The relative importance of amplitude, temporal, and spectral cues for cochlear implant processor design

Speech understanding with cochlear implants has improved steadily over the last 25 years, and the success of implants has provided a powerful tool for understanding speech recognition in general. Comparing speech recognition in normal-hearing listeners and in cochlear-implant listeners has revealed many important lessons about the types of information necessary for good speech recognition--and some of the lessons are surprising. This paper presents a summary of speech perception research over the last 25 years with cochlear-implant and normal-hearing listeners. As long as the speech is audible, even the relatively severe amplitude distortion has only a mild effect on intelligibility. Temporal cues appear to be useful for speech intelligibility only up to about 20 Hz. Whereas temporal information above 20 Hz may contribute to improved quality, it contributes little to speech understanding. In contrast, the quantity and quality of spectral information appear to be critical for speech understanding. Only four spectral "channels" of information can produce good speech understanding, but more channels are required for difficult listening situations. Speech understanding is sensitive to the placement of spectral information along the cochlea. In prosthetic devices, in which the spectral information can be delivered to any cochlear location, it is critical to present spectral information to the normal acoustic tonotopic location for that information. If there is a shift or distortion of 2 to 3 mm between frequency and cochlear place, speech recognition is decreased dramatically.