Impact of Electrode Position on the Dynamic Range of a Human Auditory Nerve Fiber

Electrodes of a cochlear implant generate spikes in auditory nerve fibers. 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. The firing efficiency of an auditory nerve fiber, stimulated by a train of pulses varies between 0 and 100%. 100% firing efficiency means every pulse elicits a spike, 50% defines threshold. The dynamic range of an auditory nerve fiber is the range of stimulus intensities that causes a firing probability between 10 and 90%. This ‘electrical’ dynamic range is quite small in comparison to the variation of spiking rates measured during acoustic stimulation. Consequently, an increased dynamic range may improve the quality of auditory perception for cochlear implant users. Electrodes are often placed as close as possible to the center axis of the cochlea. Analysis of simulated auditory nerve firing showed that this placement is disadvantageous for the dynamic range. Five times larger dynamic ranges are expected for electrodes close to the terminal of the dendrite or at mid-dendritic placement.