A noninvasive auditory prosthesis: Evaluation of a safety margin of the transtympanic laser stimulation

Abstract

Cochlear implants are widely used for compensating the sensorineural hearing loss. Surgical intervention for implanting electrodes is one of the most significant obstacle for the further widespread of these devices. We proposed a non-invasive auditory prosthesis using infrared laser because the laser can stimulate cochlea nerves from the outer ear. The purpose of this study was to evaluate the safety margin of the transtympanic laser stimulation. Head-fixed classical conditioning was performed on Mongolian gerbils. Subjects were trained using auditory stimulus, a bandpass noise, as a conditioned stimulus for a water reward, and licking behavior was recorded as a conditioned response. After the training, each subject was exposed to continuous pulsed laser irradiation of 1.6, 3.3, 6.6, 26.4, 52.8, or 105.6 W/cm2 for 15 hours. The bandpass noise with various intensities was presented without the reward before and after one-day of the laser exposure. As a result, the licking rate did not change after laser exposure of 6.6 W/cm2 or weaker, but drastically decreased after 26.4 W/cm2or above. The result suggests that the injury threshold in Mongolian gerbils for transtympanic laser stimulation is between 6.6 and 26.4 W/cm2 and the exposure over 6.6 W/cm2 could be out of the safety margin.