Cochlear implants – wiring for sound
Glossary
brainstem. The brain is like two large mushrooms side by side with a single stalk. The stalk of the 'mushrooms' is called the brainstem, and it controls many vital functions such as blood pressure and breathing. In addition, all sensory information reaching the cerebral cortex of the brain (the 'mushrooms') gets there through the brainstem.
Sound signals pass from the cochlea along the auditory nerve to the brainstem, where they activate other nerve cells that transmit the message higher up the brain. If deafness is caused by damage to the cochlea or the auditory nerve, it may be possible to restore some perception of sound by carefully stimulating the correct region of the brainstem.
deafness. There are two types of deafness: sensori-neural and conductive. In sensori-neural deafness, the defect lies in either the cochlea (the organ that converts vibrations to nerve impulses) or in the transmission of the sound signals to the brain once they have left the cochlea. This form of deafness tends to occur with age, and is accelerated by exposure to loud sounds (eg, at a disco, from a ghetto blaster, from a portable radio used with earphones, from construction projects). Workers on noisy building sites wear ear protectors. So, too, do sporting rifle shooters and army personnel on a rifle range.
Conductive deafness occurs when something prevents the sound vibrations from reaching the inner ear. This could merely be wax in the ear canal, but it could also occur if infection has caused the ear drum to become perforated so that it does not move normally under the influence of sound pressure. Alternatively, the ossicles (the tiny bones connecting the ear drum to the cochlea) might become stiff so that they lose their 'lever' action. With conductive deafness, the hearing organ is basically normal, and the problem lies in getting sound to the cochlea.
electrode. An electrical conductor. Electrochemical reactions occur on the surface of an electrode.
An electrode can be used to deliver electricity to the body or to receive electricity from it. (Delivering electricity to the body is used to stimulate; receiving electricity from the body can be used to detect and record signals.) In either case the term refers to the contact formed by the stimulating or recording device within the body.
With the multi-channel cochlear implant, the electrodes are used to stimulate the cochlea by delivering electricity to it. There are 22 electrodes at different positions along the implant so that it is possible to stimulate at many different sites. When the implant is inserted into the cochlea, the 22 electrodes allow auditory nerve fibres at different sites from the base of the cochlea to its apex to be stimulated selectively, thus enhancing the ability of the patient to distinguish different frequencies of sound.
neurotransmitter. A chemical substance, given off by the terminals of a nerve cell or nerve fibre, which affects the next nerve cell or fibre in the chain, thus allowing a message to be passed between different links in the chain. It is the arrival of the electrical impulse at the end of the nerve fibre that causes the release of a neurotransmitter into the small gap (called the synapse) between nerve cells. The neurotransmitter travels across the synapse and excites or inhibits the next nerve cell in the chain.
radio waves. Low frequency electromagnetic radiation. Radio waves have wavelengths ranging from less than a centimetre to as long as 100 kilometres. The hertz (Hz) is the unit of frequency and means one complete oscillation per second. Many frequencies are much higher than this so other units are used (eg, 1 megahertz (1MHz) = 1,000,000Hz).
We divide the radio wave part of the electromagnetic spectrum into bands that are allocated to different uses. These include AM radio (amplitude modulation), FM radio (frequency modulation) and CB radio (citizens' band), television, aircraft communications, satellites, mobile phones and pagers. Within each band, no two transmissions can use the same part of the spectrum – or frequency – at the same time. For this reason, each band within the radio wave spectrum, itself a part of the broader electromagnetic spectrum, must be managed carefully to ensure the best use of this limited resource.
The frequency of radio waves used in magnetic resonance imaging range from 1-100 megahertz, depending on the strength of the magnetic field in the scanner. This is close to the range of frequencies used for FM radio (88-108 megahertz). For more information see How the radio spectrum works (How Stuff Works, USA).
sine wave. A sine wave is the simplest and smoothest sort of wave. It looks like the sort of wave you can produce by repeatedly moving one end of a long rope up and down while the other end remains fixed. A plot of the position of a long pendulum of a clock as a function of time is a sine wave.
Mathematically we write the position y of the pendulum in the form
y = a sin 2 π f t
where a is the amplitude of the motion, t is the time, and f (measured in cycles per second, or hertz) is the frequency. Here, the angle in the sine function is measured in radians. If we were to express it in degrees, then we would write y = a sin (360° × f t).
Page updated August 2006.