Sounds from the Bionic Ear Within!

By David Kemp, Ph.D., FRS

There's lots of excitement about digital hearing aids, cochlear implants and other amazing advances in technology that are bringing great benefits to people who are deaf or hard of hearing (D/HH). But they are all just copies of nature's original creation - our biological hearing aid, called the cochlear amplifier, that is a key part of every healthy inner ear. Without it, about half of our hearing would disappear.

Inner Workings

Normally, when sound enters the ear, waves are created along the delicate spiral strip of tissue that holds the sensory cells. Deep, low-frequency sounds travel right to the end of this spiral before they reach their full size, whereas high-pitched sounds reach their peak near the beginning of the spiral strip. We might naturally assume that the sensory cells, directly stimulated by these traveling waves, send a corresponding signal to the brain - but it's not that simple.

First, the waves tend to die out and second, the sensory cells which are connected to the brain are not sensitive enough to respond to quiet sounds at all. And yet healthy ears can detect ear drum vibrations less than the size of an atom because of the cochlear amplifier. This is a mystery. We don't know why nature didn't develop super-sensitive hearing cells to detect this vibration directly, but we do know why the cochlear waves tend to die away if not supported by this amplifier. When sound hits the ear, it vibrates the waters - that is, the cochlear fluid. The friction in the tiny spaces of the inner ear acts in a thousandth of a second to deaden and absorb sound vibration. And then it stops moving around, just like the water in a bath tub eventually stops moving if it sits undisturbed long enough. These still waters de­scribe what happens in an ear with sensory deafness.

In the healthy ear, 9000 special cells (outer hair cells) in three very neat rows all along the length of the cochlear spiral create movement of their own in response to the stimulation of the wave of cochlear fluid. And it's not a lazy shrug - it's a very fast kick exactly in step with the vibrations carried by the wave. Because of the orderly array of outer hair cells and their speed of action, their individual response energies add up and, in turn, vibrate the waters of the cochlear fluid that first vibrated them. The vibration in the fluid lasts longer and builds up to the point that it can activate the true hearing cells. Some estimates say that this mechanism of kicking outer hair cells can amplify weak sound vibrations by as much as 1000 times! That's the bionic power of the cochlear amplifier.

Bionic, Yes. Invincible, No.

OAEs are widely used in newborn hearing screening programs. It takes only a minute to detect the sounds that signal all is well in the ear
Photo courtesy of David Kemp

Hearing loss due to disease, constant exposure to loud noise, toxic drugs and chemicals or simply due to aging is nearly always caused by the loss of these outer hair cells. It seems that their energetic role in amplifying weak sounds makes them especially vulnerable to strong sounds and to changes in metabolism. We can see this loss of outer hair cells under the microscope after dissection - but that's not possible in the clinic! Fortunately, nature's bionic hearing aid has a few features we can take advantage of to determine how well those outer hair cells are working while the ear is still intact.

Not surprisingly, our amplifier is not perfect. A small amount of vibration that the outer hair cells create leaks from the inner ear back into the middle ear. Upon reaching the eardrum, the escaping energy vibrates the air in the ear canal creating a sound of its own - sound actually made by the ear. The sounds are called "otoacoustic emissions," or OAEs for short. Whenever a healthy ear is stimulated with external sound, a faint echo of that sound emerges from the ear after about 1/100th of a second. It's only a faint echo because of its journey back out of the ear. Have you ever noticed that in complete silence you can still hear something? Most of that "quiet noise" is regenerated by your own cells. How about the rushing or ringing in your ears that remains for a while after you've been exposed to loud noise? It's a temporary tinnitus that is, to some extent, related to OAEs. Detecting OAEs is relatively simple, although the technology to measure them was not achieved until 1977. An ear piece or probe is placed in the ear canal and sounds such as clicks are played repeatedly. A small microphone embedded in the probe records any sound in the ear canal. Computer processing is used to differentiate sounds made by the ear, sound presented to the ear and background noise. Logically, an ear that puts out strong OAEs at each frequency must have a strong cochlear amplifier and probably good hearing too. In a very high proportion of cases, this simple procedure is a powerful and reliable test of ear function. And because it does not require any subjective reporting on the part of the listener, it's a great technology for testing infants and nonverbal people.

