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

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.