A Computational Model of the Electrically or Acoustically Evoked Compound Action Potential in Cochlear Implant Users with Residual Hearing

Objective: In cochlear implant (CI) users with residual acoustic hearing, compound action potentials (CAPs) can be evoked by acoustic or electric stimulation and recorded through the electrodes of the CI. We propose a novel computational model to simulate electrically and acoustically evoked CAPs in humans, taking into account the interaction between combined electric-acoustic stimulation that occurs at the level of the auditory nerve. Methods: The model consists of three components: a 3D finite element method model of an implanted cochlea, a phenomenological single-neuron spiking model for electric-acoustic stimulation, and a physiological multi-compartment neuron model to simulate the individual nerve fiber contributions to the CAP. Results: The CAP morphologies predicted for electric pulses and for acoustic clicks, chirps, and tone bursts closely resembled those known from humans. The spread of excitation derived from electrically evoked CAPs by varying the recording electrode along the CI electrode array was consistent with published human data. The predicted CAP amplitude growth functions for both electric and acoustic stimulation largely resembled human data, with deviations in absolute CAP amplitudes for acoustic stimulation. The model reproduced the suppression of electrically evoked CAPs by simultaneously presented acoustic tone bursts for different masker frequencies and probe stimulation electrodes. Conclusion: The proposed model can simulate CAP responses to electric, acoustic, or combined electric-acoustic stimulation. It takes into account the dependence on stimulation and recording sites in the cochlea, as well as the interaction between electric and acoustic stimulation. Significance: The model can be used in the future to investigate objective methods, such as hearing threshold assessment or estimation of neural health through electrically or acoustically evoked CAPs.