Infrared laser stimulation has been studied as an alternative approach to auditory prostheses. This study evaluated the feasibility of infrared laser stimulation of the cochlea from the outer ear, bypassing the middle ear function. An optic fiber was inserted into the ear canal, and a laser was used to irradiate the cochlea through the tympanic membrane in Mongolian gerbils. A pulsed infrared laser (6.9 mJ/cm2) and clicking sound (70 peak-to-peak equivalent sound pressure level) were presented to the animals. The amplitude of the laser-evoked cochlear response was systematically decreased following insertion of a filter between the tympanic membrane and cochlea; however, the auditory-evoked cochlear response did not decrease. The filter was removed, and the laser-evoked response returned to around the original level. The amplitude ratio and the relative change in response amplitude before and during filter insertion significantly decreased as the absorbance of the infrared filter increased. These results indicate that laser irradiation could bypass the function of the middle ear and directly activate the cochlea. Therefore, laser irradiation from the outer ear is a possible alternative for stimulating the cochlea, circumventing the middle ear.

Infrared laser stimulation of the cochlea has been proposed as a possible alternative to conventional auditory prostheses. The safety margin of laser stimulation must be determined for the practical application of the laser to a prosthesis. This study assessed the effect of laser-induced damage to the cochlea on auditory perception. To examine the effects of laser-induced injury of the auditory system on the perception of auditory stimuli, we trained Mongolian gerbils (Meriones unguiculatus) using a head-fixed classical conditioning task. White noise of 80 dB sound pressure level (SPL) was presented as a conditioned stimulus for a reward (a drop of water), and licking behavior was recorded as a conditioned response. After training, an optic fiber was inserted into the ear canal. Each subject was exposed to continuous pulsed laser exposure of 1.6, 3.3, 6.6, 26.4, 52.8, or 105.6 W/cm2 for 15 h. White noise of various intensities was presented without pairing it with water before and after laser exposure, to assess the decrease in auditory perception due to laser-induced injury. The licking rate did not change after laser exposure of 6.6 W/cm2 or weaker but drastically decreased after 26.4 W/cm2 or higher. These results suggest that the injury threshold in Mongolian gerbils for laser stimulation is between 6.6 and 26.4 W/cm2; laser exposure over 6.6 W/cm2 could be out of the safety margin. These findings will contribute to the appropriate delimitation of safe and effective laser stimulation parameters in future research.

The application of infrared laser stimulation to brain-machine interfaces, such as cochlear implants, has been discussed over the last decade because the infrared laser can activate spatially selected neural populations without delivering exogenous agents to tissues. Here, this study demonstrated for the first that the laser irradiation of the peripheral sensory organ, the cochlea, can induce an unequivocal behavioral auditory response. We proved that the animals, Mongolian gerbils (Meriones unguiculatus), can be classically conditioned with a laser stimulus in a similar way to an auditory stimulus. Additionally, the auditory masking significantly inhibited the laser-conditioned response, and the auditory-conditioned animals exhibited stimulus generalization to laser stimulation. In a subsequent experiment, simultaneous presentation of auditory and laser stimulation successfully elicited nonlinear amplification in both the auditory cortical and behavioral response. These results suggest that the infrared laser irradiation to the cochlea can evoke and possibly enhance auditory perception and has real potential to be implemented in an auditory prosthesis.

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.