Pulsed Mid-Infrared Laser Stimulation of the Auditory Nerve in the Gerbil: Implications for Cochlear Implants

Abstract

Light can artificially stimulate nerve activity in vivo and has the advantage of a higher spatial selectivity relative to that which can be obtained with electrical stimulation. An increased spatial selectivity of stimulation could improve the function of neuroprosthetics, such as cochlear implants, whose performance is limited by overlapping electric fields from neighboring electrodes. The studies reported here investigated optical stimulation of nerves in the gerbil auditory system. Pulsed, infrared light was used to elicit compound action potentials (CAPs) from the gerbil cochlea in vivo. Penetration depth was varied by changing the wavelength of irradiation, to select the tissue depth of stimulation. Experiments showed no immediate damage to the cochlea when optically stimulating at 400Hz for several hours. Immunohistochemical staining for the protein c-FOS revealed spatial specificity of the optically stimulated cochleae: only the spiral ganglion cells directly in the optical path revealed c-FOS staining, indicating that they were stimulated. In contrast, c-FOS staining of electrically stimulated cochleae demonstrated electric current spread. An electrophysiologic measurement of the spatial specificity of optical stimulation was conducted with tone-on-light masking experiments. Results from tone-on-light masking studies revealed tuning curves that were similar in extent to tone-on-tone tuning curves.