Simulating auditory nerve fiber response following micro-electrode stimulation

Abstract

The cochlear implant was the first effective and is still the most common neuroprosthetic device which is employed for people with severe to profound hearing loss. To restore auditory perception, an array of micro-electrodes that deliver electrical pulses to the auditory nerve is surgically implanted into the lower cochlea duct, the scala tympani (ST). However, implantation into the upper cavity, the scala vestibuli (SV) has been tested due to severe anatomical obstruction or ossification of the ST. Clinical results revealed similar performance and thresholds for SV and ST cochlear implant users. We present a simulation study of auditory nerve fibre response to monophasic stimulation of both polarities. Excitation profiles are compared for microelectrodes placed in the SV and equivalent positions in the ST. In total, 7 different electrode positions for 4 different fibres have been investigated in a homogenous 2D model. Results for the intact fibres predict generally higher anodic thresholds in comparison to cathodic stimulation at the same electrode position and mostly lower thresholds for the SV electrodes in comparison to their ST counterparts. In contrast, anodic thresholds are mostly lower than cathodic thresholds for the degenerated fibres. Furthermore, due to the increased electrode-fibre distance for degenerate fibres which have completely lost the dendrite, SV stimulation is less beneficial. However, for basal fibres and the clinically relevant mid scala placement of the electrode the typically high thresholds remain similar for ST and SV positions.

Keywords

• auditory nerve fibres
• extracellular stimulation
• excitation patterns
• cochlear implants
• scala vestibuli