Cochlear implants – wiring for sound

Key text

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.

Box 1. How the implant works

Reproduced with the kind permission of the Department of Otolaryngology, University of Melbourne

The diagram shows the parts of a cochlear implant in place in a user's ear. Parts a, b, c and d are external parts; parts e and f are internal.

1. microphone (worn behind the user's ear);
2. thin cord (connects microphone to speech processor);
3. speech processor (codes sounds electronically);
4. transmitting coil (sends code as radio waves);
5. receiver/stimulator (converts code into electrical signals);
6. electrode array implanted in the cochlea (stimulates auditory nerve fibres when electrical signal is received);
7. cochlea;
8. auditory nerve.

External parts: Microphone, speech processor, transmitting coil

A microphone worn just behind the patient's ear performs the function of the outer ear. It picks up the sounds of the outside world and transmits them via a thin cord to a speech processor. This looks a bit like a small transistor radio but works like a computer. It is worn externally on a belt or in a pocket or shoulder pouch (although a new product containing the microphone and speech processor is small enough to be worn behind the ear, like a hearing aid). Much of the wizardry of the cochlear implant is contained in the speech processor: it selects the sounds most useful for understanding speech and codes them electronically (Box 2).

These electronic codes are sent back through the cord to the transmitting coil, which is a plastic-covered ring about 33 millimetres in diameter. This is located a little further back behind the user's ear and is held in place by two magnets, one located under the skin and the other in the centre of the transmitting coil.

Internal parts: Receiver/stimulator, electrodes

The coil sends the electronic codes through the skin via radio waves to the receiver/stimulator. This consists of a custom-designed integrated circuit (a small computer, in effect). The receiver/stimulator converts the codes it receives into electrical signals that it sends along the electrode array, implanted in the cochlea of the user.

The electrode array consists of 22 tiny electrode bands arranged in a row inside a piece of tapered flexible silicon tubing. Each electrode has a wire connecting it to the receiver/stimulator; each has been separately programmed to deliver electrical signals representing sounds that can vary in loudness and pitch. When the electrodes receive an electrical signal, they stimulate the appropriate populations of auditory nerve fibres, which send the messages to the brain.

Some early cochlear implants provided stimulation of the auditory nerve fibres using only one electrode: these were developed in various research centres in Europe and North America. One study of 49 children implanted with single-electrode cochlear implants showed that most could discriminate syllable patterns, but only two achieved any significant understanding of conversational speech. The great advantage of the multi-channel cochlear implant is that speech can be filtered into frequency bands by the speech processor and delivered to different points along the cochlea.

The cochlea is organised so that different sound frequencies preferentially stimulate different hair cells. (As the membrane along the bottom of the cochlea resonates in time with the sound vibration, hair cells at different positions along the membrane are stimulated.) Stimulating hairs located at the base of the cochlea produces perceptions of high-pitched sounds; stimulating hairs located at the opposite end (apex) of the cochlea produces perceptions of lower-pitched sounds. The multi-channel cochlear implant has a number of electrodes at different positions on the cochlea and is designed to deliver stimuli to appropriate electrodes so that high-pitched sounds cause electrodes to stimulate hair cells towards the base of the cochlea and low-pitched sounds cause electrodes to stimulate hair cells towards the apex of the cochlea.

Is normal service resumed?

A person with a cochlear implant will hear sounds as they happen, which is why the technology can be so helpful. But, sophisticated though it is, the cochlear implant does not fully reproduce the sounds experienced by someone with full hearing.

The effectiveness of the cochlear implant varies considerably. Factors that determine the benefit recipients will gain include:

• whether they developed spoken language before going deaf (people who have learned to speak usually benefit most);
• the time since deafness first occurred;
• their level of motivation;
• the environment in which they live: an encouraging home, school or work environment will help implant patients achieve their full potential; and
• the number of surviving auditory nerve fibres in the implanted ear.

The age of recipients varies considerably – from children as young as 14 months to 80-year-olds. Studies of recipients of the 22 electrode implant have shown that adults with acquired hearing losses can understand 80 per cent of speech (on average) when using their implant (without lip reading). At this level of understanding, such things as interactive telephone conversations become possible.

It's harder for people who have been deaf since birth. They have to learn the associations between sounds and words from scratch; understandably, this can be difficult after a life of silence. It thus becomes very important that the implants are performed on deaf children as early on in their lives as possible, giving them the opportunity to develop spoken language as they grow up, just like children with full hearing.

Related site

• Cochlear implants (National Institute on Deafness and Other Communication Disorders, USA)

Box 2. The mathematics of hearing

Frequency of sound

Sound is caused by pressure vibrations, usually in air. The rate at which the vibrations repeat is called the frequency and it determines the pitch we hear ­ high frequencies (fast vibrations) cause high notes, low frequencies produce low notes. The variation in pressure, when plotted against time or against distance, forms a wave pattern. The simplest sort of wave, produced when there is only a single frequency present in the sound, is called a sine wave.

