  Objective: In cochlear implant users with residual acoustic hearing, compound
action potentials (CAPs) can be evoked by acoustic (aCAP) or electric (eCAP)
stimulation and recorded through the electrodes of the implant. We propose a
novel computational model to simulate aCAPs and eCAPs in humans, considering
the interaction between combined electric-acoustic stimulation that occurs in
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 closely resembled
those known from humans. The spread of excitation derived from eCAPs by varying
the recording electrode along the cochlear implant electrode array was
consistent with published human data. The predicted CAP amplitude growth
functions largely resembled human data, with deviations in absolute CAP
amplitudes for acoustic stimulation. The model reproduced the suppression of
eCAPs 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 considers the dependence on stimulation and recording sites in
the cochlea, as well as the interaction between electric and acoustic
stimulation in the auditory nerve. Significance: The model enhances
comprehension of CAPs and peripheral electric-acoustic interaction. It can be
used in the future to investigate objective methods, such as hearing threshold
assessment or estimation of neural health through aCAPs or eCAPs.
