As Many Exceptions As Rules: Your Ears Hear, But Can You Hear Your Ears?

The auditory mechanism is very smal

Of your 10 or 11 senses (remember last week? – sight, hearing, taste, smell, touch, hot, cold, pain, kinesthetic awareness, balance, and maybe proprioception), hearing and sight have the most complex mechanisms. This is amazing, as the entire auditory apparatus in your ear is smaller than a peanut M&M. Into this space are packed more than a dozen individual structures needed to convert waves into sound. And even more amazingly, the same structures can also produce sound!

In generation of a normal auditory response, the sound waves contact the tympanic membrane (eardrum), and vibrate the ossicles of the middle ear (the three smallest bones of the human body; the incus, the malleus, and the stapes – great crossword answers by the way). The movements of these bones transfer the vibrations to the fluid filled cochlea. The vibrations create a travelling wave.

Look at the pictures below of the cochlea along its length and in cross section. Three fluid filled chambers work together to change the fluid wave into an electrical impulse. The wave travels from the ossicles up the scala vestibuli (the top chamber) and then back down the scala tympani (the bottom chamber).

The left cartoon shows the cochlea unrolled, while the right drawing is the rolled up cochlea in cross section.

The Organ of Corti transduces waves to nerve impulses.

In the middle chamber is the Organ of Corti, laying on the basilar membrane. Depending on the frequencies contained in the travelling fluid wave, the basilar membrane is vibrated in specific places along the length of the cochlea. Where the basilar membrane vibrates, inner and outer hairs of the Organ of Corti rub along the tectorial membrane, and the rubbing triggers electrical impulses in the nerve cells attached to the hairs. This creates the nerve impulse that the brain interprets as sound of a certain frequency. I told you it was complex – and I’m giving you the simple version.

Now comes the shocker. This same mechanism can be reversed, to send a wave OUT of the inner ear, and both waves are occurring all the time. Why you ask? The outward wave is a side effect of a mechanism that allows us to hear accurately.

Here (or hear) is how it happens. The traveling wave is moving in cochlear fluid, the hair movements are stiff, and the space that the wave is moving in is cramped. All these factors cause a loss of energy in the wave, so much so that the frequencies could be lost and our hearing would be inaccurate and not very sensitive.

The inner hairs of the Organ of Corti generate the electrical impulses, but the outer hairs are attached to muscles and can actively move, as opposed to the inner hairs that are passively moved by the traveling wave.

The active movement of the outer hairs is in the opposite direction of the liquid wave in the cochlea. This keeps the wave swirling, instead of dying (no, I don’t quite understand how this happens either). The energy loss in low energy waves is therefore counteracted, and the energy of strong signals can be amplified. Hence, this mechanism is called the cochlear amplifier.

If the generation of this opposite wave by the amplifier is not uniform across the entire cochlea, which often it is not, then the wave produced will travel back to the ossicles and eardrum, and generate a new traveling wave in the cochlea. This is new wave, generated and then detected by your own cochlea, is called an otoacoustic emission (OAE).

Why would anyone search for sounds coming from the ears? It’s like conducting research to see if pigs really fly. But for OAEs, it seems there was a reason. In the 1940’s, an astronomy graduate student named Thomas Gold deduced that the damping of the traveling wave would be too much to overcome without some sort of compensatory mechanism to amplify the frequency. Although he did not discover the cochlear amplifier mechanism, he did predict that one would be found.

Gold’s ideas were loudly rejected and he soon returned to cosmology research, but not because of the controversy. Dr. Gold had many controversial ideas. His 1968 hypothesis that the newly discovered pulsars were rotating neutron stars was initially rejected, but proved correct. However, he missed the target when he suggested that the dust on the surface of the moon was many feet thick and astronauts landing on the moon would be sink in and disappear. According to Dr. Gold, “Science is no fun if you’re never wrong.”

