Simulation of a cochlear implant with current spread and varied number of activated electrodes
Cochlear implant (CI) users’ spectral resolution is inherently restricted by a limited number of excitatory spectral channels/electrodes (up to 20). Thresholds of speech intelligibility in noise (SpIN) are consequently higher in CI users than in normally hearing listeners. CI current spread along the spiral ganglia further reduces spectral resolution, which lowers the number of “effective channels” to around 8 (Friesen et al., 2001), beyond which no significant SpIN improvement is found. Simulations of CIs through tone or noise vocoding traditionally employ the same numbers of analysis bands (electrodes) and carriers (tones or noise bands) used for resynthesis. A recent extension to traditional vocoding incorporates current spread as an exponential function weighting the contribution of envelopes from each analysis band to each carrier (Oxenham & Kreft, 2014). When measuring percent-correct recognition of words in sentences as a function of the number of activated electrodes, a knee point should appear where current spread starts to counteract the benefit of further increase in activated electrode number. This occurs at the number of effective channels. Increasing the simulated current spread should shift the knee point to a lower number of effective channels. However, a vocoder incorporating current spread as per Oxenham & Kreft fails to produce the expected outcome. We propose a novel vocoder with a fixed, large number of carrier bands that more adequately models the fixed ganglion cells. The exponential weighting function is integrated over the carrier bands to ensure an improved representation of coupling between electrodes and ganglion cells. The weighting can equally be applied to carrier bands or tones. Where using a variable number of carriers fails to demonstrate the expected effect of current spread on the number of effective channels, the proposed novel approach succeeds.
Friesen, L., Shannon, R., Baskent, D., and Wang, X. (2001). “Speech recognition in noise as a function of the number of spectral channels: Comparison of acoustic hearing and cochlear implants,” J. Acoust. Soc. Am., 110, 1150.
Oxenham, A., and Kreft, H. (2014). “Speech perception in tones and noise via cochlear implants reveals influence of spectral resolution on temporal processing,” Trends Hear., 18, 1–14.