Published by Australian Academy of Science
KEY TEXT Cochlear implants – wiring for sound This topic is sponsored by the Bionic Ear Institute and the Cooperative Research Centre for Cochlear Implant and Hearing Aid Innovation.

'Can you hear me?'

These are often the first words spoken to recipients of a cochlear implant. They may be simple, but they are music to the ears of the deaf.

By 1996, more than 27,000 people in some 60 countries had received a cochlear implant (also known as a 'bionic ear'). All were profoundly or severely deaf before the implant; all owe their new hearing to technology that is being upgraded continually by Australian scientists.

Hearing loss: a significant problem

About 1 million Australians have some degree of hearing loss; that's nearly 6 per cent of the population. Of these million, about 36,000 can hear virtually no sounds from the outside world (this is called profound deafness). Three in every 1000 children are born with a hearing loss or develop a loss before learning to speak. These children have great difficulty in learning to speak intelligibly. The World Health Organization estimates that 120 million people worldwide have a disabling hearing impairment. It's a significant problem.

In your ear

Ear structure

The ear can be divided into three parts. The external ear consists of the pinna (the fleshy bit that sticks out like a satellite dish from the side of your head) and the external auditory canal (the ear hole, or ear canal). Sound is collected by the pinna and channelled along the ear canal towards a membrane at the end of it. This membrane is called the ear drum; it forms the start of the middle ear, and vibrates when struck by sound waves. These vibrations are passed to three small bones (the smallest bones in the body) called ossicles, which have a 'lever' action and amplify the vibrations as they pass them on to the inner ear.

A word in your shell-like

The role of the inner ear is to translate the vibrations into electrical impulses that the brain can receive and interpret. Central to this role is the cochlea, a seashell-like structure in the inner ear (kochlias is Greek for 'snail'). About the size of a pea, the cochlea consists of rigid bony walls and is filled with fluid. The cochlea is divided along its length by two membranes, with the cochlear duct between them. The organ of Corti, which contains auditory hair cells, is inside this duct.

The following steps describe how the inner ear translates vibrations into electrical impulses:

• Vibrations from the ossicles are passed through the 'oval window' (the entrance to the inner ear) and produce pressure waves in the fluid in the cochlea.

• The pressure waves stimulate the sensory hairs (technically known as stereocilia) attached to the auditory hair cells in the organ of Corti. Stereocilia can be thought of as keys on a piano, each one playing a slightly different 'note'.

• When one of the stereocilia is 'played', a chemical transaction takes place: potassium ions (K⁺) and calcium ions (Ca²⁺) move into the attached auditory hair cell.

• The movement of ions generates an electrical current.

• This electrical current activates the release of a chemical called a neurotransmitter across the gap (known as a synapse) between the hair cell and the adjacent auditory nerve cell.

• The auditory nerve cell responds to the neurotransmitter released by the hair cell and sets up an electrical impulse which is transmitted along its nerve fibre to the brain, and we perceive sound.

When it all goes wrong

Given the extraordinary delicacy of our hearing apparatus, it's not surprising that it sometimes goes wrong and when this occurs the person suffers a hearing loss. There are two basic types of deafness: conductive, which affects the outer and middle ear, and sensori-neural, which is caused by a malfunctioning of the inner ear or the auditory nerve.

The development of the cochlear implant

Corrective surgery or hearing aids can improve some forms of deafness. But only two or three decades ago severe-to-profound sensori-neural deafness was incurable, and many scientists considered that this would probably always be the case.

Australian scientist Professor Graeme Clark and his colleagues at the University of Melbourne began research into cochlear implants in the late 1960s. By 1978, their prototype multi-channel implant was ready for trial: Rod Saunders, an Australian who became profoundly deaf after a head injury, was the world's first recipient. He regained partial hearing; the sound barrier had been broken.

The cochlear implant replaces the function of the entire ear, directly providing any functioning auditory nerve fibres with electrical stimuli that enable the perception of sound (Box 1: How the implant works and Box 2: The mathematics of hearing).

The presence of auditory nerve fibres is essential to the functioning of the device: if these are damaged to such an extent that they cannot receive 'messages', the implant will not work. Current research is investigating ways to bypass the cochlea altogether and send electrical messages directly to the brainstem (Box 3: The bionic ear industry). Early results show promise, although the quality of hearing is less than that obtained from cochlear implants.

A sound future

The testimonies of implant recipients provide moving evidence of the role that the cochlear implant can play in improving the quality of life for the deaf (Box 4: Breaking the silence). As research in Australia and overseas continues to improve the performance of cochlear implants, the challenge for health systems around the world will be to make the implants available to all those who need them.

Boxes

1. How the implant works

2. The mathematics of hearing

3. The bionic ear industry

4. Breaking the silence

CREDITS

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