Preferential and non-preferential transmission of formant information by an analogue cochlear implant using noise: the role of the nerve threshold

Previous experiments have shown that, in principle. the addition of noise t o any vowel coded by an analogue multichannel cochlear implant can enhance the representation of formant information by the temporal pattern of evoked nerve discharges. The optimal addition of noise to some vowel stimuli caus ed a largely uniform transmission of all input harmonics, including those r elated to a formant. But for other vowel stimuli, the optimal addition of n oise caused preferential transmission of the harmonic closest to a formant compared with other input harmonics. Such preferential transmission may be useful to a cochlear implantee for formant estimation, but the basis of thi s transmission is unknown. In the present study, the nature of this prefere ntial transmission was investigated with a set of parallel discriminators ( or level-crossing detectors) to determine whether the inherent threshold of a nerve fiber was the main cause of the effect. An explicit threshold was found to account for some but not all of the previously observed preferenti al transmission. Furthermore, many discriminators were required to obtain p referential transmission. Therefore, preferential transmission of a formant -related harmonic may be best achieved by pre-processing a stimulus and usi ng methods associated with stochastic resonance. (C) 1999 Elsevier Science B.V. All rights reserved.