The Limits of Hearing and Balance in Humans
“One person’s roar is another’s whine, just as one person’s music is another’s unendurable noise.” —Henry Rollins, musician
Anyone who has ever woken up after a rough night of drinking alcoholic beverages has likely experienced the room spinning viciously. In a flurry of chatroom posts over a fourteen-hour period in May 2006, several tech nerds discussed the phenomenon and proposed “cures” for it. Here is a sample of the banter: “The only advice I can give you is to wedge yourself into a corner of your room and hold onto the walls for dear life. Then phone your folks and tell them that the world, does indeed, revolve around you.” Although overindulging in alcohol is no joke, it does serve the purpose of explaining how balance works in humans. In Chapter 5, I described the structure of the inner ear. Some of that structure exists for hearing, but the semicircular canal structures in the inner ear are there for balance.
These structures form a kind of X-Y-Z three-dimensional coordinate system known as the vestibular system. Each of the three semicircular canals has an area called an ampulla, where the semicircle of each canal meets. The canals themselves are filled with a fluid called endolymph. Inside each of the three ampullae is a gelatinous cellular structure called the cupula, which has small cilia, or hairs, emanating from its surface. These cilia are enervated and connected to the brain. If you rotate your head, the fluid moves with the rotation as a result of the inertia induced by head movement. Each cupula will lag behind like a floater on a fishing rod and will move in the opposite direction, which induces bending of the cilia in the ampulla. The bending hairs trigger an action potential in the canals, which is transmitted to the brain, where the information is interpreted to help us keep our balance. But balance isn’t just about where our heads are in space.
There is motion all around us, and we are continually moving. In addition, how we perceive where we are in space involves a lot of random motion, called Brownian motion. If all of this random Brownian motion were detected by the vestibular system, it would cause a good deal of chaos in how we balance ourselves, because it would be transmitted to the brain as false information about our position in space and more or less overload our brains. Mees Muller and colleagues developed a model to examine how our semicircular canals overcome Brownian motion effects, and it involves several structural factors of the hair cells on the cupula. Specifically, these hair cells are ten times longer than the hairs in the auditory system, ten times less compliant to bending (the cochlear hairs of the auditory system bend ten times more easily), and one hundred times harder to displace than the cochlear hairs. They postulate that the strange X-Y-Z format of the vestibular structure is a good mechanical way to overcome the effects of Brownian motion on the sense of balance (fig. 7.1).
All of this information from the semicircular canals is transmitted to the brain and integrated with information from two other sources: the eyes and the muscles and joints. The integration of these three sensory inputs is transmitted to the brain stem, where it is integrated and interpreted by further contact with the cerebellum and cerebral cortex. These two areas of the brain are important, because they help the rest of the body respond to the initial movement that originated the need to balance oneself. The cerebellum is important because it coordinates complex movement of the body, and the cerebral cortex is important because it provides information from memory and learned experience to “right the ship.”
Figure skaters are among the best balancers in human populations. They put themselves through incredible spinning routines that wreak havoc on their vestibular systems. For instance, as they start their spin, the cupula will, because of the inertia of the endolymph, move rapidly in the opposite direction of the spin. The cupula’s cilia, or hairs, will be bent, indicating to the brain that the head is moving in the direction of the spin. But as the spins increase and the hairs get more and more bent because of the spin, the body of the skater, with the visual system and muscular movements, begins to respond. Even with this extreme perturbation of the cupula, whether it be by the classic scratch spin, the Biellmann spin, an “I” spin, a camel spin, or a pearl spin, skaters are amazing at pulling spins off without falling flat on their faces. Figure skater Natalia Kanounnikova set the world record for spins per minute at 308 rotations per minute in an “I” spin and skated straight away evenly as if nothing had happened.
Figure 7.1. Structure of the inner ear. The balance organs are on the left, and the hearing apparatus is on the right.
