Published by Australian Academy of Science
Cochlear implants – wiring for sound Box 1 | How the implant works

The cochlear implant consists of external and internal parts: the internal parts are placed surgically in the bone behind the ear and in the inner ear. The external parts can be detached at any time.

Reproduced with the kind permission of the Department of Otolaryngology, University of Melbourne

The diagram shows the parts of a cochlear implant in place in a user's ear. Parts a, b, c and d are external parts; parts e and f are internal.

1. microphone (worn behind the user's ear);
2. thin cord (connects microphone to speech processor);
3. speech processor (codes sounds electronically);
4. transmitting coil (sends code as radio waves);
5. receiver/stimulator (converts code into electrical signals);
6. electrode array implanted in the cochlea (stimulates auditory nerve fibres when electrical signal is received);
7. cochlea;
8. auditory nerve.

External parts: Microphone, speech processor, transmitting coil

A microphone worn just behind the patient's ear performs the function of the outer ear. It picks up the sounds of the outside world and transmits them via a thin cord to a speech processor. This looks a bit like a small transistor radio but works like a computer. It is worn externally on a belt or in a pocket or shoulder pouch (although a new product containing the microphone and speech processor is small enough to be worn behind the ear, like a hearing aid). Much of the wizardry of the cochlear implant is contained in the speech processor: it selects the sounds most useful for understanding speech and codes them electronically (Box 2).

These electronic codes are sent back through the cord to the transmitting coil, which is a plastic-covered ring about 33 millimetres in diameter. This is located a little further back behind the user's ear and is held in place by two magnets, one located under the skin and the other in the centre of the transmitting coil.

Internal parts: Receiver/stimulator, electrodes

The coil sends the electronic codes through the skin via radio waves to the receiver/stimulator. This consists of a custom-designed integrated circuit (a small computer, in effect). The receiver/stimulator converts the codes it receives into electrical signals that it sends along the electrode array, implanted in the cochlea of the user.

The electrode array consists of 22 tiny electrode bands arranged in a row inside a piece of tapered flexible silicon tubing. Each electrode has a wire connecting it to the receiver/stimulator; each has been separately programmed to deliver electrical signals representing sounds that can vary in loudness and pitch. When the electrodes receive an electrical signal, they stimulate the appropriate populations of auditory nerve fibres, which send the messages to the brain.

Some early cochlear implants provided stimulation of the auditory nerve fibres using only one electrode: these were developed in various research centres in Europe and North America. One study of 49 children implanted with single-electrode cochlear implants showed that most could discriminate syllable patterns, but only two achieved any significant understanding of conversational speech. The great advantage of the multi-channel cochlear implant is that speech can be filtered into frequency bands by the speech processor and delivered to different points along the cochlea.

The cochlea is organised so that different sound frequencies preferentially stimulate different hair cells. (As the membrane along the bottom of the cochlea resonates in time with the sound vibration, hair cells at different positions along the membrane are stimulated.) Stimulating hairs located at the base of the cochlea produces perceptions of high-pitched sounds; stimulating hairs located at the opposite end (apex) of the cochlea produces perceptions of lower-pitched sounds. The multi-channel cochlear implant has a number of electrodes at different positions on the cochlea and is designed to deliver stimuli to appropriate electrodes so that high-pitched sounds cause electrodes to stimulate hair cells towards the base of the cochlea and low-pitched sounds cause electrodes to stimulate hair cells towards the apex of the cochlea.

Is normal service resumed?

A person with a cochlear implant will hear sounds as they happen, which is why the technology can be so helpful. But, sophisticated though it is, the cochlear implant does not fully reproduce the sounds experienced by someone with full hearing.

The effectiveness of the cochlear implant varies considerably. Factors that determine the benefit recipients will gain include:

• whether they developed spoken language before going deaf (people who have learned to speak usually benefit most);
• the time since deafness first occurred;
• their level of motivation;
• the environment in which they live: an encouraging home, school or work environment will help implant patients achieve their full potential; and
• the number of surviving auditory nerve fibres in the implanted ear.

The age of recipients varies considerably – from children as young as 14 months to 80-year-olds. Studies of recipients of the 22 electrode implant have shown that adults with acquired hearing losses can understand 80 per cent of speech (on average) when using their implant (without lip reading). At this level of understanding, such things as interactive telephone conversations become possible.

It's harder for people who have been deaf since birth. They have to learn the associations between sounds and words from scratch; understandably, this can be difficult after a life of silence. It thus becomes very important that the implants are performed on deaf children as early on in their lives as possible, giving them the opportunity to develop spoken language as they grow up, just like children with full hearing.

Related site

• Cochlear implants (National Institute on Deafness and Other Communication Disorders, USA)

Other boxes

Box 2. The mathematics of hearing

Box 3. The bionic ear industry

Box 4. Breaking the silence