Neural mechanisms relevant to the design of an auditory prosthesis. Location and electrical characteristics

A cochlear prosthesis designed to electrically stimulate eighth nerve fibers in deaf subjects was studied in acute guinea pig preparations. The prosthesis, which consisted of two pairs of platinum-iridium electrodes, was inserted into the scala tympani through the round window. The electrical impedances of different current paths and their effectiveness in evoking neural responses in the central auditory system are compared. Assuming lumped neural elements, the results indicate that the sites of neural excitation lie outside the scala tympani. Frequency-response characteristics of neural excitation are in agreement with resistive spread of current through tissue to the site of stimulation. Neural membrane characteristics similar to those previously studied in nonmammalian nerves seem to account for the response initiation.