How do skaters like Kanounnikova maintain their balance and deal with dizziness? Some of it is innate ability, because figure skaters are incredible athletes. But a lot involves using tricks to overcome the vestibular system’s quirkiness. One of the tricks in spinning is the placement of the feet and hence the placement of the central axis along which the skater rotates. If a skater keeps this axis very tight and constantly upright, and maintains the center of gravity along the spin axis, he or she will have enough support to remain upright. Of course, the technique takes years of practice to perfect. Another trick is simply to execute a superfast spin at the end of the skating program. Here the skater can recover balance quickly if the spin ends in a very stable (but dramatic) pose. Another trick is to come out of a spin and go into a wide arc during which the skater can recover whatever balance that has been lost. Although the arc is often considered a poetic and artistic move, it really is a matter of survival that the skater uses to stay balanced.
With respect to someone who feels dizzy after a night of heavy drinking, if too much ethanol is ingested into a person’s body and isn’t detoxified by the liver, it will remain in the bloodstream, where it will travel to all sorts of places in the body. One place where it ends up in sufficient amounts to affect physiology is the inner ear and especially the semicircular canals there. The cupulae in the semicircular canal aren’t used to being bathed in ethanol, and so when they are, they respond by distorting their shape and forcing the cilia to remain in constant contact with the sides of the ampullae. The nerve cells connected to the cupulae then send impulses to the brain indicating something is wrong with balance, and the brain responds by trying to compensate for the faulty information. Specifically, the brain starts the visual system spinning, and hence when someone drinks a little too much, the room might appear to be spinning. But it is also possible to get lucky and fall asleep without further discomfort. During sleep the ethanol will evaporate, and the cupulae will return to their original shape. But on waking the person might find the room still spinning. This spinning is the result of the brain remembering the bad behavior from the night before, thinking that it is spinning and setting the visual system spinning again to compensate for the memory.
Balance is one of those attributes where we more often detect defects in the sense, instead of superhumans who are very good at it. Our understanding of human variation in balance is based on anecdotes and the study of defects that certain humans have with respect to balance. Perhaps the most famous anecdotal instance of superbalance is the mythical case of Native American skywalkers (box 7.1).
And many recognized medical anomalies are rooted in the vestibular system, according to the National Institute of Deafness and Other Communication Disorders, part of the National Institutes of Health. Vertigo, for example, is a very disarming condition. It can be caused by several anatomical and physiological dysfunctions and goes by several names. Benign paroxysmal positional vertigo (BPPV) and Meniere’s disease are two of the more common disorders that result in vertigo. Many of the problems with the vestibular system are shared with the auditory system because of the nearness of the two apparatuses. In BPPV, structures in the auditory system called otoliths come loose from the saccule of the inner ear and enter the semicircular canals. These small, stonelike particles are important for hearing, but in the semicircular canals, they push on the cupulae. If the cupulae don’t compensate, then cilia on them will be bent. This state will cause the neural cells of the inner ear to send false information to the brain about the position of the head. And of course, the brain will attempt to compensate for the false information by tweaking the visual system. The cause of Meniere’s disease is not fully understood, but it is known to involve changes in the volume of the endolymph fluid in the canals.
There are many anecdotes about the super balance characteristics of different ethnic groups. Mohawks from the Iroquios tribe have long been thought to be expert balancers, and this assumption has helped them find work in high places such as skyscraper construction sites and earned them the name skywalkers. In fact, in the New York City area, nearly 10 percent of the ironworkers on skyscrapers are individuals of Iroquois ancestry. But the myth is actually backward. The Mohawks of Kahnawake who were considered the original skywalkers lived near a railroad bridge construction site in the late 1880s. The ironworking company doing the construction hired several Mohawk men to work on the bridge, and they were quite successful because apparently they showed no fear of heights. It turns out, that is exactly what was going on. Although they showed no fear, they later admitted that they were very scared of the heights but simply did not show it as part of their cultural response to danger. Because to a Mohawk male, the career of a skywalker was and still is very warriorlike, the tradition of Mohawk and other Iroquois working in iron on skyscrapers was established and exists to this day. The skywalkers are more than likely no better at balancing than other groups of people are.
