Following up on the earlier post about how a baby reacted wonderfully to the first sounds though his cochlear implant, check out this illustration of a cochlear implant and how it sits inside the head. This illustration shows the Nucleus System.
Note that there are two wires which come from the internal part of the system which is above the ear. One makes its way away from the ear, which is the ground electrode. This electrode doesn't stimulate the cochlear itself but is essential to how the system works.
The second wire, with an array of electrodes at its tip, makes its way through the bone to the middle ear where it enters the cochlear either through a natural opening opportunity, called the round window, or through a hole made by the surgeon.
On the outside of the head, is the speech processor, which sits behind the ear. Above that is the coil or transmitter, which adheres to the site of the implant on the inside of the head by magnetism. This magnet has to be strong enough to hold the coil securely to the head, but not too strong because that can cause soreness.
The processor behind the ear, like a hearing aid, has a microphone inside it, but unlike a hearing aid, doesn't make the sound louder. Instead, it changes the sound into electrical signals which are transmitted across the skin to the implant by using FM radio signals. The implant then stimulates the electrodes placed in the cochlear. That stimulation sends tiny electrical currents down the hearing nerve to the brain where they are understood as sound.
There are between 10 and 24 separate electrodes on the electrode array which is placed inside the cochlear. The number depends on the maker of the implant. The electrodes are spaced out so that they stimulate different parts of the cochlear and this makes a difference to the sound that the brain hears. For example, electrodes positioned at the entrance of the cochlear make a higher pitch sound while those deeper in make a lower pitch sound. So a lorry going past would stimulate the electrodes deeper in, while birdsong would stimulate those towards the entrance.
Or put another way, an 'mmm' sound would stimulate the electode deeper in while the 'sss' sound would stimulate those toward the entrance. Having an appreciation of all the frequencies used in speech is one of the main reasons people have cochlear implant operations.
So far, cochlear implants have been given to over 200,000 people worldwide with around one thousand implanted annually in the UK alone.
For more information and links check out Action on Hearing Loss' website