Dr. David Kemp picked up Dr. Gold’s work in the 70’s, and did publish on the existence of the amplifier, with an acoustic emission as a byproduct in 1978. He showed that the movement of the eardrum in OAE’s is very small. A movement of just 10^-10 meters (about the width of a hydrogen atom) will produce a strong OAE. He told me that some people can hear their own OAE’s, but they are too soft for other people to hear. Most people learn to ignore them, like how you don’t feel your backside against the chair after sitting for a while. This is called sensory adaptation, and we may talk more about it in a few weeks.

It is interesting enough to know that OAE’s exist, but scientists have taken advantage of them to help identify us and to track our hearing. OAE’s are also unique to each individual, so some companies are developing security identification instruments based on OAE recognition. More importantly, OAE’s can only be generated if the cochlea is functional, so you can monitor inner ear function with OAE’s.

Many studies have begun to use OAE's in research on auditory functional and health. One study published in late 2012 showed that damage to the inner ear caused by the cancer drug cisplatin could be detected by monitoring OAE's. This same study provided evidence that using gingko bilobo can reduce the ototoxicity produced by cisplatin, as revealed by OAE production.

Conventional hearing tests like that shown on the left can't be used with infants, so OAE's offer a new testing mechanism..

Remember the old hearing test where you would raise your hand when you could hear the tone? Well, babies and some people with disabilities can’t do this. Scientists can induce OAE’s by broadcasting clicks into the ear, and then listening with a microphone for the OAE. It is a new test for the function of the cochlea.

Furthermore, different frequency OAE’s are generated by the outer hairs at different distances along the Organ of Corti; therefore, a lack of certain frequencies in the OAE response would correspond to a dysfunction in that part of the cochlea.

OAE tests cannot replace hearing tests, as many hearing problems have nothing to do with the cochlea. For example, there can be dysfunctions in the nerves that carry the signals to the brain, downstream of the OAE. Tinnitus, a ringing in the ears that affects some 36 million Americans can be caused by many things, including damage to the hair cells or even antibiotics, but is not related to OAEs.

But this not to say that OAE monitoring is not helpful in tinnitus. A recent study in Poland shows that OAE function is often affected in people with tinnitus. The results suggest that damage to the basal region (low tone region) of the cochlea may result in an ear that can then hear the ringing (tinnitus).

But OAE’s do help monitor the impulse-generating portion of the system and can locate problems in specific aspects of hearing. The next time no one else hears that noise you swear was there, maybe you should just chalk it up to your ears talking to you.

Next time we will take a look at an animal that is built to be the best listener on Earth - even if it has to break a biological rule to do it.

Fabijańska A, Smurzyński J, Hatzopoulos S, Kochanek K, Bartnik G, Raj-Koziak D, Mazzoli M, Skarżyński PH, Jędrzejczak WW, Szkiełkowska A, & Skarżyński H (2012). The relationship between distortion product otoacoustic emissions and extended high-frequency audiometry in tinnitus patients. Part 1: Normally hearing patients with unilateral tinnitus. Medical science monitor : international medical journal of experimental and clinical research, 18 (12) PMID: 23197241

Cakil, B., Basar, F., Atmaca, S., Cengel, S., Tekat, A., & Tanyeri, Y. (2012). The protective effect of Ginkgo biloba extract against experimental cisplatin ototoxicity: animal research using distortion product otoacoustic emissions The Journal of Laryngology & Otology, 126 (11), 1097-1101 DOI: 10.1017/S0022215112002046

For more information, classroom activities, and laboratory activities:

Auditory mechanism –

http://www.pbs.org/wgbh/nova/teachers/activities/22s1_hearing.html

http://www.augie.edu/perry/ear/hearmech.htm

http://www.youtube.com/watch?v=GpkD8AZTFCs

http://neuroscience.uth.tmc.edu/s2/chapter12.html

http://thalamus.wustl.edu/course/audvest.html

www.rle.mit.edu/media/pr145/35.pdf

http://www.nidcd.nih.gov/health/education/

Cochlear amplifier and OAE’s –

www.oae-ilo.co.uk/downloads/advisories/the%20oae%20story.pdf

http://audres.org/cel/cochmech/cochamp.htm