The Body, page 10
The white of the eye is formally known as the sclera (from a Greek word for “hard”). Our scleras are unique among primates. They allow us to monitor the gazes of others with considerable precision, as well as to communicate silently. You have only to move your eyeballs slightly to get a companion to look at, let’s say, someone at a neighboring table in a restaurant.
Our eyes contain two types of photoreceptors for vision—rods, which help us see in dim conditions but provide no color, and cones, which work when the light is bright and divide the world up into three colors: blue, green, and red. People who are “color-blind” normally lack one of the three types of cones, so they don’t see all the colors, just some of them. People who have no cones at all, and are genuinely color-blind, are called achromatopes. Their main problem isn’t that their world is pallid but that they really struggle to cope with bright light and can be literally blinded by daylight. Because we were once nocturnal, our ancestors gave up some color acuity—that is, sacrificed cones for rods—to gain better night vision. Much later, primates re-evolved the ability to see reds and oranges, the better to identify ripe fruit, but we still have just three kinds of color receptors compared with four for birds, fish, and reptiles. It’s a humbling fact, but virtually all nonmammalian creatures live in a visually richer world than we do.
On the other hand, we make pretty good use of what we have got. The human eye can distinguish somewhere between 2 million and 7.5 million colors, according to various calculations. Even at the lower end of estimates, that is a lot.
Your visual field is surprisingly compact. Look at your thumbnail at arm’s length; that’s about the area you have in full focus at any given instant. But because your eye is constantly darting—taking four snapshots every second—you have the impression of seeing a much broader area. The movements of the eye are called saccades (from a French word meaning “to pull violently”), and you have about a quarter of a million of them every day without ever being aware of it. (Nor do we notice it in others.)
In addition, all the nerve fibers leave the eye via a single channel at the back, resulting in a blind spot about fifteen degrees off center in our field of vision. The optic nerve is fairly hefty—it is about the thickness of a pencil—which is quite a lot of visual space to lose. You can experience this blind spot by means of a simple trick. First, close your left eye and stare straight ahead with the other. Now hold up one finger from your right hand as far from your face as you can. Slowly move the finger through your field of vision while steadfastly staring straight ahead. At some point, rather miraculously, the finger will disappear. Congratulations. You have found your blind spot.
You don’t normally experience the blind spot, because your brain continually fills in the void for you. The process is called perceptual interpolation. The blind spot, it’s worth noting, is much more than just a spot; it’s a substantial portion of your central field of vision. That’s quite remarkable—that a significant part of everything you “see” is actually imagined. Victorian naturalists sometimes cited this as additional proof of God’s beneficence, without evidently pausing to wonder why He had given us a faulty eye to begin with.
HEARING
HEARING IS ANOTHER seriously underrated miracle. Imagine being given three tiny bones, some wisps of muscle and ligament, a delicate membrane, and some nerve cells, and from them trying to fashion a device that can capture with more or less perfect fidelity the complete panoply of auditory experience—intimate whispers, the lushness of symphonies, the soothing patter of rain on leaves, the drip of a tap in another room. When you place a set of $800 headphones over your ears and marvel at the rich, exquisite sound, bear in mind that all that that expensive technology is doing is conveying to you a reasonable approximation of the auditory experience that your ears give you for nothing.
The ear consists of three parts. The outermost of these, the floppy shell on the side of our heads that we call “the ear,” is formally the pinna (from the Latin for “fin” or “feather,” a bit oddly). On the face of it, the pinna would seem ill-designed to do its job. Any engineer, starting from scratch, would design something larger and more rigid—more like a satellite dish, say—and certainly wouldn’t allow hair to cascade over it. In fact, however, the fleshy whorls of our outer ears do a surprisingly good job of capturing passing sounds—and, more than that, of stereoscopically working out where they come from and whether they demand attention. That is why you can not only hear someone across the room speak your name at a cocktail party but turn your head and identify the speaker with uncanny accuracy. Your forebears spent eons as prey to endow you with this benefit.
