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

Abstract

Previous experiments have shown that, in principle, the addition of noise to 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 caused a largely uniform transmission of all input harmonics, including those related to a formant. But for other vowel stimuli, the optimal addition of noise 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 this transmission is unknown. In the present study, the nature of this preferential 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 preferential transmission. Furthermore, many discriminators were required to obtain preferential transmission. Therefore, preferential transmission of a formant-related harmonic may be best achieved by pre-processing a stimulus and using methods associated with stochastic resonance.