Simulating auditory nerve fiber response following micro-electrode stimulation: Comparing efficiency of electrode placements in the scala tympani and scala vestibul

Abstract

The cochlear implant was the first effective and isstill the most common neuroprosthetic device which isemployed for people with severe to profound hearing loss. Torestore auditory perception, an array of micro-electrodes thatdeliver electrical pulses to the auditory nerve is surgicallyimplanted into the lower cochlea duct, the scala tympani(ST). However, implantation into the upper cavity, the scalavestibuli (SV) has been tested due to severe anatomicalobstruction or ossification of the ST. Clinical results revealedsimilar performance and thresholds for SV and ST cochlearimplant users. We present a simulation study of auditorynerve fibre response to monophasic stimulation of bothpolarities. Excitation profiles are compared for micro-electrodes placed in the SV and equivalent positions in theST. In total, 7 different electrode positions for 4 differentfibres have been investigated in a homogenous 2D model.Results for the intact fibres predict generally higher anodicthresholds in comparison to cathodic stimulation at the sameelectrode position and mostly lower thresholds for the SVelectrodes in comparison to their ST counterparts. In contrast,anodic thresholds are mostly lower than cathodic thresholdsfor the degenerated fibres. Furthermore, due to the increasedelectrode-fibre distance for degenerate fibres which havecompletely lost the dendrite, SV stimulation is lessbeneficial. However, for basal fibres and the clinicallyrelevant mid scala placement of the electrode the typicallyhigh thresholds remain similar for ST and SV positions.