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 cochlear implant has previously been shown in modelling studies to enhance the representation of speech cues by the fine time structure of evoked nerve discharges. The enhancement, however, occurred only for a range of noise levels, and this range was stimulus dependent. Theoretically, fine optimization of the noise levels would be unnecessary if each implant channel stimulated a group of cochlear nerve fibres such that each fibre in the group received an independent noise waveform in addition to the same information-bearing 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. The model simulated monopolar stimulation of a cochlear nerve by 11, 21 or 41 electrodes in the scala tympani. The correlation between the effective stimuli for pairs of nerve fibres and the correlation between the corresponding evoked discharges were calculated for two noise strategies. In one strategy, an independent noise current was applied to each electrode. Less correlation between effective stimuli was obtained with the alternate strategy that used inhibition between the noise sources.