An orchestra crescendos, and deaf audience members hear every note, thanks to cochlear implants that translate the complex sounds into a rainbow of optical light. That is the vision of a team of scientists in Germany, Japan, South Korea and Singapore, who believe a device that uses optics instead of sound waves might birth a refined class of auditory prosthetics.
In people who can hear, spiral ganglion neurons in the inner ear allow for the precise discrimination of sound—we can recognize hundreds of people by the sound of their voice and distinguish between thousands of different pitches or frequencies of sound. In traditional cochlear implants the external microphone picks up sound and transmits it to these neurons via electrodes, but the resolution is very poor. The neurons are lined up like piano keys in our inner ear, and using electrodes to stimulate them is like playing a concerto with fists instead of fingers. The scientists think there is a better way.
In a study that appeared in March in the Journal of Clinical Investigation, the researchers used viruses to implant genes for light sensitivity into mouse embryos of a deaf lineage. The genes went to work in the auditory pathways of the mouse brains, creating light-sensitive patches on the membranes of their spiral ganglion neurons and other neurons. The scientists then directed LED light onto these neurons and recorded brain stem activity—an essential integration step in auditory processing.
The activity indicated the deaf mice successfully perceived the light as sound. Compared with stimulation from traditional cochlear implant electrodes, the light produced more precise neural activity in the brain stem, similar to normal hearing. The mice also exhibited a high level of sound discrimination that current prosthetics cannot achieve.
The team envisions that in the future, deaf people might benefit from gene therapy similar to the approaches currently being tested in clinical trials for other diseases. If they so choose, their cochleae could be altered to express these light-sensitive channels. A chain of LED lights could then be inserted in the ear, which would light up according to the qualities of an external sound, allowing auditory neurons to communicate its rich details to the brain.