Impact of electrode position on the dynamic range of a human auditory nerve fiber

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 of an electrically stimulated ANF is quite small. Approach. The dynamic range 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 dynamic range is reversely related to ANF diameter. An increased dynamic range is expected to improve the quality of auditory perception for cochlear implant 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 dynamic range of a selected ANF. Significance. Five times larger dynamic ranges are expected for electrodes close to the terminal of the dendrite or at mid-dendritic placement as opposed to electrodes close to the modiolus.

A simple model considering spiking probability during extracellular axon stimulation

“…More details are found in Table 3 . Our starting value of knoise = 0.00125 was from a previous study that fits reported RS values of feline auditory nerve fibers [ 24 ]. The upper part of Table 3 demonstrates that the CRRSS model needs about 10-fold knoise values of HH10 for similar RS and, second, the chosen knoise (0.00125) in the HH10 model leads to a significantly larger RS compared to Verveen’s data.…”

“…In order to show the impact of ion current fluctuations we have investigated the loudness discrimination with cochlear implants in a forthcoming study using a combined HH1 and HH10 model [ 24 ]. As mentioned above, the diameter of an auditory nerve fiber is much smaller in its peripheral part than after the soma.…”

“…As a consequence, the simulated RS is about five times larger if the implanted electrode is close to the fiber ending in comparison to a position close to the center of the cochlea where it elicits spikes in the thick axon. As loudness perception, coded by the spiking rates of the stimulated fibers, depends on the dynamic range, we predict better loudness discrimination for electrodes close to the fiber endings [ 24 ]. This effect agrees with a study on intensity discrimination of 14 cochlear implant users with two types of electrode placements: subjects with the electrodes close to the cochlea center (far from fiber ending) had an average of only 9 discriminable intensity steps as compared to 23 steps in the other group [ 45 ].…”

“…The firing probability can be calculated by dividing the number of spikes by the number of stimulating pulses, where 100% firing probability means that every pulse generates a spike and 50% is called threshold. The ’threshold’ (mean of the distribution function) as well as the ’spread’ (standard deviation of the distribution function) depend on the stimulus duration, while the RS (quotient of spread and threshold) is independent of this stimulus duration for a relatively large interval of durations [ 10 , 23 , 24 ].…”

Improving the stimulation selectivity in the human cochlea by strategic selection of the current return electrode

“…In our model, auditory nerve fibers had an assumed smooth trajectory from the basilar membrane to the modiolus, a fixed length, and regular spacing, simply because the µ-CT images are not able to capture the trajectory of individual fibers. In the human cochlea, the fibers are bundled after passing the Organ of Corti through the basilar membrane (Cakir et al, 2019;Rattay and Tanzer, 2022). The actual trajectories of these fibers can vary considerably along the turns of the cochlea.…”