Practical model description of peripheral neural excitation in cochlear implant recipients: 2. Spread of the effective stimulation field (ESF), from ECAP and FEA

This second paper of the series considers the spread of the "effective stimulation field" (ESF) produced by monopolar biphasic stimulation of an electrode within scala tympani, in subjects implanted with the Nucleus 24 cochlear implant system (three with straight and two with Contour electrode arrays). A novel measure of the ECAP was employed, using the Neural Response Telemetry (NRT) system. The ESF provides a patient-specific measure of the "ability" of the stimulation field to excite neurons at differing locations around the cochlea. The results were interpreted with the aid of simple finite element computational models of the electric field. The finite element models were used to generate template field spread functions that differed with radial distance of the stimulated electrode from the modiolus. Relative to a template field function for the appropriate radial distance, the ESF spread on average approximately twice as broadly (a scaling factor of two). The magnitude of this scaling factor was considered to be indicative of the site of excitation on the neural fibers. The relationship between two ECAP measures, the spread of ESF and the "spread of excitation" (SOE), is discussed.