The practical use of noise to improve speech coding by analogue cochlear implants

The addition of noise to speech signals coded by an analogue multichannel c ochlear implant has previously been shown in modelling studies to enhance t he representation of speech cues by the fine time structure of evoked nerve discharges. The enhancement, however, occurred only for a range of noise l evels, and this range was stimulus dependent. Theoretically, fine optimizat ion of the noise levels would be unnecessary if each implant channel stimul ated a group of cochlear nerve fibres such that each fibre in the group rec eived an independent noise waveform in addition to the same information-bea ring signal. We present results from computer simulations that suggest that current spread in the cochlea may be exploited to obtain a high degree of independence between the noise waveforms that stimulate adjacent fibres. Th e model simulated monopolar stimulation of a cochlear nerve by 11, 21 or 41 electrodes in the scala tympani. The correlation between the effective sti muli for pairs of nerve fibres and the correlation between the correspondin g evoked discharges were calculated for two noise strategies. In one strate gy, an independent noise current was applied to each electrode. Less correl ation between effective stimuli was obtained with the alternate strategy th at used inhibition between the noise sources. (C) 2000 Elsevier Science Ltd . All rights reserved.