A device that can restore a sense is an impressive feat of bionics. Cochlear implants have enabled hundreds of thousands of people around the world to hear sounds they would otherwise miss.
But how do cochlear implants actually work? And how do they function in reality? Cosmos investigates.
How do cochlear implants work?
The first thing to understand is how sound moves through the ear. Sound waves are vibrations – physical movements through the air, and other substances.
“That’s picked up by a very flexible membrane inside the cochlea,” says Associate Professor Rachael Richardson, a principal research fellow at the Bionics Institute in Melbourne.
“That movement stretches and squashes these tiny cells called hair cells. They’re kind of the sensors of sound, designed to detect these vibrations or waves. When they’re stretched, squashed and generally moving, that’s stimulating the release of neurotransmitters.”
People with hearing loss lack some, or all, of these hair cells. A cochlear implant (CI) stimulates the nerves electrically instead.
The implant has two parts: a small device fitted with an array of electrodes, which is surgically implanted in the ear; and a device worn outside the ear with microphones and processors that pick up sound, and electronically transmit it to the implant.
Do cochlear implants work exactly like ears?
Cochlear implants don’t (and can’t) do the same job as hair cells. The sound that cochlears produce isn’t as rich.
“It’s hard to describe – it sounds a bit robotic, a bit tinny,” says Richardson.
“[For example], if there’s some music playing, you wouldn’t be able to tell if it’s played by a piano or violin.”
There are a couple of limitations with the technology that are hard to overcome. The implants have a maximum of 22 electrodes, each of which cover a range of pitch, but can’t be as precise as hair cells.
“It’s like playing piano with your elbow – you’re going to get a mishmash of notes together,” says Richardson.
The other problem is to do with electricity. “You’re using electrical current to stimulate the nerves,” says Richardson. “If you think about electrical current, in a fluid environment, especially in a salty fluid environment, it spreads out.”
This makes it very hard to activate individual nerves.
“Even with a really tiny electrode, it’s just going to spread out and overlap,” says Richardson.
So those 22 electrodes can’t make 22 independent channels of information: in reality, there’s more like eight to 10 channels.
What this results in is a device that’s useful for some things – like hearing one-on-one speech – but not so good with other sounds. It also picks up all the sound in a room, so if someone’s in a noisy environment, it’s difficult to focus on one voice.
It’s also important to note that cochlear implants don’t instantly restore hearing in the same way that putting on a pair of glasses can correct vision – whatever videos you may have seen online to the contrary.
Jen Blyth, chief executive of Deaf Australia, points out that there is “intensive and long-term therapy involved for the cochlears to function.
“It can take months and years for the brain to learn the meaning of new sounds.”
The surgery to get the implant, while straightforward, isn’t completely risk-free – it’s surgery, after all.
“The operation itself carries intra-operative and post-operative risks, including infection, malfunction, neurological impairment, tinnitus, facial palsy, balance issues, vertigo and poor auditory outcomes,” says Blyth.
The implant prevents some activities, such as MRI scans and scuba diving. There is also some evidence that it can affect the structure and function of the ear.
These are a couple of the reasons (there are many others) that some people choose not to be implanted, or choose not to use their cochlear implants.
So it’s a limited technology – are there ways to improve it?
Since the invention of cochlear implants four decades ago, there have been a number of things that have improved their function.
“A lot of the improvements have come from the speech coding strategies,” says Richardson. “That’s the process on the outside of the ear, picking up the sound and filtering it and sending that information.”
But there is some basic physics that limits the development of cochlear implants.
“We’ve kind of hit this plateau now about how much further we can take the technology as it is,” says Richardson. “And a lot of that comes down to current spread.”
To make a better hearing tool, you’d need to use something other than electricity. Richardson and her colleagues are investigating using light, instead of electrical current, for instance, which can be focussed much more finely.
“The trouble is, the nerves in the ear don’t naturally respond to light,” says Richardson. So, any technique that uses light in the ear will need some modification to the nerves, as well. This idea is still in very early stages, and nowhere near testing in people yet.
Do cochlear implants always help their users?
You might assume that someone born with complete hearing loss would automatically benefit from having a cochlear implant. While they’ve been useful for many people, especially those who have lost hearing later in life, it’s not always necessarily the case.
Blyth says that one Danish study has even found that the implants hinder people born with hearing loss.
“Deaf individuals who grew up with CIs were less likely to report participating in mainstream (hearing) activities than deaf individuals without CIs and, furthermore, reported higher levels of feeling limited,” says Blyth.
“The CIs were designed with the intention of reducing barriers to accessing spoken language and there, by implication, the broader society. Yet this research suggests that there is a cohort reporting a decrease in positive perceptions of their own abilities and self-efficacy.”
Other research has suggested that it’s more important for their literacy that children born with hearing loss learn any language – whether it’s Auslan or another sign language – than that they learn to speak.
“Most people working within this space are not aware of, or do not endorse the existence of, the Deaf community, its culture and Auslan as a language with its own grammar and syntax,” says Blyth.
She says there’s no evidence that learning sign language hinders the ability to learn a spoken language, nor that cochlear implants hinder the ability to learn to sign. It’s just as easy to be bilingual with a signed and spoken language, as it is with two spoken languages.
“Because of the varying degrees of success in post-implantation, regardless of whether a deaf baby is implanted or not, it is Deaf Australia’s position, based on numerous peer-reviewed research, that every child should have the opportunity to learn both Auslan and English and be immersed in both the deaf community and their spoken language communities,” says Blyth.
Something to watch
Cochlear implants featured in the 2021 SCINEMA International Science Film Festival film Hear Me Out. It was selected to feature in the Australian, Body and School categories, and can be viewed on RiAus’ Education site.
Originally published by Cosmos as How cochlear implants work…and when they don’t
Ellen Phiddian is a science journalist at Cosmos. She has a BSc (Honours) in chemistry and science communication, and an MSc in science communication, both from the Australian National University.
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