It’s hard to imagine what Count Alessandro Volta was thinking in the late 1700s when he inserted two metal rods into his ears and flipped on the electricity. But he certainly didn’t know that the “crackling and popping sounds” he heard would later help to cure the deaf and hard of hearing – it was just a wild supposition.
“I received a shock in the head and some moments after… I began to hear a sound… It was a kind of crackling with shocks as if some paste or tenacious matter had been boiling.”
The cochlear implant (or “bionic ear”) that later evolved from these experiments – and from the earlier investigations of Giuseppe Veratti and Benjamin Wilson – has already empowered over 600,000 people around the globe to overcome their hearing challenges. And this is where science merges with the miraculous. Witnessing the reaction of someone turning on a cochlear implant for the first time would bring anyone to tears (try not to cry).
But how do cochlear implants achieve this miracle? And, what’s in store for this technology in the years ahead?
In the first part of this two-part series, we’ll look at the fascinating science behind cochlear implants and how they work. In the second part, we’ll investigate the tragedy of how expensive these devices are ($80,000 in the United States on average). Finally, we’ll explore some groundbreaking strategies and technologies that could make cochlear implants more affordable for people living in the developing world.
So, without further ado, let’s dive in...
The Science of How the Ear Converts Sound Vibrations Into Electrical Impulses
To understand the science of how cochlear implants work, we need to understand how the human ear serves as an “interface” between two worlds: (1) The mechanical world of sound vibrations and (2) the electro-chemical world of the brain. We also need to understand what goes wrong in the ear to cause deafness, hearing impairment, and hearing loss.
Diagram of the Inner Ear: Sourced from Wikipedia
As an interface that converts mechanical sound vibrations into electrical impulses the brain can understand, the ear does its job through an electrochemical process. If you’re not familiar with this process, reading the following steps will blow your mind:
Sound vibrations travel through the air: Sound waves are nothing more than a series of air molecules bouncing off each other. Coming from the source of the sound, these vibrations travel through the air like waves in a body of water.
Sound vibrations enter the ear: The bowl-shaped structure of the outer ear (auricle) captures the sound waves. Next, the vibrating air molecules enter the ear canal (external auditory canal) until they make contact with the eardrum (tympanic membrane).
The eardrum moves with the sound vibrations: Once the sound vibrations make it to the eardrum, the sound waves cause the eardrum to vibrate in harmony at the same frequency.
The eardrum causes the “ear bones” to vibrate: There are three small ear bones in the middle ear (malleus, stapes, and incus). As the eardrum vibrates, it transfers its vibrations to the ear bones.
Sound vibrations become fluid vibrations in the cochlea: As the ear bones vibrate, particularly the stapes bone, they transfer the vibrations to the fluid inside the snail-shaped cochlea.
Fluid vibrations move the hair cells in the cochlea: The tiny hair cells responsible for hearing are found in the fluid-filled cochlea. The fluid vibrations in the cochlea cause these hair cells to move in harmony with the vibrations. Look at this incredible colorized image of the hair cells!
The hair cells transfer the fluid vibrations into electrical impulses: The moving and bending of the hair cells cause tiny pore-like channels to open and close near the hair cells. The opening and closing channels release special chemicals that create an electrochemical reaction and electric charge. The electric charge stimulates the auditory nerve. This is the principle that Count Volta experienced in his investigations. According to the current scientific explanation of this process, stimulating the narrowest part of the cochlea causes you to perceive lower sound frequencies. Stimulating the widest part of the cochlea causes you to perceive higher sound frequencies. Dr. Chip Goldsmith of the AllHear Foundation gave us the following perspective for hearing loss patients: “The most important sounds are the high frequencies, the consonants of speech. When you’re missing that, the speech quality of the speaker [and understanding] will be diminished.”
The auditory nerve transfers the electrical impulses to the brain: As the auditory nerve receives the electrical impulses, it delivers them to the brain. The brain processes and interprets the electrical signals as the sounds we hear.
Image of an Uncoiled Cochlear Bone with Corresponding Sound Frequencies: Sourced from Wikipedia
Now that we understand how the ear serves as an interface that converts mechanical sound vibrations into electrical signals that the brain can understand, let’s see what can go wrong in this process to cause deafness and hearing impairment. Then we will explain what cochlear implants are doing to restore hearing ability.
What Causes Deafness and Hearing Impairment?
Illustration of Damaged Hair Cells: Sourced from CDC.gov
Aside from hearing loss caused by cognitive impairment, most hearing loss cases – and incidents of deafness and severe hearing impairment – result from one or more problems that interfere with the above-described process, preventing the ear from serving as an “interface." Here are the most common causes of deafness and hearing loss that don’t relate to cognitive impairment:
Hair cell damage: When the hair cells in the ear become damaged – due to loud noise, the aging process, or another reason – they can’t produce a strong enough electrical signal for the auditory nerve to send a sufficiently strong electrical signal to the brain. This interferes with clear hearing.
