognition, resulting in upward (i.e., basal) spectral shift. Chapter 4 discusses such effects of such implant related frequency-place mismatch on lexical tone perception and consonant confusion with a unilateral cochlear implant simulation using a noise-excited vocoder. Chapter 5 presents a novel speech processing strategy that uses dichotic stimulation to improve spectral resolution in bilateral implants. The dichotic stimulation assigns odd- and even-number spectral channels that carry complementary information in an interleaved manner to bilateral implants. The effects of binaural frequency-place mismatch as well as the effects of increased spectral resolution delivered via dichotic stimulation were examined for music perception, lexical tone perception, frequency discrimination, and speech perception in noise. Results of the study indicated that dichotic stimulation significantly improved speech perception in noise due to better auditory segregation. Customized frequency-to-electrode maps that minimize spectral mismatch may provide better speech perception in monaural or dichotic stimulation.