What do music, the sound of the breeze, the chirping of birds, the waves of the sea, the caresses of a loved one or the warm words of a friend have in common? Sound and our ability to perceive it .
Just as vision depends on a process that involves several organs (eyes and brain), hearing also depends on an organ (in addition to our brain): the ear.
It seems obvious, yes, but what is not so obvious is how the ear works. This is because it depends on several parts, and all of them are very (very) small.
Lets start by the beginning…
You see, the human ear is fascinating. Really, just think that we can distinguish between 7,000 different tones and identify the source of sounds (from knowing where that crying baby is or where that menacing mosquito is coming from).
As I was saying, hearing depends on 2 organs: the brain and the ear. The ear’s job is to convert sound energy into neural signals; the brain’s is to receive and process the information contained in these signals.
Any sound creates vibrations that travel as pressure waves through the air (that’s why sound doesn’t travel in space), but it can also be transmitted through solids or liquids.
Those sound waves that may correspond to the next lottery winning numbers, or a message from that person who loves you is, a priori, incomprehensible. This is because at the moment sound is just waves traveling through the air, entering through the tube in the ear (external auditory canal) to a membrane (tympanic membrane).
The first piece of a chain reaction
This is really nice, because the function of this membrane is to vibrate on contact with sound waves. Why? Good question! Because just behind that membrane, attached to it, there is a chain of 3 small ossicles. The function of these small bones is to transmit these sound vibrations to the innermost part of our ear, to a snail-shaped structure (the cochlea) , which is filled with a liquid that is very similar to seawater in composition. Interesting, right? In other words, sound travels in the form of vibrations, usually through the air, and then, thanks to the design of our ears, through a solid (those little bones) until it reaches a liquid medium. Thus, the next problem that these still incomprehensible sound waves encounter is how to reach our brain so that it can translate them into something we understand.
The connection between the ear and the brain
Well, the answer is that a surface called the basilar membrane runs the length of the cochlea. This is lined with hair cells that have specialized components (stereocilia), which move with the vibrations of the cochlear fluid and the basilar membrane.
This movement triggers a signal that travels through the hair cell, into the auditory nerve, then to the brain, which interprets it as a specific sound . When a sound vibrates the basilar membrane, not all hair cells move, only selected ones, depending on the frequency of the sound.
Which comes down to good engineering. At one extreme, the basilar membrane is rigid, vibrating only in response to high-frequency, short-wavelength sounds.
The most common type of deafness
Unfortunately, not everyone has normal hearing. Hearing loss is the third most common chronic disease in the world . On the other hand, 80% of people with hearing loss suffer from something called sensorineural hearing loss. That is, losses caused by dysfunctions of the cochlea, that snail-shaped structure that we talked about earlier.
You will see, as I was saying, in the cochlea there are fine hair cells, responsible for the transmission of sound. Unfortunately, these cells can be damaged , either by exposure to loud sounds, wear and tear, disease, or some drugs.
As a result, if we have fewer hair cells stimulating the auditory nerve, that nerve that must carry information to the brain, the impulse may be insufficient or distorted, making the person unable to hear and/or understand clearly.
But then, is it possible to “cure” deafness?
If you have been attentive, you will have noticed that in these cases, although the hair cells are damaged, the auditory nerve is intact , its only problem is that it is not receiving the signal well, and this is fantastic! You know why?
Well, because there is something, a device, which we call a cochlear implant . This is a “miracle” of technology, a device we can use to restore hearing . And now that you know how hearing works, and how we often lose it, understanding how to restore it is a piece of cake.
I tell you. Cochlear implants have two parts. One part is implanted in the inner ear and the other goes on the outside, but is connected to the inner part.
This device captures sound and converts it into electrical impulses that directly stimulate the auditory nerve. It is like a “by-pass”, a “bridge” of the damaged inner ear.
With this idea, we can refine much more. For example, early cochlear implants consisted of the external device (primarily a microphone and sound processor, which converted sound into a voltage wave) and a single electrode attached to the cochlea. Therefore, most of the early experiments with cochlear implants focused on single channels or single electrodes to excite the auditory nerve. However, this gave poor results in speech perception, the patients could hear the sounds, but they could not interpret them, they did not understand them.
It was a few years later that we made great strides, putting in not one but seven electrodes, thus creating the first multi-channel device . With each electrode placed in a different place within the cochlea, the device could operate at multiple frequencies to help produce audible sounds. As you will understand at this point, by stimulating different parts of the cochlea, we produce different tones.
And while that mysterious message at the beginning probably doesn’t contain winning lottery numbers, there are few things quite like the smile of someone once again feeling connected to the world and its sounds.
References:
The Cochlear Implant Web (sf). What is a cochlear implant? Implantecoclear.org. http://implantecoclear.org/index.php?option=com_content&view=article&id=76&Itemid=82&showall=1
Marsh, A. (2022, January 27). The Long Road to Today’s Cochlear Implant. IEEE Spectrum. https://spectrum.ieee.org/cochlear-implant-history
Mayo Clinic (2020, August 11). Cochlear implants . Mayoclinic.org. https://www.mayoclinic.org/es-es/tests-procedures/cochlear-implants/about/pac-20385021
MedlinePlus Medical Encyclopedia (2018). Cochlear implant. Medlineplus.gov. https://medlineplus.gov/spanish/ency/article/007203.htm
National Institute on Deafness and Other Communication Disorders (2021, March 30). Implantes cocleares. NIDCD. https://www.nidcd.nih.gov/es/espanol/implantes-cocleares
Ramos-Macías, Á., Borkoski-Barreiro, S., Falcón-González, JC, & de Miguel, Á. R. (2016, November). Cochlear implants. Current and future state. Revista Médica Clínica Las Condes, 27 (6), 798–807. DOI: 10.1016 / j.rmclc.2016.11.011