Auditory nerve damage: When the auditory nerve degrades, it cannot deliver enough electrical impulses to the brain for clear hearing. In this case, the cochlea could be receiving enough fluid vibrations, and it could be producing enough electrical signals, but the degradation of the auditory nerve prevents the delivery of those signals. Individuals with this type of hearing loss may not respond well to cochlear implants.
Injuries: After a serious injury, one or more parts of the ear – such as the ear bones or eardrum – may not function properly and hearing loss or deafness could result.
Poor development: Due to poor development or genetic conditions, different parts of the ear may not function properly – or they could be completely missing – which can inhibit hearing or cause deafness.
Illnesses and infection: A serious illness or infection could cause temporary or permanent loss of hearing when the condition affects the structures of the ear.
In all of the above cases, the brain is not receiving enough electrical stimulation for clear hearing. While surgical procedures can correct certain problems with the ear bones and eardrums – and the effects of an infection could be temporary – most cases of hearing loss and deafness are permanent and require treatment through hearing aids or cochlear implants. Hearing aids work by providing sound amplification to overcome the functional deficiencies in the ear. A cochlear implant may be able to help when sound amplification from a hearing aid isn’t enough.
Image of Healthy Hair Cells: Sourced from Wikipedia
How Cochlear Implants Work
A cochlear implant bypasses the ear’s job of converting sound vibrations to electrical signals. Instead of relying on the movement of the hair cells in the ear to create an electrochemical reaction, the cochlear implant delivers electrical sound information (by way of electrical stimulation) directly into the cochlea. To understand how this process works, we need to look at the different components of a cochlear implant:
Microphone and speech processor: The microphone and speech processor rest behind the ear. These components pick up sound information and convert it into digital sound like a hearing aid does. The speech processor converts the sound into digital signals, separating them into multiple channels based on the frequency of the sounds.
Image Source: Edited image from Centreforhearing.org
Transmitter: After separating the sounds into different frequency channels, the speech processor sends the digital sound information to the transmitter. The transmitter holds itself in place on the outside of the skin by using magnets to connect to the implanted receiver/stimulator. The transmitter turns the digital sound information into electrical signals that it sends through the skin to the receiver/stimulator.
Image Source: Edited image from Centreforhearing.org
Receiver/stimulator: As for the receiver/stimulator, it is completely hidden beneath the skin – and contrary to the concerns of some patients, there are zero wires or connectors that emerge from the skin. When placing the receiver/stimulator, a surgeon creates a shallow indentation in the bone of the skull, so the receiver/stimulator lies flush and even with the skull. The surgery is performed behind the ear. After the surgery, you cannot detect that someone has the implant unless they are wearing the external microphone/processor and transmitter.
Image Source: Edited image from AAOCI
Electrode array: Extending out of the implanted receiver/stimulator is a thin, flexible “electrode array.” A surgeon implants the electrode array into the snail-shaped cochlea. The surgeon enters this area from an incision point just behind the ear. When activated, the electrode array stimulates different parts of the cochlea with the electrical signals it receives from the receiver/stimulator.
These days, virtually all cochlear implants use a “multi-channel” electrode array. Multi-channel means that there are different contact points that send electrical stimulation to specific parts of the cochlea. Each contact point is responsible for a range of sound frequencies. In this image, you can see 16 contact points along the electrode array. Each contact point is responsible for delivering electrical stimulation for a specific sound frequency range.
Image Source: Edited Image from Advanced Bionics
Bringing It All Together
As you can see, the miracle of the cochlear implant – and its ability to restore hearing – has come a long way from Count Volta’s experiment of putting metal rods into his ears and flipping on the electricity. By using the above components, the cochlear implant does the job of the ear – skipping steps 1 through 7 above – to deliver electrical sound information directly to the cochlea. To achieve this, the various parts of the cochlear implant do the following:
The microphone picks up sounds from the environment.
The speech processor converts the sound into digital information.
The speech processor organizes the sound information into different frequency channels.
The speech processor sends the sound information for each channel to the transmitter.
The transmitter sends a corresponding electrical impulse for each sound channel to the receiver/stimulator.
The electrical impulses pass through the skin to the implanted stimulator.
The impulses for each separate channel travel through the electrode array to the cochlea.
The contact points along the electrode array stimulate different areas of the cochlea.
The hearing nerve receives the electrical impulses from different parts of the cochlea and passes the electrical information along to the brain.
The brain interprets and understands the different electrical impulses as sound.