A single frequency wave pattern represents a pure sound like that produced by a tuning fork. Two or more sounds occurring at the same time make a new wave pattern that is simply the individual wave patterns added together. For example, these three wave patterns of different frequencies add up to give the fourth, much more complex, wave pattern. Most of the sounds we hear have complex wave patterns that are created in this way.

What Fourier discovered is that the reverse is also possible – any complex wave pattern can be analysed into a number of simple, component wave patterns. This is called Fourier analysis and is a vital tool in mathematics, physics and engineering. It is used to study the vibrations of machinery, the structure of human speech, the sounds made by dolphins and the images of distant galaxies collected by radio telescopes.

Fourier analysis in the cochlea

Fourier analysis closely resembles the process by which we hear and understand sounds. A complex sound is spread out in frequency along the basilar membrane of the cochlea, and different nerves react to different frequencies. Our ears are natural Fourier analysers and sometimes (especially with training) you can hear the different components that make up a complex sound.

The speech processor used in the bionic ear is essentially a Fourier analyser, converting sounds into 22 different components. Signals are then sent to the electrodes distributed along the basilar membrane in the cochlea to stimulate the appropriate nerves (Box 1).

Box 3. The bionic ear industry

Growth of the industry is backed by research and development conducted in Australia and overseas. In Australia, the Cooperative Research Centre for Cochlear Implant, Speech and Hearing Research, the Bionic Ear Institute, the University of Melbourne Department of Otolaryngology and the Human Communication Research Centre are all involved in cochlear implant research. Ongoing research aims to:

• improve the cochlear implant by increasing understanding of how electrical stimulation by the cochlear implant is perceived by the users, and how best to present speech information to them;
• minimise the impact of noise on the clarity of the speech signal provided by the cochlear implant;
• maximise the benefit that young children gain from the cochlear implant;
• improve the understanding of how the auditory nerve fibres and brain respond to the electrical stimulation of the cochlear implant, including the testing of newer modes of electrical stimulation;
• develop new speech processing strategies through computer simulations of the response of the auditory system to acoustic and electrical stimulation;
• improve the design of the cochlear implant electrodes;
• improve pre- and post-operative clinical management;
• improve surgical procedures;
• develop a technique for direct electrical stimulation of the brainstem for deaf people who are unable to use the cochlear implant - particularly those with few residual auditory nerve fibres;
• make the cochlear implant suitable for people who still have some hearing;
• combine cochlear implant and hearing aid strategies, so that people – particularly those with some hearing – can continue to benefit from a hearing-aid once they have a cochlear implant;
• investigate the use of two microphones – one behind each ear - to improve the perception of speech in noisy environments;
• develop a cochlear implant or hearing aid that resides entirely under the skin: the first stage of this research is the development of a microphone system that can be completely implanted; and
• develop ways of initiating auditory nerve regeneration to enhance the effectiveness of existing cochlear implant systems.

Even at its current level of sophistication, the cochlear implant has improved the lives of thousands of people. As science and technology continue to push the frontiers of hearing research, it seems inevitable that the capacity of the deaf to hear will only increase.

Related sites

• Cooperative Research Centre for Cochlear Implant, Speech and Hearing Research

• The Bionic Ear Institute

• Cochlear Limited

Box 4. Breaking the silence

I was 24 years old when I suddenly lost my hearing. I was basically deaf for the next nine years. During those years I knew a part of me was gone and I thought that part was gone for good. I had a good job, a family, and was satisfied with my life. But, there was that part of me that wanted more, and that more was to hear my daughter, talk to my family, enjoy my surroundings again. I am sure I withdrew somewhat because you all know what silence does to your daily activity. Silence is a restriction that keeps you from being a whole person. I hated that silence but had come to accept the fact I was not going to hear again. Then it happened.

I had been going to a university hospital and they told me about the implant process. I accepted some material from them and read it, but I did nothing about it. Why? Because I was afraid. Why was I afraid? Because you are always afraid of the unknown. It's the boogie man under the bed, or the eyes that follow you in the dark. I was afraid and had accepted my silent world. I did not want things to change.

The doctors were telling me the implant was a good option for me. But I waited, I did not want them to give me something that would be half hearing. I wanted to be whole again and this was not for me. It took months of pushing, for my family to talk me into having this surgery. When I agreed the doctors told me of another device they planned to insert in my head. I was basically prepared for this, then two weeks before the surgery they changed their minds. They told me of a doctor in Australia and his research. They told me about a new implant called a 22-channel cochlear implant. When asked if I could talk to someone that had this device, they told me there was no one in this country that had this implant. I was terrified and the doubt instantly returned. I asked why I would be the first. They said I was the best local candidate for this implant and I finally consented.

A doctor from Australia and a doctor from New Zealand, plus their support team were at the hospital when I went in for this surgery. They were very reassuring and I went into that room full of hope, full of fear and full of the understanding that if it was not to be, so be it. Three weeks later I was hooked up to the processor for the first time. The team from Australia was there to help and to show the university staff how to do the programming. Dr McCabe, the doctor that I had spent so many years with and had never heard his voice before, was to be the first to speak to me.

