Modeling auditory-nerve responses to electrical stimulation

Cochlear implants (CI) directly stimulate the auditory nerve (AN), bypassing the mechano-electrical transduction in the inner ear. Trains of biphasic, charge balanced pulses (anodic and cathodic) are used as stimuli to avoid damage of the tissue. The pulses of either polarity are capable of producing action potentials (AP) whereby the sites of initiation of the AP differ for the two polarities . A cathodic pulse triggers an AP in the peripheral axon, whereas an anodic pulse triggers an AP in the central axon. The latency difference between the APs initiated at the different sites is about 200μs, which is large enough to affect the temporal coding of sounds and hence, potentially, the communication abilities of the CI listener. In the present study, two recently proposed models of electric stimulation of the AN [1,2] were considered in terms of their efficacy to predict the spike timing for anodic and cathodic stimulation of the AN of cat [3]. The models’ responses to the electrical pulses of various shapes [4,5,6] were also analyzed. It was found that, while the models can account for the firing rates in response to various biphasic pulse shapes, they fail to correctly describe the timing of AP in response to monophasic pulses. Strategies for improving the model performance with respect to correct AP timing are discussed.