It’s important to note that we don’t fully understand how the brain processes and understands electrical sound information. But we do know that the brain is highly adaptive. Even though the electrical impulses from a cochlear implant are different from those in natural hearing, the brain quickly adapts to this new form of electrical stimulation. In the beginning, implant recipients usually report that they hear “robot sounds” or “Mickey Mouse voices.” After several months or a year, these experiences tend to fade and hearing feels more natural.
Michelle Harris, writer/owner of the blog Deaf Be Not Proud – and a cochlear implant recipient herself – recently gave us this intimate look at what the cochlear implant experience is like:
“When I first got my cochlear implant, nothing sounded as I remembered before I started losing my hearing. Initially, everyone sounded exactly the same, like Donald Duck. It took weeks to months before I could recognize the voices of my family members. It took even longer to relearn all of the new but common sounds my ears were picking up like dripping faucets, beeping electronics, rustling leaves, etc. In fact, I was constantly asking my kids, ‘What’s that sound?’
“Getting used to a cochlear implant takes a lot of time, patience, and support from family. I’d say it took a good year or more before I felt like things were sounding normal again. The longest period of adjustment involved hearing projected voices such as people speaking through a phone, on a microphone, on TV, and in movies at the theatre. It’s still a challenge at times. After 10 years of having a cochlear implant, music still doesn’t sound quite the same, and it’s difficult to understand the lyrics. In spite of the difficulties, getting a cochlear implant has been life-changing, to say the least.”
The Potential Downsides of Cochlear Implants
For most patients with mild to severe hearing loss, doctors recommend the use of hearing aids that amplify specific sound frequencies. For one, hearing aids are a lot less expensive than cochlear implants, starting as low as $399 a pair if you go with an affordable manufacturer like MDHearingAid. Also, hearing aids sidestep the potential downsides and risks associated with cochlear implants. These downsides and risks include:
Extremely expensive: The average cochlear implant procedure costs $80,000 in the United States. These costs are often covered by Medicare, Medicaid, or private health insurance in the United States. While patients can receive a cochlear implant for less money in a developing nation, they are still far outside most people's budgets if the country has the medical infrastructure necessary for the procedure. Ultimately, the high cost of implantation means that these devices are largely unaffordable and inaccessible for the uninsured and those living in developing economies.
Don’t restore “natural” hearing: Cochlear implants do not restore “natural” hearing. They replace it. As Michelle Harris noted in the quote above, the hearing experience of a cochlear implant user is dramatically different from that of someone with natural hearing or a hearing aid.
Loss of remaining hearing ability: The surgical procedure that installs the cochlear implant’s electrode array may disrupt and permanently damage the hair cells and other structures inside the cochlea. Doctors try to preserve natural hearing, but many patients lose their remaining hearing ability as a result of the procedure.
Risk of facial nerve damage: Although uncommon, cochlear implant surgery comes with the risk of facial nerve damage, which could lead to the paralysis of certain facial muscles.
Doesn’t always work: A cochlear implant might not work to restore your hearing. For example, individuals who were born deaf and did not receive the implant before the age of speech development have a lower success rate. According to the study Cochlear Implants and Brain Plasticity: “The level of performance of pre-linguistically deaf adults generally remains well below that of post-linguistically deaf adults.” Furthermore, individuals with hearing nerve damage, traumatic brain injuries, and cognitive problems could have lower success rates.
Are You a Candidate for Cochlear Implants?
The best way to determine if you’re a candidate for cochlear implants is to visit an audiologist. An audiologist can perform a professional hearing test, diagnose your condition, and offer treatment suggestions. Nevertheless, if you simply want to check the severity of your hearing loss – and figure out whether an affordable pair of direct-to-consumer hearing aids will fit your condition – a free, five-minute online hearing test is an excellent place to start.
After taking this free hearing test from MDHearingAid, you will know (1) if you have a hearing loss, (2) how severe the hearing loss could be, and (3) if a pair of affordable, direct-to-consumer hearing aids could help. Here’s an example of what your test results will look like:
Read more about hearing tests in my Discovery article on free online hearing tests.
At this point, you should have a general understanding of the science behind cochlear implants, but nothing summarizes the miracle of this technology like videos of real people experiencing sound for the first time while turning on their implants (here’s another video I loved). After seeing the beauty of these moments, it is heartbreaking to consider that millions of people around the world cannot access this technology. Indeed, Dr. Sandra Porps, an audiologist and cochlear implant specialist who works with the affordable hearing aid manufacturer MDHearingAid, says that “only about 6% of the people who can benefit from cochlear implants currently have one.”
In part two of this article, we’ll explore why cochlear implants are so expensive. We’ll also look at an experimental yet controversial form of cochlear implant – and some exciting non-profit work – that could make this technology more economically accessible for everyone in the world. Stay tuned...