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What is a cochlear implant?

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What is the difference between a cochlear implant and a hearing aid?

A cochlear implant is a type of hearing aid that turns sound into electrical signals. Instead of simply making sounds louder like a conventional hearing aid would, the cochlear implant provides a sensation of hearing by directly stimulating the auditory nerve using electrical signals. The cochlear implant has two parts – an internal receiver, which is implanted surgically, and an external part, which is worn like a hearing aid.

Cochlear implant systems

What are the parts of a cochlear implant?

There are several makes and models of cochlear implant but basically they contain the same parts.

The internal receiver is implanted under the skin behind the ear and a number of electrodes are fed through into the cochlea. So to have a cochlear implant your child will need to have an operation.

The external part is made up of a speech processor, a lead, a transmitter coil and a microphone. The speech processor is either worn in a harness or belt on your child’s body or behind their ear (like a hearing aid). The microphone sits behind the ear. The transmitter coil is held in place over the implanted receiver by a magnet. This external part is only attached once the scar from the operation has healed. The speech processor is tuned in to suit each child.

How does a cochlear implant work?

To understand how a cochlear implant works, it is useful to know about how the ear works.

The ear has two main functions:

  • It receives sound and converts it into signals the brain can understand.
  • It helps us to balance.

The two functions are closely connected. 

The ear is the first part of the hearing system. The pinna is the outer part of the ear that we can see. It gathers sound waves and directs them down the ear canal. The waves then cause the eardrum to vibrate. These vibrations are passed across the middle ear by three tiny bones. These are called:

  • the malleus (also known as the hammer);
  • the incus (also known as the anvil); and 
  • the stapes (also known as the stirrup).

The bones increase the strength of the vibrations before they pass through the oval window into the cochlea.

The cochlea looks like a snail’s shell. It is filled with fluid and contains thousands of tiny sound-sensitive cells. These cells are known as cilia or ‘hair cells’. As the vibration of the bones in the middle ear enters the cochlea, it causes the fluid to move. This causes the hair cells to bend. The movement of the hair cells is like the movement of seaweed on the seabed when waves pass over it.

As the hair cells move they create a small electrical charge. The charge moves along the auditory nerve to the brain where it is converted into signals that can be understood.

For an ear to work fully, and let us hear the full range of sounds, all of the hearing system must work well. Deafness happens when at least one part of this system is not working effectively.

Cochlear implants work as follows.

  • Sound is picked up by the behind-the-ear microphone and is sent to the speech processor. The sounds are then processed, changed into electrical signals and passed along the lead to the transmitter coil.
  • The signals are sent by radio waves through the skin to the implanted receiver and down the wire to the electrodes in the cochlea. The number of electrodes for each child may be different.
  • When the electrodes receive the signal, they make a tiny current that travels along the auditory nerve to provide a sensation of hearing.
How will changes in technology affect my child’s cochlear implant?

The internal part of a cochlear implant (the part that is surgically implanted) is unlikely to need upgrading, unless there are problems.

The external parts (the part that is worn like a hearing aid) may change as technology develops, but this should not mean further surgery.

Should my child have one implant or two?

Most people hear better using both ears rather than just one. This is because listening with two ears makes it easier to understand speech when there is background noise, and we are better able to work out which direction sound is coming from. Hearing with both ears is known as binaural hearing.

Today most deaf children use hearing devices in both ears. The cochlear implant team may recommend having two cochlear implants, known as bilateral fitting, or recommend having one cochlear implant and using a hearing aid in the other ear, known as bimodal fitting. This will depend on the level of hearing and the expected benefits of both options.

Once a cochlear implant has been implanted, your child will probably not have the same level of hearing left in that ear. The implant operation can cause changes to the cochlea which would make it unlikely that your child could go back to using a hearing aid in the implanted ear. This is why it is very important for you to talk to your specialist implant team about the options.

If it is recommended that your child has one implant and continues to use a hearing aid in the other ear, you will need to discuss which ear is to be implanted with the surgeon and audiologist. It is important for you to know why the implant team thinks that a particular ear should be implanted instead of the other. It may be because of medical reasons (for example, the scan may show why one ear rather than the other should be implanted), or it could be because of differing levels of hearing in each ear. For children with physical difficulties, the ear to be implanted may be chosen for practical reasons.

For people who do not use one it is very difficult to imagine what hearing is like through a cochlear implant. Most people who become deaf and then go on to use an implant agree that at first, the sound they hear through a cochlear implant is very different from sounds they heard before they became deaf. However, after a while sounds become clearer and more natural to the user. To use an implant effectively, the user needs to learn how to identify and interpret different sounds. For a young child, who has never heard before, adapting to the new signal may be easier.

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Information supplied by the National Deaf Children’s Society. Reproduced with permission.
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