Labyrinthitis and vestibular neuronitis can be caused by viral infections. Labyrinthitis causes parts of the inner ear to swell and causes loss of balance because the inflammation changes the relation of the cupulae to the ampullae. Vestibular neuronitis is an inflammation of the nerve leading from the vestibular apparatus called the vestibular nerve. Apparently, the inflammation of the nerve interferes with the positional information coming from the cupulae on their way to the brain. Like BPPV, the disorder known as perilymph fistula can be caused by head injury. The head injury in perilymph fistula causes the fluid of the middle ear to leak into the semicircular canals, which disrupts the positioning of the cupulae and results in lack of balance and dizziness. Those readers who have been on long cruises will be familiar with mal de débarquement syndrome. This syndrome usually manifests itself after being on a boat for long periods. Apparently, the vestibular system compensates for the up-and-down motion of a ship sailing on the ocean and continues to compensate after reaching land.
Balance has also been characterized across ethnic groups and age groups through the National Health and Nutrition Examination Survey (NHANES). These surveys, which began in the 1960s, have been conducted on U.S. populations regularly since 1999 and provide trends for such health issues as anemia, cardiovascular disease, diabetes, environmental exposures, eye disease, hearing loss, infectious disease, kidney disease, nutrition, and obesity. From 2001 to 2004, an NHANES survey was conducted on balance dysfunction to understand the loss of balance in aging populations and among varying ethnic groups. Balance is an important area and relevant to human health because the loss of balance can lead to falls that are often very injurious, if not fatal.
The NHANES balance survey uses a standardized balance test called the Romberg Test of Standing Balance in Firm and Compliant Support Surfaces. Although the Romberg Test has been shown to have shortcomings, it can describe the ability to balance quite well. The test is based on a simple principle. A subject is asked to stand erect, then to close his or her eyes. If the subject falls, then he or she scores positive on the test. When Friedrich Romberg developed this test in 1846, a positive score would have indicated a lesion of the nervous system. Any test that has the potential result of falling and becoming hurt is not good for the health of the subject, so the test was changed a bit. The original Romberg Test morphed into the very same test that police administer when they suspect someone of being inebriated.
The NHANES balance survey found that no single ethnic group balanced better than others. Also, no differences were found between the sexes, between smokers and nonsmokers, or between people with hypertension and those without. Oddly, individuals with a high school education scored 40 percent better on the Romberg Test than individuals without the diploma. In addition, people with diabetes mellitus scored 70 percent worse than individuals without the disease and hence were diagnosed with vestibular dysfunction. People with high school diplomas probably read more on the average, and diabetes can cause problems with vision. This information suggests that the last two correlates (a high school diploma and diabetes) might be explained by some vision phenomenon, but establishing causation is another story.
The survey’s most important finding is that vestibular dysfunction increases with age, and not just linearly. People between the ages of fifty and fifty-nine were twice as likely to fail the balance test, whereas people over age eighty were twenty-five times more likely to fail. Not surprisingly, the survey also shows a correlation with failing the Romberg Test and falling. So it would seem that we are pretty good at balancing and that some of us can get better by practice. Sadly, though, we lose our acumen for balance as we age, probably as a result of wear and tear on our vestibular system throughout our lifetime.
Speaking of the impact of aging on the senses, hearing is one of the most talked about senses that worsens with age, and it also harbors tremendous variation in human populations. Hearing starts with the ear and the collection of sound waves by the outer ear that are then fed into the middle and inner ears. The middle and inner ears have evolved to detect specific characteristics of sound waves and transmit these to the brain, where they are interpreted and perceived as different sounds.
All waves have two basic properties: height or amplitude and frequency. Observe waves in a bathtub or the ocean; the more force you put behind the wave in the bathtub or the more force the Moon creates to make waves in the ocean, the higher the tide and the bigger the wave, so the greater the height or amplitude. Then note how frequently the waves occur. This characteristic is related to the frequency with which the wave has been generated. The closer the peaks of waves are to each other, the higher the frequency. If the waves are coming in slowly, then they are coming in at a lower frequency. The same principles apply to sound, except instead of water the waves consist of displaced air.
