The influence of stimulus intensity on spike timing and the compound action potential in the electrically stimulated cochlea: a model study

In order to get more insight in the fundamentals of electrical stimulation of the auditory nerve by cochlear implants we developed a rotationally symmetric volume conductor model of the implanted inner ear (cochlea), using the boundary element method. The neural responses are calculated with a multiple nonlinear node model of auditory nerve fibers. In the present paper this model is used to study the relative timing of the spikes elicited in different locations in the (tonotopically organized) cochlea. This timing information is used in conjunction with the so-called unit response to synthesize compound action potentials of the auditory nerve. It is concluded that increasing the intensity of biphasic pulses on longitudinal bipolar electrodes significantly affects the time relations between spikes elicited in fibers at different positions in the cochlea. This might well have perceptual consequences, while it is poorly reflected in the compound action potential as measured at the round window