Although all outer ears function in the same way, each set, it appears, is uniquely built and as distinctive as the owner’s fingerprints. According to the British scientist and author Desmond Morris, two-thirds of Europeans have free-hanging earlobes and one-third have attached lobes. Whether tethered or flapping, the earlobes make no difference to your hearing or indeed anything else.
The passage beyond the pinna, the ear canal, ends in a taut and sturdy piece of tissue known to science as the tympanic membrane and to the rest of us as the eardrum, which marks the boundary between the outer ear and the middle ear. The tiny quiverings of the eardrum are passed on to the three smallest bones in the body, collectively known as ossicles and individually known as the malleus, incus, and stapes (or hammer, anvil, and stirrup, because of their very vague resemblances to those objects). The ossicles are perfect demonstrations of how evolution is so often a matter of make-do. They were jawbones in our ancient ancestors and only gradually migrated to new positions in our inner ear. For much of their history, those three bones had nothing to do with hearing.
The ossicles exist to amplify sounds and pass them on to the inner ear via the cochlea, a snail-shaped structure (cochlea means “snail”) that is filled with twenty-seven hundred delicate hairlike filaments called stereocilia, which wave like ocean grasses as sound waves pass across them. The brain then puts all the signals together and works out what it has just heard. All this is done on a sublimely modest scale—the cochlea is no bigger than a sunflower seed, the three bones of the ossicles would fit on a shirt button—yet it works incredibly well. A pressure wave that moves the eardrum by less than the width of an atom will activate the ossicles and reach the brain as sound. You genuinely cannot improve upon that. As the acoustics scientist Mike Goldsmith has put it, “If we could hear quieter sounds still, we would live in a world of continuous noise, because the omnipresent random motion of air molecules would be audible. Our hearing really could not get any better.” From the quietest detectable sound to the loudest is a range of about a million million times of amplitude.
To help protect us from the damage of really loud noises, we have something called an acoustic reflex, in which a muscle jerks the stapes away from the cochlea, essentially breaking the circuit, whenever a brutally intense sound is perceived, and it maintains that posture for some seconds afterward, which is why we are often deafened after an explosion. Unfortunately, the process is not perfect. Like any reflex, it is quick but not instantaneous, and it takes about a third of a second for the muscle to contract, by which point a lot of damage can be done.
Our ears are built for a quiet world. Evolution did not foresee that one day humans would insert plastic buds in their ears and subject their eardrums to a hundred decibels of melodic roar across a span of millimeters. The stereocilia tend to wear out anyway as we age, and they do not, alas, regenerate. Once you disable a stereocilium, it remains lost to you forever. There isn’t any particular reason for this. Stereocilia grow back perfectly well in birds. They just don’t do it in us. The high-frequency ones are at the front and the low-frequency ones farther in. This means that all sound waves, high and low, pass over the high-frequency cilia, and this heavier traffic means they wear out more quickly.
In order to gauge the power, intensity, and loudness of different sounds, acoustic scientists in the 1920s came up with the concept of the decibel. The term was coined by Colonel Sir Thomas Fortune Purves, chief engineer of the British Post Office (which in those days was in charge of the British telephone system, hence the interest in sound amplification). The decibel is logarithmic, which means that its units of increment are not mathematical in the everyday sense of the term but increase by orders of magnitude. So the sum of two 10-decibel sounds is not 20 decibels but 13 decibels. Volume doubles about every 6 decibels, which means that a 96-decibel noise is not just a bit louder than a 90-decibel noise but twice as loud. The pain threshold for noise is about 120 decibels, and noises above 150 decibels can burst the eardrum. For purposes of comparison, a quiet place like a library or the countryside is about 30 decibels, snoring is 60 to 80 decibels, a really loud nearby thunderclap is 120 decibels, and standing in the wash of a jet engine at takeoff would be 150 decibels.
