Chris Smith: First up Ben, scientists have found a way to use lasers to improve certain types of hearing aid. Tell us more.
Ben Valsler: Well this is the discovery that laser light interacts with nerve cells in such a way that we can actually use it to greatly improve cochlear implants - these are implants inside the inner ear. Now a healthy inner ear will contain many thousands of hair-like cells which detect sound, and they pass that sound signal on to nerve cells. The nerve cells transport the signal to the brain where it can be decoded and you know what you’ve heard.
If these cells get damaged, as a result of illness, accidents or something like a genetic defect, you wind up being deaf. A cochlear implant takes the place of these hair cells. It communicates directly with the nerve cells so that someone who was formerly deaf can hear. And at the moment cochlear implants are actually very effective. Deaf children with implants can develop speech that is very close to that of hearing children.
Chris Smith: But how do they actually work? How are they getting the signal back into the nerve cells?
Ben Valsler: Well, at the moment, they use about 20 electrodes to connect to the nerve so they directly electrically plug into those nerve cells. Now that’s very few connections compared with the 3,000 or more hair cells that you find in a healthy ear. So that means that although an implant will certainly improve hearing, the hearing will still be quite poor, and many people with implants, they don’t enjoy music, they find it very difficult to converse in noisy environments and, actually, they find it quite hard to understand tone or languages like Mandarin or Thai. Now due to the way that tissue conducts electricity, you find that you just can’t increase the number of electrodes more than about 20.
Chris Smith: Is that because you get a sort of spill-over from one stimulated area to an adjacent one if you put more electrodes in, and that means the signal blurs rather than getting more precise?
Ben Valsler: Exactly, yes. The fact that the tissue will actually send some of the electricity through to the neighbouring electrodes will put a limit on the density of electricity that you can put into that tissue, and that in turn puts a limit on the quality of cochlear implants. But now a team from Northwestern University in Chicago lead by Claus-Peter Richter have announced at the Medical Bionics Conference in Victoria, Australia that they found a way to shine some light upon the problem - boom boom - using infrared laser light to be precise.
Now, for reasons that are not yet fully understood, laser light will stimulate neurons, even though they do not contain light sensitive proteins. Richter’s team decided to see if this would be a better way of communicating with the neurons in the inner ear, and knowing that the laser would not spread through the tissue, you wouldn’t get this spill-through in the tissue that an electric pulse would, lasers could be a way to achieve much higher definition for implants.
They tested this out by shining a laser of infrared light onto the neurons in the inner ear of deaf guinea pigs. And they also monitored the electrical activity in the region of the brain responsible for passing information from the ears to the brain cortex - that’s called the inferior colliculus. Now not only did the lasers register a signal which shows that the deaf guinea pigs could definitely hear, so they’re working just as the cochlear implants do, but the brain activity actually looked incredibly similar to that seen in guinea pigs that can hear.
So not only does it show that it works but actually it shows that you may be able to almost restore hearing completely, using lasers.
Chris Smith: Amazing to think that you can use light to hear a sound. If you’d said you could do that a few years back people would have said you were mad, for various reasons, but very encouraging work from Australia, the home of course of the cochlear implant where it was all invented.
This article was originally broadcast as part of Breaking Science in November 2008. Listen to the full episode.