Speech perception with the Vienna extra-cochlear single-channel implant: a comparison of two approaches to speech coding.

Although it is generally accepted that single-channel electrical stimulation can significantly improve a deafened patient's speech perceptual ability, there is still much controversy surrounding the choice of speech processing schemes. We have compared, in the same patients, two different approaches: (1) The speech pattern extraction technique of the EPI group, London (Fourcin et al., British Journal of Audiology, 1979,13,85-107) in which voice fundamental frequency is extracted and presented in an appropriate way, and (2) The analogue 'whole speech' approach of Hochmair and Hochmair-Desoyer (Annals of the New York Academy of Sciences, 1983, 405, 268-279) of Vienna, in which the microphone-sensed acoustic signal is frequency-equalized and amplitude-compressed before being presented to the electrode. With the 'whole-speech' coding scheme (which they used daily), all three patients showed an improvement in lipreading when they used the device. No patient was able to understand speech without lipreading. Reasonable ability to distinguish voicing contrasts and voice pitch contours was displayed. One patient was able to detect and make appropriate use of the presence of voiceless frication in certain situations. Little sensitivity to spectral features in natural speech was noted, although two patients could detect changes in the frequency of the first formant of synthesised vowels. Presentation of the fundamental frequency only generally led to improved perception of features associated with it (voicing and intonation). Only one patient consistently showed any advantage (and that not in all tests) of coding more than the fundamental alone.