What is a Cochlear Implant and how does it work?

Royal Hobart Hospital

What is a Cochlear Implant and how does it work?

What is a Cochlear Implant and how does it work?

A cochlear implant is an electronic medical device that replaces the function of the damaged inner ear. Unlike hearing aids, which make sounds louder, cochlear implants do the work of the damaged parts of the inner ear (cochlea) to provide sound signals to the brain.

A cochlear implant is comprised of both internal and external components.

An external speech processor is worn behind the ear and picks up sound, analyses it, and turns it into digital code. These signals are sent to a transmitting coil, which is circular, about 2 cm in diameter and held onto the side of the head with a magnet. The transmitting coil sends the coded signals through the skin to the receiver which is under the skin. The sound processor has a battery that powers the entire system.

The surgically implanted device converts the digital code into electrical impulses and sends these along the electrode array in the inner ear. The electrode array stimulates the hearing nerve, which then sends the impulses to the brain, where they are interpreted as sound.

You can visit the Cochlear website for further information and videos about cochlear implants:

What can you expect from a Cochlear Implant?

The sound from a cochlear implant does not restore “normal” hearing and it is difficult to predict how much benefit each person will receive. It takes time and practice before people become comfortable with the sound. Adults who have previously had normal hearing tell us that voices and sounds are different to what they remember. Speech with a cochlear implant has been described as robotic, scratchy, hollow, fuzzy, or just not clear. The sound seems to be different for different people and implantees find it hard to describe.

Many things affect how much each person will be able to hear with a cochlear implant. These include how long they have had a hearing loss, the degree and type of hearing loss they have had and their ability to make sense of a degraded signal. For some people, speech is easy to understand and they are able to follow a conversation without lip-reading, for others the sound is not clear and they will continue to rely on lip-reading and other cues to understand conversations. All implantees have difficulty following conversation in busy noisy places.

How long will it take to get used to the new sound?

While implantees may recognise some sounds when their speech processor is first turned on, others will become clearer over time and with practice. It is natural to expect to immediately enjoy the new sounds and understand conversation with an implant. However for many people the initial sound is confusing and even irritating. Gradually the sound begins to make sense. The longer an implantee has been deaf the longer this will take. Like the muscles of a broken limb immobilised in a cast, the hearing nerves don’t work as well due to lack of use. It takes time and practice to strengthen these nerve pathways. Recipients who lost their hearing later in life will have more memories and neural connections for sound in the brain. After they receive a cochlear implant, these memories can support them as they re-learn to hear with a cochlear implant.

For people who have been deaf for a long time or children who have been born deaf, there is little, if any hearing memory. The new sounds are meaningless until this hearing memory is built up. Perhaps it is like being in a room full of people speaking a foreign language - it takes time to sort through the noise, attach meaning to the sounds and make sense of what is heard.

Why is the sound different?

An implant works by electrically stimulating sections of the cochlear or inner ear. Different sections of the cochlea are stimulated for different sounds. When a high pitch sound such as “s “or “sh” is detected by the speech processor microphone, the electrodes at one end of the cochlea are stimulated. When the microphone detects a low pitch sound e.g. “b”, electrodes at the other end are stimulated. Where the electrode array is sitting in the cochlea varies from one person to another. Sounds are frequency shifted. This is similar to playing the same tune on a piano at different places on the keyboard.

The ability to tell the difference between similar sounds is also difficult. Normal hearing ears have thousands of hair cells tuned to different frequencies; the implant only has 22 channels. Sounds which are similar e.g. “s”, “sh”, “z” may stimulate the same electrodes and therefore sound the same. It does not matter how hard an implantee listens, these sounds may always sound the same. In order to understand conversation, cochlear implantees need to lip-read and use context.

What you will hear with a cochlear implant is also affected by your previous experience of sound. When a person has a hearing loss, the area of the brain which normally processes sound gets assigned to another function, as the brain is highly adaptable, or "plastic. The longer and more severe the hearing loss, the more neural reorganization occurs and the longer it will take for the brain to adapt to the new signal.

Recent research suggests that the better implantees are at integrating the new sound with other cues, such as lip-reading, the more benefit they will receive from their cochlear implant.

Rehabilitation and consistent use is critical to making the most of a cochlear implant.