Practical model description of peripheral neural excitation in cochlear implant recipients: 5. Refractory recovery and facilitation

In this paper the neural response to electrical stimulation with short inter-pulse intervals was examined. The refractory recovery of the electrically-evoked compound action potential (ECAP) was recorded using masker pulses with a wide range of currents relative to the probe pulse. The ECAP was recorded in subjects implanted with the Nucleus® 24 cochlear implant system (three with straight and two with Contour™ electrode arrays), using the Neural Response Telemetry™ (NRT™) system. Employing the non-refractory parameters previously obtained in the fourth paper of the series and a two-parameter neural refractory recovery function, the model was fitted to ECAP recovery data for a case where the masker current was high relative to that of the probe and masking assumed to be almost complete. A single pair of refractory parameters, fitted at one probe current, allowed the model to describe quite well the ECAP recovery functions for different probe currents. Facilitatory contributions to the recovery functions, for differing current of masker relative to probe, were quantified by comparing experimental recovery functions with modeled functions that included only refractory behavior. The model should provide the means to improve speech processing algorithms for cochlear implants, by allowing the systematic incorporation of additional information concerning the neural response to electrical stimulation.