Trans-tympanic optical stimulation of the cochlea evokes reliable auditory responses

A new study, using Mongolian gerbils as an animal model, demonstrated for the first time that trans-tympanic infrared laser stimulation of the cochlea evokes reliable, auditory-guided behavior in awake animals without the need for genetic modification or invasive surgical implantation. The study was led by Dr. Yuta Tamai, a researcher at the Acoustic Navigation Research Center, Doshisha University, Japan (who also holds positions as a JSPS Postdoctoral Research Fellow at Keio University and as a Research Fellow at Eberhard Karls University of Tübingen), along with Professor Kohta I. Kobayasi from the Faculty of Life and Medical Sciences at the same university, and Associate Professor Koji Toda from the Department of Psychology, Keio University, Japan. The findings were published online in the journal iScience on June 30, 2026.

"My research motivation arises from observing family members who have age-related hearing loss and struggle to engage in conversations due to the limitations of conventional hearing aids. Their experience highlighted the need for more effective solutions, as traditional cochlear implants require invasive procedures and have technical drawbacks. This inspired me to investigate non-invasive, optical alternatives that offer a natural auditory experience. My goal is to bridge the gap between neuroscience and technology by developing a contactless device to restore the joy of communication for those underserved by existing hearing aids," says Dr. Tamai, while discussing the motivation behind this study.

The research team used a classical conditioning procedure in which gerbils learned that a cue was associated with a reward, which was water for this experiment. While the cue was a conventional sound stimulus for one group, for another, it was infrared laser stimulation directed through the eardrum toward the cochlea.

Post-training, gerbils exposed to laser stimulation began licking in anticipation of the water reward, showing that the laser cue was behaviourally meaningful. Their learning pattern was comparable to that of animals trained with sound, although sound-trained animals showed a stronger overall response. This highlighted that trans-tympanic laser stimulation could act as a conditioned stimulus to support auditory-guided behaviour.

The researchers introduced white-noise masking, testing whether the laser-evoked response was truly auditory-related, which led to reduced responses to ordinary sound. It also significantly reduced laser-evoked behavioral responses, while visual cue responses were not significantly affected. This suggests that the laser-induced perception was processed through auditory pathways rather than being a general sensory distraction.

The study also found that laser-evoked perception could be controlled by changing radiant energy, which mirrored the way stronger sound pressure levels produced stronger auditory responses. This indicates that laser stimulation could modulate the strength of auditory-like perception.

In another test, animals trained only with acoustic cues responded when laser stimulation was presented for the first time. This "stimulus generalization" suggests that laser stimulation produced perceptual features overlapping with sound. However, the response was weaker than the response to the trained acoustic cue, indicating that the laser-evoked percept was similar to, but not identical to, a clicking sound.

"In the next 5–10 years, this technology could revolutionize the treatment of hearing impairment. By perfecting trans-tympanic optical stimulation, we aim to provide a clinical alternative that minimizes surgical risks and complications. It may also open new avenues for sensory substitution devices, improving the quality of life for millions suffering from communication challenges due to hearing loss," mentioned Prof. Kobayasi, highlighting the real-life applications of this technology.

Overall, this study demonstrates how contactless optical stimulation of the cochlea could become a useful platform for studying auditory perception and may inform the future development of non-invasive or less invasive auditory prosthetic technologies.