Intelligibility of words by reduced-channel and cochlear implant listeners

One of the differences in the way a cochlear implant, versus a normally functioning cochlea, delivers sound to auditory centers in the brain is that the implant delivers far fewer channels of frequency information. This experiment investigated how speech intelligibility in cochlear implant (CI) listeners may be related to reduced frequency resolution as opposed to other factors. A closed-response set Modified Rhyme Test was administered to CI listeners [House et al., J. Acoust. Soc. Am. 37 (1965)] using naturally produced words spoken in isolation. The stimuli presented to the listeners without implants were a reduced-channel (RC) version of exactly the same stimuli, which were filtered and processed in software in a manner similar to the hardware method described by Shannon [Science 270 (1995)]. The effect of the signal processing was to divide each word into four frequency channels for the first group of RC listeners and into eight channels for the second group. Results indicated final phoneme identification appeared to be more difficult for CI listeners than for four- or eight-band RC listeners. Results were more variable for phonemes in the initial position. CI listeners were better than both four- and eight-band listeners on affricates and better than eight-band listeners for weak fricatives.