Energy leaks out of the ear in another way too, specifically in response to distorted sound; we can use that emission to confirm healthy functioning of the cochlea. As music techies will attest, distortion is something to be absolutely avoided in high-fidelity amplifiers and even the minutest distortion in equipment detracts from listening pleasure.

In excess, it destroys intelligibility. Oddly, the ear's amplifier creates a lot of distortion, yet is somehow able to cope with internal distortion much better than externally generated distortion. That's because of the clever way the ear works - dividing sound stimulation into frequency bands before processing it. Distortion occurs only within narrow frequency bands, which is not so unpleasant. Furthermore, the design of the ear's frequency distribution system prevents the worst distortions from traveling to places where they might be heard. Musicians are often aware of our internal distortion, something they call combination tones. A small amount of the distortion the ear produces leaks out to produce sounds in the ear canal which we call distortion product otoacoustic emissions, or DPOAEs. These too can be easily recorded with an OAE probe when two close tones are applied to the ear. DPOAEs are used to measure the functioning of the cochlea.

A Case for Calculated Risk-Taking

The OAE signal from a healthy newborn ear. L: The waveform of the OAE. R: The frequencies that have been strongly amplified. The discovery of these ear sounds helped spur universal newborn hearing screening.
Photo courtesy of David Kemp

The discovery of otoacoustic emissions took the hearing researcher community by surprise. In fact, they didn't believe it at first. This is a prime example of why research funding should go to not only popular topics that follow "safe," well-trodden pathways, but also to visionary projects. It isn't easy to spot a visionary research project but there is a clear difference between oddball ideas that are poorly substantiated and of no value and those ideas that are potentially revolutionary. Projects that present well-documented, reproducible, new findings or clear logic that challenges accepted scientific theories need to be considered carefully and funded.

The first proposal that the ears contained an advanced amplifier mechanism was made in 1948 by Thomas Gold - an idea that came from pure logical deduction. His ideas were dismissed by hearing researchers of the time and sadly, he left the hearing field to become an internationally revered cosmologist. When I discovered otoacoustic emissions in 1977, my research was fortunately subsequently funded by the British Medical Research Council but for many years the practical applications were not recognized by audiologists or the manufacturers of hearing test instruments. For a decade, hearing researchers remained very skeptical and now we look to this once-suspect concept to help us identify newborns with hearing loss.

In 1989, otoacoustic emission measurements were adopted out of necessity for a federally funded pioneer trial of universal newborn hearing screening at Women and Infants Hospital in Providence, R.I. The study proved that OAEs could be relied on to detect inner ear problems in well babies and that the technique was so efficient and cost-effective that universal newborn hearing screening became a practical reality. In 1993, the National Institutes of Health recommended that every newborn be tested with OAEs or the alternative, the auditory brainstem response technique. Today both techniques provide very effective screening methods and almost every baby has the opportunity of a hearing test. It takes only a minute to fit the small OAE probe and wait until the instrument automatically identifies the tell-tale signals that come from healthy outer hair cells indicating the inner ear is in good condition.

OAEs are not only of use in the nursery. They are widely used in the clinic to assess inner ear status and to provide diagnostic information. The auditory system is complex and hearing loss can be the result of many different pathologies. To test the mobility of the middle ear, we use tympanometry. Middle ear disorder is the most common cause of hearing loss, especially in school-age children. Inner ear disorder (sensory loss) accounts for the vast majority of other hearing losses and is detected through a lack of OAEs. To test the auditory pathway from the inner ear to the brain, we record the auditory brainstem response. Hearing losses of neural origin are quite rare and were once identified mainly in patients with tumors impinging on the auditory nerve. However, when OAE testing became widely used in the clinic in the 1990s, it was discovered that some children with hearing loss have normally functioning inner ears but poor auditory nerve signals. Called "auditory neuropathy," people with this interesting condition have limited use for hearing aids which just magnify the sound going into the ear. They still have their biological hearing aid, so they don't need more sound! What they lack is a good signal transmission to the brain.

