Hearing restored in rats by modifying ear cells to respond to light

Cochlear implants that restore hearing could be improved by genetically modifying the nerve cells in people’s ears so that they respond to light instead of electricity, a study in rats has shown.

“This is so much better than what we currently have with electrical implants,” says Tobias Moser at the University Medical Center Göttingen in Germany.

Our hearing relies on hair cells inside the cochlea of our ears detecting sounds of different frequencies and then stimulating the right auditory nerve cells. Damage to these hair cells is a common cause of deafness.

Cochlear implants can partly restore hearing by electrically stimulating nerve cells, bypassing the hair cells. But the electrical signals stimulate lots of nerve cells at once, meaning the resulting sound is less detailed – the audio equivalent of a low-resolution image.

People with cochlear implants may not understand speech in noisy environments or enjoy music in the way they did before, says Moser.

The problem is that the salty fluid inside the cochlear conducts electricity, so it is hard to confine the signal. Instead, Moser’s team is developing optical cochlear implants that use light to stimulate the nerve cells.

Of course, nerve cells don’t usually respond to light. But they can be genetically modified to respond to it. Optogenetics, as this is called, is widely used for research and is also being tested as a way to restore sight or treat neural conditions. It appears to be safe from the animal and human studies done so far.

Moser’s team has previous shown that the concept works by using a single optical fibre to stimulate the cochlea in deaf animals that have modified auditory nerve cells. Now the researchers have tested an implant with 10 LED chips in rats.

After the implant was inserted into deaf rats with modified auditory nerve cells the animals responded to a sound they had been trained to respond to before being deafened. This shows that what they heard via the implant was sufficiently similar, says Moser. “I think this is a great achievement.”

For people, the team will create implants with 64 light sources, or channels. The sound clip below starts with a spoken sentence and then simulates how it would sound with a standard cochlear implant and then with an optical implant with 64 channels.

The team hopes to begin human trials around 2025.