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 cochlea.

"Tonotopic organisation" is the way the ear sorts out the different frequencies so that our brain can process that information. High frequency sounds (which are high pitched sounds) do not pass very far along the cochlear, but low frequency sounds pass further in. In a normal ear, the sound causes vibration of hair cells which are located all along the cochlear. The movement of the hair cells creates an electrical disturbance that can be picked up by the surrounding nerve cells.

If hearing loss is caused by a problem in the hair cells or any other problem related to the changing of sound energy to nerve pulses, then the problem may be able to be rectified by a cochlear implant.

The implant is capable of creating an electrical pulse. It does this at a certain point along the cochlea. This electrical pulse is interpreted by the brain as a sound at a certain frequency.

Table of contents

1 Who can use a Cochlear Implant? 2 The Device 2.1 Processing 2.2 Transmitter and Receiver 2.3 Electrode array 3 Objections to use

Who can use a Cochlear Implant?

When a cochlear implant is used, the conduction of sound via the normal processes can not be used again. Sound will no longer be able to 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 cannot use a cochlear implant. Patients must have a sensoneural hearing loss in order to benefit from the implant.

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.

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:

• Receiever
• 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.

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.