Learning while teaching, hearing while chewing
Jeune pêcheur à la coquille by Jean-Baptiste Carpeaux, Musée du Louvre, Paris (photo by me)
Yesterday, we filmed the hearing and communication unit for my Coursera course on Understanding the Brain. The short version of this post is that as I was explaining the tensor tympani reflex, I realized that my explanation did not make sense. This is a beautiful thing about teaching – the teacher gets to learn!!! So I regrouped, figured out the correct story, we re-shot the segment and today I understand something that I did not understand at the start of yesterday. Teaching leads to learning which is a major reason that I like it so much. Being part of the teaching-learning axis makes me so happy that I want to break into verse, “O frabjous day! Callooh! Callay!” (from Lewis Carroll’s Jabberwocky)
For those of you interested, here is the long version of this story, filled with digressions (the most egregious ones are marked by indented text) and arcane details along with philosophical thoughts:
The hearing and communication unit starts in the external ear, moves through the middle ear, and spends a great deal of time in the inner ear before ending up in the neocortex, the inevitable destination for all perceptual pathways. I was happily talking about the middle ear reflexes. There are two such reflexes. The middle ear reflex involving the stapedius muscle is fairly straightforward:
The stapedius muscle contracts reflexively in response to loud sounds. The effect of stapedius muscle contraction is to pull back on the stapes which is the middle ear bone (or ossicle: middle ear bones are such light and tiny bones that they are called by the Latin diminutive form for bone) that ultimately contacts the cochlea. The stapes is the drum stick that beats on the cochlear drum (the oval window). You could say; while we may not march to the beat of the stapes, that is what we “hear.” If the stapes is pulled back (away from the oval window and toward the tympanic membrane), it strikes the oval window more softly and the ensuing perception of the loud sound is muted. The stapedius reflex is akin to someone taking the arm of a drummer and pulling it away from the drum head. Because of the harm that loud noises cause to hearing, the stapedius reflex serves a protective role.
The middle ear reflex that involves the tensor tympani is not actually a reflex: it is not activated in response to a stimulus. Instead, tensor tympani activation accompanies chewing and certain other orofacial movements. When chewing starts, the same signal that is sent to chewing muscles also goes to the tensor tympani. Conveniently, the motoneurons that innervate the tensor tympani muscle sit in the same nucleus (the motor trigeminal nucleus) as do the motoneurons that innervate chewing muscles (or muscles of mastication as we say in MD-speak).
So what is the point of tensor tympani activation? Well, the “action” of the tensor tympani muscle is to pull back on the malleus which in turn attaches to the tympanic membrane, aka the ear drum. Thus the net effect of tensor tympani activation is to tighten the ear drum. What happens if you tighten a drum head? The frequency of the sound emitted from that drum goes up. Conversely, striking a loose drum will produce a relatively low frequency sound. So, this is where I was when I realized that I did not understand the effect of tensor tympani contraction on hearing. I was explaining that contraction of the tensor tympani would increase the frequency of the chewing sounds when I realized that that made no sense.
Luckily, in MOOC-land, when I make a mistake, I can correct it. I quickly figured out what my error was and we immediately re-shot the segment. Here is the way that it works. Chewing produces sounds that are conducted through bone directly to the cochlea. No banging on the oval window required. The resonant frequency of the skull is low (about 1 kHz, which is roughly two octaves above middle C) which means that chewing sounds will occupy low registers (two octaves above middle C may seem high but our hearing range goes up to 18-20 kHz at birth and up to about 12 kHz in adults). Contracting the tensor tympani has no effect on the bone conduction of chewing sounds!!!! Bone conduction bypasses the external and middle ears altogether and goes right to the cochlea in the inner ear.
This is the key to the Rinne hearing test. In the Rinne test, a vibrating tuning fork is either held just outside of the ear or placed in contact with the mastoid bone. In the former case, the sound from the tuning fork is borne through air to the tympanic membrane. Sound that passes through the ear canal increases in intensity. Then, even though some of the increase in sound magnitude is lost in the middle ear, an air-borne sound arrives at the oval window of the cochlea with a magnitude or intensity that is very close to that with which it started out. In contrast, the magnitude of the tuning fork vibration will dissipate as the sound travels through bone. Note that tuning forks vibrate at about 250 Hz, far from the resonant frequency of bone and thus will not increase in magnitude but rather decrease.
Because of the amplification of air-borne sounds and the lack thereof of bone-conducted sounds, an individual with normal hearing will perceive an air-borne sound as louder than the same sound when it is bone-conducted. In contrast, a person with a conductive hearing loss (due to a problem in either the external or middle ears) will hear the bone-conducted sound as louder. Let’s consider one clear example. If a person’s ear canal is plugged up with wax, then the air-borne sound will go nowhere whereas the bone-conducted sound will be conducted normally.
Let’s return to the tensor tympani. While contracting the tensor tympani has no effect on bone-conducted sounds, it does affect the frequency of incoming air-borne sounds. Sounds will arrive at the cochlea at a slightly higher frequency than that at which they arrived at the external ear. In essence, we have changed the acoustic stimulus by chewing. This allows more separation between incoming sounds (that we want to pay attention to) and our own chewing sounds (that we want to ignore).
I imagine that some of you are thinking, “Well, as long as I don’t eat while listening to music, there is no problem. I’ll hear the sounds accurately, just as they are played.” However, as it turns out, the tensor tympani is also activated, to one extent or another, during coughing, laughing, swallowing and even breathing. So now what? How do we hear the true sound? The answer is that we don’t because we can’t. The brain does not “do” absolute truths. We are not cameras or tape recorders. I say this a lot but it is worth repeating as it is such a deep and important, and even philosophical, truth. Our entire construct of reality depends on perception which changes with every minute and every shift in the external world or internal condition. That our perceptual world does not contain any absolute benchmarks or reference points is profound.
Teach, learn, do, live, and repeat!!!