Pulsed mid -infrared laser stimulation of the auditory nerve in the gerbil: Implications for cochlear implants

Light can artificially stimulate nerve activity in vivo. A significant advantage of optical neural stimulation is the potential for 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. Cochlear implants restore a sense of hearing and communication to deaf individuals by directly electrically stimulating remaining neural cells in the cochlea. However, 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 (Holmium:YAG, 2.12 μ m; diode laser, 1.844-1.873 μm) was used to elicit compound action potentials (CAPs) from the gerbil cochlea in vivo . A monotonic increase in optical energy induced a monotonic increase in the amplitude of the evoked CAP. Penetration depth was varied by changing the wavelength of irradiation, to select the tissue depth of stimulation. As the pulse duration of the laser decreased, a lower optical energy was required to elicit a CAP. Experiments showed no immediate damage to the cochlea when optically stimulating at 400 Hz 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 exhibit best frequencies between 6-11 kHz and Q10dB ratios between 2-7. The tone-on-light tuning curves were similar in extent to tone-on-tone tuning curves.;The results from these studies are promising for the construction of an "optical cochlear implant". With optical stimulation, it may be possible to build an implant array to provide a significantly improved spatial selectivity of stimulation, with minimal overlap of stimulated neural cells. An increased selectivity of stimulation could improve cochlear implant patients' performance, especially in noisy conditions.