To understand how our ears perceive sound, we need to consider three characteristics of sound: intensity, pitch, and tone. Intensity is related to amplitude, or wave height, and pitch is related to the frequency of the wave. Tone is more complex. Most sources of sound do not make pure sounds, meaning that the sound source will produce waves with multiple frequencies. Tone is related to the purity of the source. For instance, the tone of a band with three instruments is purer when the three instruments have similar pitch or frequency. A band producing sounds with several pitches will sound very different from the band that has a more uniform tone. Notice that I have avoided judging whether pure tone (our three instruments using the same pitch) or a mixed tone (three different pitches) is more pleasing than the other. How the brain interprets the tones tells the individual what is more pleasing.
Although humans can hear over a range of three hertz orders of magnitude (20 Hz–20,000 Hz; see box 1.3), we hear best in the range between 1,000 and 4,000 hertz (fig. 7.2). Sounds with this frequency are relatively high pitched to begin with. Some singing and very high pitched speaking are in this range, but other sounds we process are off the charts.
When Mariah Carey whistle sings at the end of “Emotions,” she hits a G7 note at 3,135.96 hertz. She can also hit a low note at 82.41 hertz. To give you some perspective, that annoying test pattern sound you hear every month if you are unlucky enough to be awake when radio stations run the Emergency Alert System test is exactly 1,000 hertz. At the lower end of our range, at 20 hertz, we hear vibrating sounds like the vibration of wind blasting through an open car window when the car is speeding. At the upper end, 20,000 hertz, are bizarre rare sounds like high-pitched whistles. Mariah Carey, though impressive, doesn’t come close to matching the highest-pitched sound produced by a human and certainly doesn’t approach the range a human can traverse with respect to pitch.
Figure 7.2. Range of sound in hertz, including human audible frequencies.
BOX 7.2 | PITCH AND MUSICAL SCALES
Although pitch is measured in hertz, it can also be quantified by the notes that musicians use. The seven notes in the diatonic musical scale are do, re, mi, fa, so, la, and ti, starting over again with do. These traditional notes can be correlated with notes musicians use, A through G. Each set of eight notes, do, re, mi, fa, so, la, ti, do (C, D, E, F, G, A, B, C), make up an octave. The highest octave on an eighty-eight-key piano is numbered 8 and the lowest is numbered 0. So, musical scales on a piano can span eight octaves. This makes sense because only one note of octave 8 and three notes of octave 0 are found on the piano. Notes do exist that are off the chart with respect to the eighty-eight-key piano. The frequency of the lowest-used musical note, B8, is only 4.37 hertz, whereas the highest note, C0, is 2,109.89 hertz. Note that this highest note is considerably lower than what Mariah Carey hits when she whistles sings in the song “Emotions.” There is an interesting aspect of notes and their frequency that you may have noticed or already know. If the lowest note (B8) is 4.37 and the highest note (C0) is 2,109.89, then the scale cannot be linear (try dividing 2,109 by 4 and you will not get eight octaves). This is because as musicians make sounds that go from octave 0 to octave 1, they double the frequency or hertz output. So, the note B7 is 8.74 hertz, and B6 is 17.46 hertz, and so on. What this means is that even though musicians use these eight octaves, we can also have a higher octave than 8 and a lower octave than 0 (designated with minus signs).
Although singers in some cultures can traverse the musical scales, such as the Tuvan throat singers, whose vocalizations are at the low part of the scale of human capacity, the extent of our range of notes is impressive. What then, are the highest and lowest notes ever sung by a human and the greatest range a single human can traverse? The lowest note is seven octaves below zero at B−7 and a paltry 0.189 hertz. And although Mariah Carey can hit G7, Georgia Brown, a Brazilian singer, can hit G10, or 25,088 hertz, well outside our hearing range. Brown’s range exceeds Carey’s by four octaves by going from G2 to G10, or from 3,135 to 25,088 hertz. But this isn’t the broadest range a human can belt out. This distinction belongs to Tim Storms, an American who can span ten octaves from G−5 to G5 (0.7973 Hz—807.3 Hz). Storms is the person who hit the lowest note ever recorded (G−5). Most humans do not hear such sounds at these extreme ends of the normal range of hearing.
