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The cochlear implant
What does the cochlear implant do?
How the cochlea works
The Cochlear Implant System
The hearing process using a cochlear implant in detail
External parts of the Cochlear Implant System
Internal parts of the Cochlear Implant System
Nucleus® 24 Contour
Surgery and getting an implant

Cochlear - MAIN
What does the cochlear implant do?
Many people with hearing loss can use hearing aids to amplify sounds and help them hear. For some people with severe or profound sensori-neural deafness hearing aids do not help. These people may make use of the cochlear implant.

The cochlear implant replaces the function of the entire ear. It uses a microphone and directly stimulates any remaining hearing nerves using electricity to enable the brain to perceive sound.

How the cochlea works
The cochlea is arranged so that different sound frequencies stimulate different hair cells and nerve fibres. Stimulating hair cells at the base of the cochlea produces perceptions of high pitched sounds. Stimulating hair cells at the apex of the cochlea produces low pitched sounds. The cochlear implant is designed with electrodes at different positions so as to stimulate the appropriate hair cells and improve the perception of sound.

 

The Cochlear Implant System
The Cochlear Implant System is made up of two separate parts:

  • the receiver-stimulator and electrodes are implanted surgically
  • the microphone, speech processor and transmitting coil are worn outside the body and can be removed at any time.

The Cochlear Implant System receives information in the form of sound and processes this information into a format suitable for interpretation by the brain. It uses several different processes and methods of transmission as shown in the diagram. The entire process takes only milliseconds so the person hears sounds virtually as they occur. The processor can filter out background noise.

The current Cochlear Implant System is the Nucleus® 24. Click here for specifications.

The Cochlear implant system  - click to view larger PDF document
Click here to view detailed pdf document

1. The hearing process using a cochlear implant in detail.
Sounds and speech are detected by the microphone.
2. The information from the microphone is sent to the speech processor.
3. The speech processor analyses the information and converts it into an electrical code.
4. The coded signal travels via a cable to the transmitting coil in the headset.
5. Radio waves from the transmitter coil carry the coded signal through the skin to the implant inside.
6. The implant package decodes the signal. The signal contains information that determines how much electrical current will be sent to the different electrodes.
7. The appropriate amount of electrical current passes down the appropriate lead wires to the selected electrodes. The position of the stimulating electrodes within the cochlea will determine the frequency or pitch of the sounds. The amount of electrical current will determine the loudness of the sounds.
8. Once the nerve endings in the cochlea are stimulated, the message is sent up to the brain along the auditory nerve. The brain can then try to interpret the stimulation as a meaningful sound.
(Cochlear Pty Ltd)

Activity
Watch an animation of how a cochlear implant works from the US documentary: Sound and Fury. http://www.pbs.org/wnet/soundandfury/index.html then draw a flow diagram to illustrate the hearing process using a cochlear implant
.

External parts of the Cochlear Implant System
The external parts are:

  • microphone
  • speech processor
  • transmitting coil.
ESPrit behind the ear
ESPrit behind the ear (BTE) speech processor and transmitting coil
(Cochlear Pty Ltd)
 



SPrint

 

SPrint digital body worn speech processor, microphone and transmitting coil
(Cochlear Pty Ltd)

The microphone is worn behind the ear and replaces the function of the outer ear by picking up sound. The speech processor can be either a small box like a cassette player that can be worn on the waist, or included in the microphone and worn behind the ear like a hearing aid. It acts like a computer and converts sound into electronic codes. The transmitting coil is about 30mm (3cm) in diameter and is worn on the head slightly behind the ear. It is held in place using a magnet (the other magnet is on the plate electrode under the skin). The transmitting coil sends the electronic codes through the skin to the receiver-stimulator by FM radio waves.

For the mathematics of hearing and more about radio waves visit this site: http://www.science.org.au/nova/029/029box02.htm

Internal parts of the Cochlear Implant System
The internal parts consist of a receiver-stimulator and electrodes. The receiver-stimulator is another small computer that converts the codes received by radio waves into electrical signals that are sent along the electrode array. The electrode array is implanted surgically into the cochlea of the person. It receives electronic signals and stimulates the auditory nerve so that the brain perceives sound.

 

The electrode array has 22 tiny electrodes arranged in a row inside a tapered piece of flexible tubing. Each electrode is separately connected to the receiver-stimulator and is programmed separately. The electrodes deliver different signals that represent loudness and pitch of sound by stimulating the appropriate nerve fibres that send messages to the brain.

cochlear implant
The internal component of the cochlear implant (Cochlear Pty Ltd)

cochlear implant
Enlargement of part of the electrode array (Cochlear Pty Ltd)

Nucleus® 24 Contour
Cochlear Ltd have produced the Nucleus® 24 Contour which has an electrode that is curved in the shape of the cochlea before being implanted. It is straightened during surgery and then as it is inserted into the cochlea it returns to its original curved shape. This improves the sound quality and simplifies surgery.

Activity
Watch the animation to see the placement of the electrode array in the cochlea. It's like a blade of grass being placed into a shell - just like Graeme Clark imagined.
click to view animation
view animation - windows media player required. (Cochlear Pty Ltd)

Surgery and getting an implant
The process of getting a cochlear implant requires careful consideration. There is a process to determine the suitability of each individual for having an implant. The surgery only takes a few hours and is generally fairly simple, however there are dangers to any surgery.

For further information on cochlear implant surgery:
Pre-operative selection procedures
Post-operative rehabilitation: What happens after the operation?

Equipment
Equipment needed for cochlear implant operation
(Cochlear Pty Ltd)

 

 

The costs
For those people involved in a research project there may be no cost for the procedure - it depends on the project. Some implants are free to the disadvantaged, elderly and children from State government grants. Most people, however, can recoup most of the costs through Private Health Funds if they are in one. The costs of the procedure, the implant and the training afterwards varies, and although production techniques are improving all the time each implant is handmade so the cost for a cochlear implant system is thousands of dollars.

For more information about getting an implant see:
Cochlear Implant Clinic
Cochlear Ltd

For more information on how cochlear implants work visit:
Cochlear Ltd
East Melbourne Hearing Research Group
NOVA: Science in the News


 

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