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LESSON 235

How does a cochlear implant work?

Year 3
Spring semester
Auditory Implants
Cochlear implants
Duration: 11:54

How does a cochlear implant turn sound into an electrical signal and stimulate the auditory nerve? In this lesson we cover the external unit (microphone, sound processor, transmitting coil), the internal unit (receiver-stimulator, electrode array), the signal processing steps, tonotopic stimulation, and how implant hearing differs from natural hearing.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn the parts of a cochlear implant and how it turns sound into electrical stimulation.
Then we will discuss how hearing with an implant differs from natural hearing.
A hearing aid amplifies sound and delivers it acoustically to the ear.
For this to help, some of the cochlear hair cells must still be working.
In severe and profound losses most hair cells are lost, so however much sound is amplified, it may not be clear.
A cochlear implant takes a different route here: it bypasses the damaged hair cells.
Electrodes placed inside the cochlea stimulate the auditory nerve fibers and spiral ganglion cells directly with electrical current.
So the implant does not amplify sound; it converts sound into an electrical code the nerve can understand.
This is why implants were developed for people with severe to profound loss who get limited benefit from hearing aids.
Who is a suitable implant candidate is a separate evaluation topic, which we will cover in the next lesson.
In this example, both ears show a hearing loss between severe and profound.
Thresholds start around 80 dB at low frequencies and fall to 110 dB or worse at 4000 Hz; there is no response at 8000 Hz.
Average speech sounds fall roughly between 20 and 60 dB HL on the audiogram, far above these thresholds.
With such a loss, even a powerful hearing aid may not make speech sounds, especially high-frequency consonants, audible enough.
The sounds that are heard may also be distorted, because the remaining hair cells cannot process sound in detail.
This is where the implant comes in, designed to deliver sound to the nerve by bypassing the damaged region.
The values here are examples; real decisions rest on a much more comprehensive evaluation.
The first experiments in electrically stimulating the auditory nerve date back to the late 1950s.
Multichannel implants similar to today's entered clinical use in the 1980s.
Researchers such as Graeme Clark in Australia and the Hochmairs in Austria played pioneering roles in this development.
Today the cochlear implant is considered the most successful neural prosthesis, partly restoring a sense.
More than one million cochlear implants are estimated to have been fitted worldwide to date.
A cochlear implant consists of an external unit worn outside and an internal unit placed surgically.
The external unit holds the microphones and the sound processor; it is often worn behind the ear.
A thin cable from the processor connects to the transmitting coil, which sits on the skin on the side of the head.
The coil sends the signal and power across the skin to the internal unit by radio frequency; a magnet holds the coil in place.
The internal unit has a receiver-stimulator placed under the skin behind the ear; it has no battery of its own.
The electrode array leads from the receiver-stimulator into the cochlea.
The array is usually advanced into the scala tympani through the round window or a small opening made near it.
Current from the electrodes stimulates nearby auditory nerve fibers, and the signal is carried to the brain.
The external unit picks up and processes sound; all the smart processing happens here.
It is powered by a disposable or rechargeable battery and also passes this power to the internal unit through the coil.
The external unit can be removed at will, usually for sleep or bathing, and can be replaced with newer models over the years.
The internal unit consists of the receiver-stimulator body, a magnet and the electrode array inside the cochlea.
The number of electrodes inside the cochlea usually ranges from 12 to 22, depending on the manufacturer.
There is also a reference electrode outside the cochlea, providing the return path for the current.
The internal unit is permanent and has no power source of its own; it does not work without the external unit.
So when the external unit is removed, the person hears nothing through the implant.
Now let us follow how a sound is processed inside the implant.
Sound picked up by the microphone is split into many frequency channels by a filter bank in the sound processor.
In each channel the amplitude change over time, the envelope, is extracted; most of the fine timing detail is lost here.
The wide dynamic range of the natural ear, about 120 dB, is compressed into a much narrower electrical range.
This compression is done between two individual levels measured for each electrode.
These are the T level, where stimulation is first noticed, and the C level, where it is comfortable; together they form the map.
Each channel's information is then delivered to its electrode as brief electrical pulses, usually in turn, without overlapping.
Delivering pulses in turn reduces interaction between the currents of neighboring electrodes.
The implant makes use of the cochlea's natural frequency map; this is called tonotopy.
The base of the cochlea, the part near the oval window, corresponds to high frequencies.
The apex, in turn, corresponds to low frequencies.
So high-frequency channel information goes to electrodes near the base, and low-frequency information to deeper electrodes.
The electrode array enters near the round window and advances from the base toward the apex.
The cochlea makes about two and a half turns, and the electrode array often does not reach the apex.
This can create a mismatch between the frequency delivered and the region of nerve stimulated.
So at first, sounds may seem high-pitched or mechanical; over time the brain largely adapts to this new code.
In the natural ear, frequency resolution comes from thousands of hair cells and nerve fibers.
In an implant the number of electrodes is limited, and because current spreads, effective channels are often fewer than electrodes.
The wide dynamic range of the natural ear is compressed into a narrow electrical range in the implant.
The temporal fine structure of sound is largely not transmitted by the implant; this information matters for pitch and timbre.
This is why listening to music, and telling melodies and instruments apart, is often hard for implant users.
Understanding speech in noise is also much harder than in quiet.
Even so, many users reach a level where they can understand speech in quiet without lipreading.
Knowing these differences is important for setting realistic expectations with users and their families.
The external unit is usually fitted and switched on for the first time a few weeks after surgery; this is called activation.
The audiologist sets threshold and comfort levels for each electrode; this is called programming, or mapping.
The map is updated often in the first months, because levels change as the person gets used to electrical stimulation.
Users often describe the first sounds as robotic, metallic or high-pitched.
Over time the brain learns to make sense of this new signal; this can take weeks to months.
Regular use and auditory rehabilitation are the foundation of this learning process.
Outcomes vary between people; duration of deafness, age at implantation and auditory nerve status are among the key factors.
Some people have usable residual hearing at low frequencies, and careful surgical technique aims to preserve it.
In that case, in the same ear, high frequencies can be stimulated electrically and low frequencies acoustically.
Using electrical and acoustic stimulation together is called electroacoustic stimulation, or a hybrid fitting.
When a person uses an implant in one ear and a hearing aid in the other, this is called a bimodal fitting.
The pitch and timing information carried by acoustic hearing can fill in some details the implant leaves out.
So assessing and preserving residual hearing is an important part of the implant process.
A tip from the field: for daily listening checks in children with implants, the Ling six sound test is a practical tool.
The test uses six sounds spoken with the lips hidden: m, oo, ah, ee, sh and s.
These sounds cover most of the speech frequency range, from low to high.
If the child does not respond to a sound they detected before, there may be a hardware fault or a need for a map update.
Teaching the family to do this test every morning helps problems to be noticed early.
In this lesson we saw that a cochlear implant bypasses damaged hair cells and stimulates the auditory nerve directly.
The external unit picks up and processes sound; the internal unit receives power and data across the skin and passes them to the electrodes.
The electrodes stimulate according to the cochlea's frequency map: high frequencies at the base, lower toward the apex.
Few channels, a narrow dynamic range and lost fine timing set implant hearing apart from natural hearing.
Test yourself: in a cochlear implant, which electrode receives the high-frequency channel's information?
The correct answer is B: an electrode near the base of the cochlea.
The base of the cochlea corresponds to high frequencies and the apex to low; the implant uses this tonotopic order.
In the next lesson we will examine cochlear implant candidacy in children and adults, and the expanding criteria.
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