And much like balance, hearing gets more difficult as we age, because as humans age, part of the mechanism with which we detect sound gets worse for wear. As with balance, tiny hairs called cilia are involved. As we age, some of the hairs get broken, and as a result they are nonfunctional. I can attest to this problem with aging. As I get older I tend to not hear my wife as well as when I first met her (or at least that’s my story, and I am sticking to it). I like to blame this on her high-pitched voice and not on any lack of attention on my part.
How do our ears perceive the amplitude of the wave? The height of a wave can be translated mechanically into its power. The more powerful the sound, the more of it we detect. So, the mechanical sensors in our ears need to detect not only the peaks of the sound waves but the power the waves generate. They do this by measuring how much of the mechanical parts of the inner ear is displaced. This displacement is what is transmitted to our brain and what our brain interprets as intensity.
The intensity of a sound is measured in decibels, which is a measure of the power per unit of area that hits our inner ears. As the distance from our ears increases, the power of the sound decreases by one over the square of the relative distance moved. So, if a sound is being made 1 meter away, it will have a relative effect of one. But if we move away to 4 meters and make the same sound, its relative effect will deliver power one-sixteenth of the first sound made 1 meter away. If the initial sound was 160 decibels at 1 foot away, then at 4 feet away, it will have a decibel level of one-sixteenth of 160 decibels, or 10 decibels. Unlike frequency, intensity has a threshold of hearing set at zero decibels, which is when the waves being generated by the sound have no power. The sound of a whisper hitting our eardrums is in the range of 20 decibels, and the sound waves hitting our ears from the whisper have the power to displace particles in the air 10–4 millimeters, or 0.0001 millimeters. A normal conversation at about 60 decibels produces sound waves with the power to displace particles in the air 10 millimeters.
Sporting events bring out the best and sometimes the worst of people, but they certainly are venues for making noise. The loudest recorded roar at an indoor sport was 126 decibels at the Sacramento Kings home basketball court. Although double the decibels of a conversation, this basketball roar is about a million times more powerful than a conversation. The loudest soccer stadium is probably Türk Telekom Arena, home to the Galatasaray football club in Istanbul, Turkey. Soccer fans there have produced roars at 131 decibels, generating a sound with power right at the pain threshold. Türk Telekom Arena is partially enclosed, and so it is pretty astonishing that the fans for a 131-decibel performance have been beaten by an American football stadium that is completely open. The fans at Arrowhead Stadium in Kansas City, Missouri, generated roars at an ear-damaging 142.2 decibels in 2014. This level of sound is well over the threshold of pain and would be much like standing ten feet away from a jet engine without ear mufflers. Rock concerts don’t get this loud, even if they turn their amps up to eleven (because as legendary guitarist and Spinal Tap band member Nigel Tufnel points out, eleven is “one louder”).
Humans are inundated with sound waves all the time. They are everywhere, and our ears are continually picking them up because our middle and inner ears are built for detecting sound. Our outer ears, those skin and cartilage structures that reside on both sides of our heads, act like funnels for focusing sound into our middle and inner ears, replete with a tubelike channel called the ear canal that focuses the sound waves on the actual biological contraption that collects the sound waves. The structures in our middle ears are every bit as intricate as the balance components of the inner ear. At the end of the ear canal lies a membranous structure called the eardrum. This structure is where the sound waves are collected. As sound waves hit the membranous surface of the eardrum (tympanic membrane) they cause the eardrum to vibrate. Next to the eardrum are the three bones discussed in Chapter 2: the hammer (malleus), anvil (incus), and stirrup (stapes), in that order from outer to inner ear. When the membrane of the eardrum vibrates, it causes movement of the hammer, which is in contact with the back surface of the eardrum. This vibration in turn moves the hammer relative to the anvil, which in its turn moves the stirrup. High-pitched sounds vibrate the eardrum at a higher rate than low-pitched ones. The movement that is transmitted through the hammer, anvil, and stirrup (also known as ossicles) reflects the degree of vibration that affects the eardrum. This whole contraption moves in concert with the vibrations of the eardrum, where the stirrup is connected to the cochlea in the inner ear. The connection of the stirrup to the cochlea is like a piston fitting into a cylinder. Its job is to move the fluid of the cochlea in concert with the information from sound waves that initially entered the ear.
