The hearing aid of the future
You might want to hear this! Scientists in Germany have invented what’s being dubbed the hearing aid of the future: it uses light to deliver information about sound to the nervous system. Chris spoke to Marcus Jeschke, from the University Medical Centre in Göttingen, who is one of the developers.
Marcus - So normally, sounds that reaches our ears are transmitted through what's called tympanic membranes. So that’s a small membrane that uses the vibration that's caused by sound and is transmitted into the structure in our inner ears called the cochlea. Here there is very tiny hairs cells who transmit this movement into electrical signals that our brain can understand. Close to the base of the cochlea neurons and hair cells preferentially respond to high frequency sounds, whereas at the apex or tip of the cochlea neurons and hair cells preferentially respond to low frequency sounds.
Chris - And when someone has hearing impairment, what has gone wrong with that system you just described?
Marcus - So the most common form is the loss of sensory hair cells. This is usually caused by very loud sounds. This can also be caused by antibiotics and a couple of other factors.
Chris - And how do the present generation of hearing aids surmount the loss of those hair cells then?
Marcus - the current version, which is the electrical cochlear implant uses electrical current to stimulate the auditory neurons that are sitting behind the hair cells. These nerves are still excitable, so we can activate them by inserting a small cable with multiple electrodes that are sitting along different parts of the cochlea and they stimulate different parts of the auditory nerve, which is responsible for different pitches.
Chris - So the cochlear implant is listening to sounds coming to you from outside, and when it hears a high frequency it sends electricity to the wire that stimulating the high frequency bit of your cochlear, and the converse if you hear low frequency it sending signals preferentially down the wire that stimulates the low frequency bit.
Marcus - That is absolutely correct.
Chris - So what's wrong with that. Why do we need a next generation hearing aid?
Marcus - There's actually nothing wrong with that per se. It's the most successful neuroprosthesis we have. It's an amazing success story, it allows around half a million people that are implanted with cochlear implants to understand speech and quiet environments. However these patients still have problems understanding speech and noise and typically, don't appreciate music. The reason for that is hair cells in the cochlea sit in a very saline environment and that means that if you stimulate a certain part of the cochlea, this electrical stimulation spreads to neighboring parts of the cochlea and co-activates them. In other words it's as if you were playing the piano with your forearms.
Chris - Yes you're going to take down lots of notes at the same time rather than one finger, one note.
Marcus - Yes.
Chris - So what can you do that's better?
Marcus - One way to do this, is to use light. Light can be much more precisely confined. So you could use a very small dot of light, which is just a few microns, which is on the order of single neurons in our brain. And this allows many many more frequency channels or individual stimulation channels and this in turn could mean that you have access to the individual keys on the piano.
Chris - So what you're saying is you could thread, like a miniature string of fairy lights down the inside of the cochlea rather than just individual electrodes, and illuminate bits of the cochlea. But how do you make it sensitive to light? Because at the moment it's sensitive to electricity.
Marcus - That's correct. In order to do this, we use what's called optogenetics. So what we're doing is packing the genetic information of, if you want, lights switches into the neurons of the auditory nerve and therefore make them light sensitive. So what they will do is, upon light stimulation, these lights switches will activate, and in turn activate the auditory neurons.
Chris - And does it work? It sounds ingenious.
Marcus - It does work indeed. That's the cool thing about this. We have been able to show that in Mongolian gerbils, we can actually use these light switches and put them into the auditory nerve of adults, and show that upon light stimulation in the cochlea, these animals actually can use this light information to perform a behavioral task. So in other words, these animals,now hear light.
Chris - And you can prove that they do genuinely respond as though they're hearing a sound?
Marcus - That's right. We trained animals to respond to auditory stimuli before they were actually optically stimulated. And they learned to respond to certain sound, and react on this. And upon light stimulation, after they've learned this auditory behavior of sounds, a few animals transferred this to light stimulation.
Chris - Do you think this could be translated to humans?
Marcus - That's actually our goal, yes. Of course there's a lot to be done before you can actually safely try this in humans but in general, the idea is that this could work in humans, yes.