Cochlear implants are hearing devices that can help people with certain kinds of hearing impairment or who are entirely deaf. The implant works by using the tonotopic organisation of the basilar membrane of the inner ear.

"Tonotopic organisation" is the way the ear sorts out different frequencies so that our brain can process that information. In a normal ear, sound vibrations in the air lead to resonant vibrations of the basilar membrane inside the cochlea. High-frequency sounds (i.e. high pitched sounds) do not pass very far along the membrane, but low frequency sounds pass farther in. The movement of hair cells, located all along the basilar membrane, creates an electrical disturbance that can be picked up by the surrounding nerve cells. The brain is able to interpret the nerve activity to determine which area of the basilar membrane is resonating, and therefore what sound frequency is being heard.

In individuals with sensorineural hearing loss, hair cells are usually few in number. Hair cell loss or absence may be caused by a genetic mutation or an illness such as meningitis. Hair cells may also be destroyed chemically by an ototoxic medication, or simply damaged over time by excessively loud noises. The cochlear implant by-passes the hair cells and stimulates the cochlear nerves directly using electrical impulses. This allows the brain to interpret the frequency of sound as it would if the hair cells of the basilar membrane were functioning properly (see above).

Who can use a Cochlear Implant?

Cochlear implants are usually recommended for individuals who have severe to profound sensorineural hearing loss and who do not benefit sufficiently from hearing aids. Individuals with auditory neuropathy may also benefit from cochlear implants. Cochlear implant centers determine implant candidacy on an individual basis and take into account a person's hearing history, cause of hearing loss, amount of residual hearing, and speech recognition ability.

Once a cochlear implant is put in place, any residual hearing a person has in that ear may be destroyed. For this reason, people with mild or moderate sensorineural hearing loss or conductive hearing loss are generally not candidates for cochlear implantation. After the implant is put into place, sound no longer travels via the ear canal and middle ear but will be picked up by a microphone and sent through the device's speech processor to the implant's electrodes inside the cochlea.

If an individual has been deaf for a long period of time, the brain may begin using the area of the brain normally used for hearing for other functions. If such a person receives a cochlear implant, the sounds can be very disorientating, as the brain may interpret these signals not as sounds but as other things.

Children's brains are quite plastic. Therefore, children who receive cochlear implants at a young age can adapt to them very well. Many are able to learn to listen without having to rely on speechreading (lipreading) and to speak very intelligibly with excellent voice quality and using age-appropriate language. Generally, the earlier a child receives a cochlear implant, the better the outcome will be in terms of using the implant to its full capacity. However, how well the child functions with the implant also depends on other factors such as the child's prior use of hearing aids and cause of deafness. Children may receive cochlear implants as young as 12 months of age, though some even as young as 4 months have received cochlear implants.

A critical component of success in using a cochlear implant is appropriate therapy.

The Device

The device can be divided into two parts, the part which is implanted, and the part worn outside the body. The external part of the device contains:

• Microphone
• Processing
  • Speech Processing
  • Filters
• Transmitter

The internal part of the device contains:

• Receiver
• Electrode Array of up to 22 electrodes wound through the cochlea

Processing

The sound wave received by the microphone must be processed to determine which electrodes should be stimulated. The simplest way of processing would be to divide the sound into however many electrodes there are, and apply the resulting voltage to the appropriate electrode. More sophisticated processing algorithms are used in practice because applying voltage to each of the electrodes at the same time would cause currents to flow between the electrodes, which would stimulate the nerves in undesirable ways.

Waveform processing strategies use bandpass filters to divide the signal into different frequency bands. The algorithm chooses a number of the strongest outputs from the filters. The number depends on the algorithm, and can also depend on whether the sound is determined to be a consonant or a vowel sound.

Feature extraction strategies use features which are common to all vowels. Each vowel has a fundamental frequency (the lowest frequency peak) and formants (peaks with higher frequencies). The pattern of the fundamental and formant frequencies is specific for different vowel sounds. These algorithms try to recognise the vowel and then emphasise its features.

Transmitter and Receiver

These are used to transmit the processed sound information over a radio frequency link. This is so that no physical connection is needed, which reduces the chance of infection. The transmitter attaches to the receiver using a magnet that holds through the skin.

Electrode array

The electrode array is made from a type of silicone rubber, while the electrodes are platinum or a similarly highly conductive material

Programming the Speech Processor

The cochlear implant must be programmed individually for each user. This process is called "MAPping" and is performed by an audiologist trained to work with cochlear implants. The audiologist sets threshold and comfort levels for each electrode in the array based on information given by the user.

Objections to use

The use of cochlear implants is objected to by some, particularly in the signing community (people who use sign language to communicate). They do not want deafness to be considered a disability that has to be "fixed" by cochlear implants, but rather just a different way of living. Cochlear implants for children work best when implanted at a young age, when the brain is still learning to interpret sound, and hence are implanted before the recipients can decide for themselves. There is a debate about the age that would be the best or safest for children to receive cochlear implants. However, as cochlear implants have improved and proven themselves to work well, the objections are getting less tenable. Proponents of cochlear implants believe that, since mammals are meant to have a hearing sense, deafness is a disability to be corrected. To them, objecting to a cochlear implant is akin to objecting to a heart transplant, prosthetic arm or eyeglasses.