The potential of optogenetic cochlear implants
One in six people in the UK have some form of hearing loss. Severely or profoundly deaf people can often assist their hearing with the use of hearing aids, cochlear implants or a combination of these technologies, called electroacoustic cochlear implants. There are currently around 10,000 people in the UK with either standard or electroacoustic cochlear implants.
Cochlear implants rely on a receiver surgically implanted in the mastoid bone behind the ear with electrodes inserted into the inner ear. Externally a microphone and speech processor converts sound into an electrical signal which is sent to the electrodes. These electrodes stimulate innervation of the Spiral Ganglion Neurons (SGNs) and send a signal through the auditory nerve to the brain, where it is perceived as sound. In an electroacoustic implant the exterior part of the device also incorporates a hearing aid to amplify lower frequency sound.
Spinal Ganglion Neurons (SGNs) are the sensory neurons of the auditory system. Their cell bodies lie in the cochlea where they detect sound waves. When functioning normally they will then send an electrical representation of the sound wave to the central nervous system.
Cochlear implants are currently the best option for high frequency hearing loss that cannot be helped by hearing aids alone. However, the cochlear is a very sensitive organ with around 30,000 highly organised SGNs. In comparison, cochlear implants are not nearly as sensitive. Cochlear implants normally have between 12 and 24 stimulation electrodes and their diffuse electrical fields can lead to crosstalk, blurring people's ability to distinguish tonally distinct sounds.
A group from the University Medical Centre Göttingen, Germany, are investigating the possibility of using an optogenetic alternative to electrical cochlear implants – using light to stimulate SGNs in the inner ear. Eventually they want to provide options for improving frequency and intensity resolution of auditory coding, helping those that are severely or profoundly deaf distinguish a wider variety of sounds.
(For an introduction to optogenetics read our article – Optogenetics: shedding light on the brain's secrets.)
Victor Hernandez and colleagues have provided the first proof of concept in their paper published in The Journal of Clinical Investigation. Using a combination of electrophysiology and optogenetic techniques the group found that optogenetic cochlear stimulation was able to activate the auditory pathway. Furthermore, they provided evidence that the optogenetic stimulation was able to better establish the frequency and intensity of the sound.
The group states that optical stimulation has "the potential to improve auditory prosthetics, because a larger number of independent stimulation channels would enable innovative strategies for coding frequency and intensity in order to enhance the perception of speech, prosody, and music."
Hernandez V.H., Gehrt A., Reuter K., Jing Z., Jeschke M., Mendoza Schulz A., Hoch G., Bartels M., Vogt G., Garnham C.W., Yawo H., Fukazawa Y., Augustine G.J., Bamberg E., Kügler S., Salditt T., de Hoz L., Strenzke N., Moser T. (2014) Optogenetic stimulation of the auditory pathway Journal Clinical Investigation, 124(3):1114–1129 doi: 10.1172/JCI69050