Abstract
Objective. Electrodes of a cochlear implant generate spikes in auditory nerve fibers (ANFs). While the insertion depth of each of the electrodes is linked to a frequency section of the acoustic signal, the amplitude of the stimulating pulses controls the loudness of the related frequency band. However, in comparison to acoustic stimulation the dynamic range (DR) of an electrically stimulated ANF is quite small. Approach. The DR of an electrically stimulated ANF is defined as the interval of stimulus amplitudes that causes firing probabilities between 10% and 90%. A compartment model that includes sodium ion current fluctuations as the stochastic key component for spiking was evaluated for different electrode placements and fiber diameters. Main results. The DR is reversely related to ANF diameter. An increased DR is expected to improve the quality of auditory perception for CI users. Electrodes are often placed as close to the center axis of the cochlea as possible. The analysis of the simulated auditory nerve firing showed that this placement is disadvantageous for the DR of a selected ANF. Significance. Five times larger DRs are expected for electrodes close to the terminal of the dendrite or at mid-dendritic placement as opposed to electrodes close to the modiolus.
Highlights
Version of Record: A version of this preprint was published at Journal of Neural Engineering on February 1st, 2022
While the insertion depth of each of the electrodes is linked to a frequency section of the acoustic signal, the amplitude of the stimulating pulses controls the loudness of the related frequency band
These details have less impact for synaptic stimulation in the healthy ear in comparison with electrical stimulation where (i) spike initiation site depends on electrode position, (ii) thin degenerated dendrites may not be excited, and (iii) polarity sensitivity depends on cochlear status[1,2,3,4]
Summary
Version of Record: A version of this preprint was published at Journal of Neural Engineering on February 1st, 2022. A single synaptic connection between an inner hair cell and the distal end of an auditory nerve fiber (ANF) converts this analog signal into a digital signal as a train of action potentials (spikes). Each element of the data bus is a so called type-1 spiral ganglion cell consisting of a dendrite, a soma, and an axon, where the diameters and the degree of myelination changes along the cell axis These details have less impact for synaptic stimulation in the healthy ear in comparison with electrical stimulation where (i) spike initiation site depends on electrode position, (ii) thin degenerated dendrites may not be excited, and (iii) polarity sensitivity depends on cochlear status[1,2,3,4]. For difficult neural decoding tasks such as speech understanding in noisy environment, every single ANF crucially contributes with the own spiking rate and its temporal changes following the loudness variations in the corresponding frequency region of the acoustic source signal
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