He asked, 'Can you hear me?'

I said, 'Yes!!'

My mother was there in the room and started to cry. The doctor asked her to speak to me. Choking back tears, she said, 'I can't.'

I looked at her and said, 'I have waited nine years to hear you again and you can't talk!'

The whole room was crying but me, I wanted it all and I wanted to be sure this was not going to be snatched away, as some cruel joke.

Then the audiologist from Australia said, 'So mate, tell me everything you know about Australia.'

I said, 'You do funny things with kangaroos don't you?'

The doctors erupted with laughter, it was working.

Those nine years were so terribly hard for me and I tried so hard to make my life whole but it was not until that day I felt like me again. I got back something that I had lost. It was not perfect but it was more than half a loaf. I reflect on those years and I look at the last ten years and I am amazed at my life. My daughter and I can talk whenever we want, I can enjoy my family, I can be a part of my work team and take part in most all activities. I still have restrictions but I am a resolute person and know that as time goes by it will only get better.

I am here tonight because of Dr Graeme Clark, as are many of you. I want to take this moment to say to Dr Clark, I am so terribly in debt to you sir, for giving me back so much of my life. I applaud you for your perseverance and years of hard work. To us in this room tonight it is a miracle and you can be satisfied with having achieved so much. To the staff that helped make your work a reality, we don't know their names, but they are just as important.

Related site

• Personal stories (Cochlear Limited)

Activities

• The Standards Site (Department for Education and Skills, UK)
  • Sound and hearing – activities are listed under headings such as 'How does sound travel through solids, liquids and gases?' and 'Can sound be dangerous?'

• Newton's Apple (USA)
  • Hearing – students identify the type and location of a sound while blindfolded.

• Science and Mathematics Initiative for Learning Enhancement (Illinois Institute of Technology, USA)
  • How sound travels – students investigate whether sound travels more effectively through a solid, liquid or gas.

Activity 1. Vibration and sound

Materials (for each person)

1 plastic ruler

Procedure

1. Hold the ruler firmly on the edge of a desk or table so that about 25 centimetres extends over the edge.

2. Flick the free end of the ruler with your fingers and listen for any sound it makes.

3. Look closely at the way the ruler is moving.

4. Move the ruler back so that only about 15 centimetres extends over the edge and repeat the procedure.

5. Compare your observations.

Teachers notes

Students should observe the following:

Flicking the ruler resulted in an up and down motion (vibration).

The second distance (15 centimetres) produced a faster vibration and should have generated a sound that was easier to hear.

Activity 2. The ear and hearing

1. As with many organs in the body, the structure of the ear is related to its function.
   • Draw up a two-column table. List the parts of the ears in the left-hand column and in the right-hand column list the functions of each part.

2. Sound is produced by a vibrating object that generates pressure waves.
   • Outline how a stimulus of sound is sent to the brain.

3. What structure(s) of the ear would you expect to be most vulnerable to damage by the 'noise pollution' that we are exposed to in today's environment.

4. Explain why a temporary hearing loss is produced when the external ear is blocked by ear wax.

5. Explain why a loud explosion or a blow on the side of the head that ruptures the eardrum causes a loss of hearing until the ear drum has healed.

6. Explain why having two ears enables us to more easily locate where a sound comes from.

7. Sounds must reach a certain intensity (volume) before they can be detected by the human ear.
   • Use the ticking of a watch or clock as the sound stimulus and the distance of the watch from the ear as the measure of intensity. Compare the hearing distance when the watch is moved away from the subject with the hearing distance when the watch is moved toward the subject. (Blindfold the subject and carry out the test in a quiet room.)

Further reading

Useful sites

• What is a cochlear implant?
  A description, with photographs, of one type of cochlear implant.
  http://www.bionicear.org/mhg/cicaboutcochlearimplants.html

• History of the cochlear implant
  Describes the history of electrical methods to stimulate hearing.
  http://www.bionicear.org/bei/AboutHistory.html

How hearing works (How stuff works, USA)

Provides an introduction to the hearing system.
http://health.howstuffworks.com/hearing.htm

Cochlear implant (The Science Show, 8 May 2004, Australian Broadcasting Corporation)

Researchers describe how a cochlear implant works and how we know what a deaf person hears when they receive an implant.
http://www.abc.net.au/rn/scienceshow/stories/2004/1100196.htm

Sound from silence: The development of cochlear implants (Beyond Discovery, National Academy of Sciences, USA)

Describes the centuries of scientific research that provided the foundations for the development of cochlear implants. (A PDF file of the complete article is available.)
http://www.beyonddiscovery.org/content/view.article.asp?a=252

Genetic silencer (Access Excellence)

Describes a genetic defect that appears to underlie the most common cause of deafness.
http://www.accessexcellence.org/WN/SUA02/genetic_silencer.html

Glossary

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 p f t