Around one million people around the world currently use a cochlear implant (CI). The CI is a surgically implanted device that restores hearing perception in cases of profound hearing loss or deafness. Microphones on the outside of the device convert sound into electrical signals, which then directly stimulate the auditory nerve in the cochlea.
This structure in the inner ear converts the sound information into nerve impulses and sends them to the brain. Although cochlear implants have been very successful in restoring speech understanding in silence, their limited spectral resolution limits the ability to follow conversations in background noise and limits the enjoyment of music.
The new approach builds on the expertise of Prof. Tobias Moser and his team in the field of optogenetics. The Göttingen-based hearing research team has developed a technique that uses genetic manipulations to control nerve cells with light and connect their activity to a light-emitting cochlear implant.
"We have developed an alternative way to couple light and electrical activity in nerve cells that does not require genetic manipulation. To do this, we use a substance that chemically attaches to a receptor protein in the auditory nerve cells like a 'molecular prosthesis' and activates them when they are illuminated," says co-first author Prof. Dr. Carlo Matera, who synthesized the drug. The fact that several ways of stimulating the auditory nerve are now available could make the optical CI accessible to a wider range of users.
The new substance developed by the researchers is activated by a single wavelength of blue light. "After testing and characterizing the compound in vitro on nerve cells from the hippocampus, we conducted in vivo experiments in gerbils," says co-first author Dr. Aida Garrido Charles, a scientist at IBEC and UMG. "We were able to show that our light-activated drug triggers an electrophysiological response in the cochlea. This is the first time that this has been achieved in a pharmacological way in an experiment."
Light-activated neural control overcomes some of the disadvantages of the classic cochlear implant, which is driven by electrical stimulation. "The main reason why implant users have difficulties perceiving music and speech in noisy environments is that the cochlea is filled with fluid. If it is stimulated with electricity, the excitation is widely dispersed," says Dr. Antoine Huet, co-author of the study and MBExC Junior Fellow, UMG. "Since light can be better spatially confined in liquid, our technique can stimulate the auditory nerve cells in the cochlea with much greater precision. This would mean that future potential users could regain 'near physiological hearing'. For the perception of music and conversation in background noise, excellent frequency resolution of sound is the technical key, which cannot be achieved with electrical stimulation."
Future studies will improve the drug and test how accurately it restores hearing. "Our computer predictions and animal studies suggest that hearing with light has the potential to provide an almost physiological auditory impression," says Dr. Huet.
Fast photoswitchable molecular prosthetics control neuronal activity in the cochlea. Journal of the American Chemical Society (2022), https://doi.org/10.1021/jacs.1c12314.
