Cochlear implant
Cochlear implants can help people with certain kinds hearing impairment or who are deaf to hear. 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.
If hearing loss is caused by a problem in the hair cells, which are frequently damaged over time by excessively loud noises, or any other problem related to the changing of sound energy to nerve pulses in the ear, then hearing may be improved by a cochlear implant. The implant analyses the impinging sound and creates an electrical pulse at a certain point along the cochlea, allowing the brain to interpret the frequency of sound as it would if the hair cells of the basilar membrane were functioning properly (see above).
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2 The Device 3 Objections to use |
Who can use a Cochlear Implant?
Once a cochlear implant is implanted, the conduction of sound via the normal processes cannot be used again. Sound will no longer be conducted to the inner ear, but will have to be picked up by the microphone used with the implant. For this reason, people with mild or conductive hearing loss should not use a cochlear implant. Implants are generally only indicated in patients with sensoneural hearing loss.
The brain is adaptive. If someone has not had hearing for a long period of time the brain will adapt and use the area of their brain normally used for hearing for other functions. If a person has lost their hearing for a long time and then regains it using an implant, the sounds can be very disorientating as the brain is reading these signals not as sounds but as other things.
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.
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:
The internal part of the device contains:
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.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. Electrode array
The electrode array is made from a type of sillicone rubber, while the electrodes are platinum or a similarly highly conductive material. 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 to be "fixed" by cochlear implants, but rather just a different way of living.
When used for children who are born deaf (as opposed to those who lose their hearing later in life), cochlear implants work best in those who are implanted at a pre-speaking age, and hence are implanted before the recipient can decide for themselves.