The cochlea is an amazingly convoluted structure. Its three-dimensional structure is like a spiraling snail shell, but even more complex (fig. 7.3). If we could cut across the cochlea to reveal the channels that the spiraling produces, we would see two channels per spiraling arm filled with fluid (the paralymph again). These channels are one on top of the other. In between the two channels lies a duct called the cochlear duct, where the information from the sound waves that initially hit the eardrum is ultimately collected. The duct has two sensitive and movable membranes that lie between the cochlear channels. The duct is also filled with fluid and is connected to an intricate structure called the organ of Corti, which is in turn connected to nerves that run to the brain. The reason there are two channels per spiral is that the paralymph fluid needs to be recirculated. So, if we could run through the cochlea, we would start at the point where the stapes articulates like a piston. We would run through the top canal, and when we reached the very tip of the spiraling cochlea, we would head on our way back out toward where we started in the bottom canal.
The fluid going through the cochlear channels will expand and compress the fluid in the cochlear duct. Such movements of the cochlear duct will impact the intricate organ of Corti. This organ is lined with two kinds of hairs that can bend as the fluid flows in the cochlear canals. One kind of hair is called inner, of which there are about 3,500 cells, and the other is logically called outer, of which there are about 20,000 cells. Bending the inner hairs produces most of the neural response in the neural cell to which the organ of Corti is connected. The bending of the hairs again works much like the mechanosensory bending hairs of the vestibular system and transmits electrical impulses to the brain that are then interpreted by the brain as sound. The outer hairs apparently amplify whatever sound wave signal is coming into the inner ear. The bending inner hairs are kind of like light switches, but they are not uniform in the amount of pressure it takes to flip on the nerve cell. The hair cells near the spiral tip of the cochlea are more susceptible to bending by high-pitched sounds, and the hairs near the base of the cochlea are more susceptible to being bent by low-pitched sounds. Different tones will register on specific hair cells in the organ of Corti. There are a lot of moving parts, but the apparatus works quite well most of the time, as anyone who loves music can attest.
Figure 7.3. The cochlea and its relation to the stapes.
This Rube Goldberg–like apparatus is another wonderful example of how the evolutionary forces at play in forming specialized structures like the one we use for hearing are imperfect. An intelligent engineer would certainly have designed this biological contraption differently.
What would an expert engineer consider good design rules? Amy Smith, an instructor of engineering at the Massachusetts Institute of Technology, has dictated seven simple rules for engineering design. One of Smith’s rules is to be frugal and produce the least expensive but most efficient product possible. This rule suggests that getting rid of extra moving parts is wise. The more moving parts there are, the more energy is used. In addition, the more moving parts there are, the more parts there are that could tend to break down and need replacement. Another of Smith’s rules is to engineer with transparency so that others understand the product with ease. I don’t think anyone who has just read the description of how the middle and inner ears work would say that the design is transparent. Smith’s final relevant rule quotes Leonardo da Vinci: “Simplicity is the ultimate sophistication.” Her rule is “Do the hard work needed to find the simplest solution.”
Evolution simply does not work that way. Evolution works with the raw materials it is given and is somewhat lazy in that respect. It settles on solutions that are the best given the raw materials. In addition, evolution does not allow do-overs or mulligans. In the case of the middle ear, three bones (the hammer, anvil, and stirrup) existed in this area in the common ancestor of mammals and were subsequently affected by the evolutionary process and molded into a single nontransparent, complex, but efficient contraption that in no way approximates the perfect organ that an intelligent designer might engineer.
I have already discussed age as a factor in hearing sounds with high pitches. And what about perfect pitch or the ability to reproduce sounds with perfect frequency or pitch without a frame of reference? Humans vary considerably with respect to this auditory capacity. Using genome survey techniques, researchers have implicated several genes in musical ability, and perfect pitch is one of them. The lack of ability to detect tones, also called tone deafness or amusia, also apparently has a genetic basis. It appears that the variation in these genes is substantial, indicating that, at least tonally, humans constantly hear different things from the same sources (see Chapter 12 for more discussion about the genetics of complex traits involving our senses).