It is in the research lab that OAEs are now beginning to have their greatest impact and where exciting new applications are being developed. Prior to the discovery of OAEs, no one knew the function of the three lines of outer hair cells that run all along the cochlear spiral. It was known that these cells die and that their loss brings hearing loss but the textbooks erroneously taught that the hair cells themselves signaled the weakest sounds to the brain despite any evidence to support the claim. It was one of those reasonable assumptions that obstructs progress.

A map of the distortion products by an individual ear. With other OAE measures, maps like this could play a part in assessing an individual ear's robustness - its measure of susceptibility to damage.
Photo courtesy of David Kemp

When it was discovered in the mid-1970s that nerves from the outer hair cells to the brain were extremely feeble, there was no explanation for their role in sensitive hearing. When OAEs were discovered a few years later, researchers were fascinated but nearly everyone assumed them to be a curious but unimportant byproduct of the ear and of no consequence to mainline theories. In fact, OAEs were the clue to what we had misunderstood. In retrospect, we can see that denial of the importance of facts that challenge established theories is symptomatic of overly cautious science and results in missed opportunities. Scientists have a responsibility to protect against corruption from oddball ideas that can so easily gain popular support without any substantiation; however, that same good line of defense makes it easy to overlook revolutionary new ideas. In the late 1970s and early 1980s, spurred on by the discovery of otoacoustic emissions, a handful of anatomists and physiologists began to look for self-powered motion in the ear's sensory cells. This led to significant research advances like how outer hair cells convert an electrical analog of the sound stimulation into physical vibration, a process known as electromotility. Its mechanism and speed of action seem to be unique in nature. Then in 2002, the protein named Prestin was identified as being responsible for this motility. Though the whole picture is still not clear, we now know that electromotility is the source of the ear's own hearing aid which amplifies weak sounds so they can be detected by the inner ear hair cells and converted to brain signals.

In turn, the discovery of Prestin has intensified the long-established effort to understand why outer hair cells are so vulnerable. Through brilliant research in scores of laboratories around the world, including here at the UCL Ear Institute, we are slowly beginning to understand the biochemistry of it all. It's already clear that there is a big genetic component in an individual ear's susceptibility to damage although we have a long way to go to understand and correct for this. But it does encourage the belief that soon there will be pharmaceuticals to protect our outer hair cells from further damage by noise, drugs or aging and even regenerate new hair cells. And for these future innovations, we'll come back to OAEs to provide a careful analysis of the sounds which the ear produces - by far the most precise way we have of examining the intact cochlea, specifically the health of our outer hair cells - to safely and objectively monitor the action of new, ear-specific pharmaceuticals of the future.

OAEs may also be helpful in further exploration of ear robustness, a measure of the resistance of the ear to damage. Some people are more susceptible to hearing loss than others given the same level of noise exposure. There is evidence that certain light complexions, hair colors and certain racial types are more susceptible to noise damage. We also know there is a significant genetic component in ear robustness. However, we've not yet developed a way to measure robustness, which is where OAEs come in. Preliminary experiments indicate that "exercising the ears" with mild noise may "strengthen" them, making them more robust. OAEs may be able to measure the effects of very small acoustic challenges. My lab is developing new OAE techniques for use in accurately profiling ears and identifying phenotypes with differing susceptibilities and discovering their genetic origins. More research needs to be focused on learning to measure ear robustness, over and above hearing sensitivity. This would be an important step toward enhancing preventative hearing care.

The message of OAEs for hearing research is clear. Hearing science is an exciting and continually surprising area of research which involves an extraordinary range of disciplines and technologies from engineering to genetics. We did not and still do not understand all we need to know about the ear but good research in this field has been and will be rewarded by progress in our ability to preserve hearing and to detect and treat hearing impairment.

David Kemp, Ph.D., is a Fellow of the Royal Society of London, professor of auditory biophysics at the UCL Ear Institute in London and the physicist well known for his discovery of otoacoustic emissions and the pioneer of otoacoustic hearing test technology. He is the founder of Otodynamics Ltd. Learn more about his research by visiting www.ucl.ac.uk/ear.

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