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.

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Infrared neural stimulation has been studied for its potential to replace an electrical stimulation of a cochlear implant. No studies, however, revealed how the technic reliably evoke auditory cortical activities. This research investigated the effects of cochlear laser stimulation from the outer ear on auditory cortex using brain imaging of activity-dependent changes in mitochondrial flavoprotein fluorescence signal. An optic fiber was inserted into the gerbil's ear canal to stimulate the lateral side of the cochlea with an infrared laser. Laser stimulation was found to activate the identified primary auditory cortex. In addition, the temporal profile of the laser-evoked responses was comparable to that of the auditory responses. Our results indicate that infrared laser irradiation from the outer ear has the capacity to evoke, and possibly manipulate, the neural activities of the auditory cortex and may substitute for the present cochlear implants in future.

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.

When two brief sounds are presented with a short flash of light, we often perceive that the flash blinks twice. This phenomenon, called the “sound-induced flash illusion”, has been investigated as an example of how humans finely integrate multisensory information, more specifically, the temporal content of perception. However, it is unclear whether nonhuman animals experience the illusion. Therefore, we investigated whether the Mongolian gerbil, a rodent with relatively good eyesight, experiences this illusion. The novel object recognition (NOR) paradigm was used to evaluate the gerbil’s natural (i.e., untrained) capacity for multimodal integration. A light-emitting diode embedded within an object presented time-varying visual stimuli (different flashing patterns). The animals were first familiarised with repetitive single flashes. Then, various sound stimuli were introduced during test trials. An increase in exploration suggested that the animals perceived a flashing pattern differently only when the contradicting sound (double beeps) was presented simultaneously with a single flash. This result shows that the gerbil may experience the sound-induced flash illusion and indicates for the first time that rodents may have the capacity to integrate temporal content of perception in a sophisticated manner as do humans.