Abstract:
Cochlear implants produce highly unnatural temporal and spatial (tonotopic) patterns of neural activation in the auditory system, and the patterns may vary considerably from patient to patient. In spite of these abnormal patterns, psychophysical measures of temporal processing are relatively normal in patients with cochlear implants and brainstem implants. A high level of speech recognition is possible with only three bands of modulated noise [Shannon et al., Science 270, 303--304 (1995)]. What parameters of the peripheral activity pattern are critical for the transmission of speech pattern information? Four-band speech processors were constructed which reduced the spectral information in speech to four, amplitude-modulated noise bands. Variations were constructed that altered the location, spectral extent, overlap, and amplitude compression of the four noise bands relative to the original speech. Manipulations of the band overlap and the cut-off frequencies dividing the bands had relatively little effect on speech recognition. However, amplitude compression and the tonotopic misalignment of noise bands with the speech cues resulted in severe reductions in speech recognition. These experiments and the performance levels of present cochlear implant patients demonstrate that normal temporal information and considerable speech information can be received with relatively poor tonotopic selectivity. [Work supported by NIDCD.]