Cochlear implants
The majority of children with a hearing loss have some residual hearing (Bezold & Siebenmann, 1908; Goldstein, 1939; Urbanschitsch, 1982). When this residual hearing is appropriately amplified or replace by the implant signal, 90% of these children can hear all the sounds in the speech spectrum.
The devices that are used to aid a child with a hearing loss include the hearing aid and cochlear implant. The cochlear implant is most often used in New Zealand for children with a profound hearing loss and, under special circumstances, children with a severe hearing loss. Before a child receives a cochlear implant they will undergo an assessment to ensure that this is the appropriate technology for them. The majority of the children who attend The Hearing House have a cochlear implant.
To understand how a cochlear implant works it is first necessary to understand how hearing ears hear and process sound. In hearing ears, sound waves cause changes in air pressure in the canal, which make the eardrum vibrate. These vibrations pass into the inner ear, or cochlear, which is filled with fluid.
The cochlear is like a snail in shape. If you imagine it rolled out, it resembles a piano keyboard in that it has low notes at one end and high notes at the other. All along the cochlear there are thousands of very sensitive cells called hair cells.
When a sound is made the fluid in the cochlear moves and stimulates these hair cells to send a small electrical charge to the auditory (hearing) nerve. This small electrical current passes along the nerve to the brain where it is understood as sound.
If the cochlear has damaged hair cells it will not be able to effectively change sound vibrations into an electrical current. A cochlear implant makes up for the lack of intact hair cells by placing an electrode array (with several electrode contacts) within the cochlear. This delivers sound and speech information directly to the auditory nerve by way of small electrical currents.
Unlike hearing aids, which make sound louder, a cochlear implant bypasses the non-functional parts of the ear and delivers small electrical signals directly to the auditory nerve.
The cochlear implant consists of two parts: the internal part and the external part. The internal part is implanted surgically under the skin behind the ear. The external part is worn on the outside of the body. The external part includes a microphone, speech processor and transmitter coil. The speech processor is worn on the body for small children because it is more robust and gives more information to alert the parent when it is not working. A smaller ear-worn processor can be used when the child is older.
1. Sound waves are picked up by the microphone (M)
2. The sound is sent to the speech processor (P)
3. The speech processor codes the sound
4. The code is sent to the transmitter coil (C), which is held in place on the head by a magnet
5. The code is then sent through the skin to the internal part (E)
6. The code is then changed into electrical signals and is sent to the electrical array
7. The electrode array stimulates the auditory nerve
8. The nerve impulses are sent to the brain
