Cochlear implant uses light to send sound to the brain

16 July 2018
Posted by Chris Smith.

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Deaf adult gerbils can hear again with optogenetic hearing restoration.

A hearing aid system that uses light to send sound signals into the nervous system has been developed by researchers in Germany...

The present generation of hearing aids work in a range of ways. The most common varieties make sounds louder in the the range of frequencies that the user struggles to hear.

But this sort of device is not suitable for all forms of hearing loss, which is why over half a million people worldwide have received a cochlear implant, also known as the bionic ear. 

These involve threading a series of fine electrodes into the snail-shaped inner ear, which is called the cochlea. In a healthy cochlea, specialised "hair cells" convert sound vibrations into nerve signals that the brain can understand, with different regions of the cochlea responding best to different pitches of sound: high frequencies are detected towards the front of the cochlea and lower frequencies towards the back.

If the hair cells become damaged, which can occur for a variety of reasons, the person loses their ability to detect sounds. But because the nerves that carry the signals from the hair cells into the brain nevertheless remain intact, the cochlear implant can directly recruit these nerves electrically. So a system of electrodes is used to cover a range of frequencies, in particular those that make speech comprehensible.

The problem is that the electrical signals spill over from one region of the cochlear to another, "blurring" the tonal quality of the perceived sound. "It's like playing the piano with your elbows," says University Medical Centre, Göttingen, scientist Marcus Jeschke. "You take down lots of notes at the same time."

Instead, Jeschke has come up with an alternative system that he thinks "will give you every note on the piano in future."

His approach is to use a technique called "optogenetics" to render the nerve cells light sensitive. "We add a gene that codes for a light-sensing molecule connected to an electrical switch," explains Jeschke. "Then, we shine a tiny blue light on the appropriate patch of the cochlea and it activates the nerve cells there, very discretely."

The idea is to create a miniature series of light sources resembling a microscopic string of fairy lights. This is threaded into the cochlea like the electrodes of a cochlear implant. Then, rather than deliver electricity, the device briefly illuminates the relevant cochlea territory, triggering a sympathetic burst of nerve firing in the relevant nerve fibres.

In a proof of concept experiment using Mongolian gerbils, chosen because they have a cochlea quite similar in size to a human, the animals were trained to perform a task whenever they heard a sound. The team were then able to elicit the same trained behaviour using only light signals transmitted into the animals' ears. "This proves that they were 'hearing' the light," says Jeschke.

Encouragingly, the indications are that the approach would translate readily, and rapidly, to humans. "There are already approved gene therapy systems in use for the treatment of human eye disorders that could also be used to deliver the light-sensing gene to the ear; so getting this into humans is our goal..."


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