Speech intelligibility as a function of the number of channels of stimulation for signal processors using sine-wave and noise-band outputs

Abstract

Vowels, consonants, and sentences were processed through software emulations of cochlear-implant signal processors with 2-9 output channels. The signals were then presented, as either the sum of sine waves at the center of the channels or as the sum of noise bands the width of the channels, to normal-hearing listeners for identification. The results indicate, as previous investigations have suggested, that high levels of speech understanding can be obtained using signal processors with a small number of channels. The number of channels needed for high levels of performance varied with the nature of the test material. For the most difficult material--vowels produced by men, women, and girls--no statistically significant differences in performance were observed when the number of channels was increased beyond 8. For the least difficult material--sentences--no statistically significant differences in performance were observed when the number of channels was increased beyond 5. The nature of the output signal, noise bands or sine waves, made only a small difference in performance. The mechanism mediating the high levels of speech recognition achieved with only few channels of stimulation may be the same one that mediates the recognition of signals produced by speakers with a high fundamental frequency, i.e., the levels of adjacent channels are used to determine the frequency of the input signal. The results of an experiment in which frequency information was altered but temporal information was not altered indicates that vowel recognition is based on information in the frequency domain even when the number of channels of stimulation is small.

Channel interaction limits melodic pitch perception in simulated cochlear implants

“…Although phoneme and sentence recognition performance has been shown to be similar with sinewave and noise-band CI simulations (Dorman et al, 1997), sinewave simulations have been shown to better emulate real CI performance for pitch-related speech tasks (Luo et al, 2007). Sinewave carriers offer better stimulation site specificity and better temporal envelope representation than noise-band carriers.…”

Effects of source-to-listener distance and masking on perception of cochlear implant processed speech in reverberant rooms

“…These reductions are particularly severe for listeners with impaired hearing, who typically require less noise and reverberation to achieve the same intelligibility as listeners with normal hearing ͑Finitzo-Hieber and Tillman, 1978;Duquesnoy and Plomp, 1980;Helfer and Wilber, 1990͒. The temporal effects of reverberation on speech may pose a particular challenge for cochlear implant ͑CI͒ users, who receive their auditory cues from temporal envelope modulations in a limited number of spectral channels. Temporal envelope modulations suffice to provide intelligible speech for as few as four spectral channels under ideal conditions ͑Shannon et al., 1995;Dorman et al, 1997͒. However, when the channels' envelope modulations are low-pass filtered with cutoff frequencies in the 8-16 Hz range, intelligibility decreases ͑Fu and Shannon, 2000;Xu and Zheng, 2007͒.…”

Use of Amplitude Modulation Cues Recovered from Frequency Modulation for Cochlear Implant Users When Original Speech Cues Are Severely Degraded

“…When turning off electrodes, the frequency allocation for each electrode gets redistributed and the bandwidth for each electrode changes. The results of previous studies investigating the effects of number of channels were generally interpreted in terms of limited frequency resolution (e.g., Dorman et al 1997;Loizou et al 1999;Friesen et al 2001) and limitations of multichannel stimulation caused by channel interaction (e.g., Bingabr et al 2008;Strydom and Hanekom 2011). A narrower bandwidth is desirable (i.e., more processing channels) because a more accurate analysis of the frequency spectrum is made possible.…”