Electrical equivalent circuit for analyzing the effect of signal shape on power distribution in cochlear implant electrodes and surrounding tissue

Abstract

Cochlear implants are a well-established solution for restoring hearing in severe impairment and profound deafness. However, cochlear implants still have limitations, such as speech recognition in noisy environments caused by intra-cochlear current spread across different auditory spiral ganglion neurons as a consequence of, e.g., the large distance of the stimulation electrodes to the target cells in a highly conductive environment. Stimulation in cochlear implants is typically done with charge balanced biphasic rectangular current pulses in a monopolar arrangement. However, several studies have shown that a rectangular stimulation pulse is not optimal for stimulating spiral ganglion neurons. For example, stimulation with a ramped pulse, such as a sawtooth pulse, has been shown to be more energy-efficient and achieves a similar threshold profile in spiral ganglion neurons. In this study, a new but simple equivalent electrical circuit model is introduced that describes the complex impedance between two stimulation electrodes of a cochlear implant with high accuracy (mean relative error ≤ 8%). Based on this bipolar model, a monopolar equivalent electrical circuit model is developed to describe the stimulation between one stimulation electrode and a counter electrode located outside the cochlea. These two models now allow for analyzing the effect of stimulation pulse shape on power distribution in cochlear implant electrodes and surrounding tissue providing a tool for investigating stimulation efficiency with respect to energy losses in the cochlear implant electrode.