Electrode independence in intraneural cochlear nerve stimulation

Objectives/Hypothesis: We have previously reported feline electrophysiological and anatomical studies focused on the development of an intraneural auditory neuroprosthesis. Because only the tips of the electrodes implanted in the cochlear nerve are the stimulating elements that abut the nerve axons, we hypothesize that intraneural stimulation will be highly focal in nature. In this article, we report the electrophysiological characterization of the selective activation of subpopulations of cochlear nerve fibers via electrodes implanted in feline cochlear nerve. Study Design: We have used a forward-masking paradigm to estimate the extent of stimulation overlap produced by pairs of electrodes implanted into the cochlear nerve. Methods: The technique uses sequential stimulation via masking and probe electrodes and monitoring of the electrically evoked auditory brain stem response as an index of cochlear nerve fiber recruitment. We investigated overlap in all possible electrode pair combinations. Results and Conclusion: Many electrode pairs manifest virtually no overlap in the subpopulations of fibers excited by perithreshold stimuli, whereas most had considerable overlap at higher stimulation levels. However, we also noted that our measured overlap was similar across electrodes possibly because of lack of specificity of the whole nerve electrically evoked auditory brain stem response as an assay for this parameter. These findings indicate that direct cochlear nerve stimulation via intraneural electrodes provides selective excitation of small subpopulations of cochlear nerve fibers, and suggest that cochlear nerve stimulation may selectively evoke narrow-band frequency percepts.