The ear is also responsible for keeping you balanced thanks to a tiny but ingenious collection of semicircular ducts and two tiny associated sacs called otolith organs, which together are called the vestibular system. The vestibular system does everything that a gyroscope does on an airplane, but in an extremely miniaturized form. Inside the vestibular channels is a gel that acts a little like the bubbles in a carpenter’s level, in that the gel’s movements from side to side or up and down tell the brain in which direction we are traveling (which is how you can sense whether you are going up or down in an elevator even in the absence of visual clues). The reason we feel dizzy when we jump from a merry-go-round is that the gel keeps moving even though the head has stopped, so the body is temporarily disoriented. That gel thickens as we age and doesn’t slosh around as well, which is one reason why the elderly are often not so steady on their feet (and why they especially shouldn’t jump from moving objects). When loss of balance is prolonged or severe, the brain doesn’t know quite what to make of it and interprets it as poisoning. That is why loss of balance so generally results in nausea.
Another part of the ear that intrudes upon our consciousness from time to time is the Eustachian tube, which forms a kind of escape tunnel for air between the middle ear and the nasal cavity. Everyone knows that uncomfortable feeling you get in your ears when you change heights rapidly, as when coming in to land in an airplane. It is known as the Valsalva effect, and it arises because the air pressure inside your head fails to keep up with the changing air pressure outside it. Making your ears pop by blowing out while keeping your mouth and nose closed is known as the Valsalva maneuver. Both are named for a seventeenth-century Italian anatomist, Antonio Maria Valsalva—who also, not incidentally, named the Eustachian tube, after his fellow anatomist Bartolomeo Eustachi. As your mother doubtless told you, you shouldn’t blow too hard. People have ruptured eardrums from doing so.
SMELL
SMELL IS THE sense that nearly everyone says they would give up if they had to give up one. According to one survey, half of people under the age of thirty said they would sacrifice their sense of smell rather than part with a favored electronic device. I hope it isn’t necessary for me to observe that that would be a little foolish. Smell is, in fact, a lot more important to happiness and fulfillment than most people appreciate.
At the Monell Chemical Senses Center in Philadelphia, they are devoted to understanding smell, and thank goodness because not very many others are. Housed in an anonymous brick building alongside the campus of the University of Pennsylvania, the Monell is the largest research institution in the world dedicated to the complex and neglected senses of taste and smell.
“Smell is something of an orphan science,” said Gary Beauchamp when I visited in the autumn of 2016. A friendly, soft-spoken man with a trim white beard, Beauchamp is president emeritus of the center. “The number of papers published on vision and hearing is in the tens of thousands every year,” he told me. “On smell, it is a few hundred at most. It is the same with research money, where funding is at least ten to one in favor of hearing and vision over smell.”
One consequence of this is that there is a great deal that we still don’t know about smell, including exactly how it works. When we sniff or inhale, odor molecules in the air drift into our nasal passages and come into contact with the olfactory epithelium—a patch of nerve cells containing some 350 to 400 types of odor receptors. If the right kind of molecule activates the right kind of receptor, it sends a signal to the brain, which interprets it as a smell. How exactly this happens is where the controversy lies. Many authorities believe the odor molecules fit into the receptors like a key into a lock. A problem with this theory is that sometimes molecules have different chemical shapes but the same smell, and some have almost matching shapes but dissimilar smells, which suggests that a simple shape explanation is not enough. So there is a competing, rather more complicated theory which is that the receptors are activated by something called resonance. Essentially, the receptors are stimulated not by the shape of molecules but by how they vibrate.
For those of us who are not scientists, it doesn’t really matter, because the outcome is the same in either case. What is important is that odors are complex and hard to deconstruct. Aroma molecules typically activate not one type of odor receptor but several, rather like a pianist playing chords—but on an enormous keyboard. A banana, for example, contains three hundred volatiles, as the active molecules in aromas are called. Tomatoes have four hundred, coffee no fewer than six hundred. Working out how and to what degree these contribute to an aroma is not straightforward. Even at the simplest level, results are often wildly counterintuitive. If you combine the fruity odor of ethyl isobutyrate with the caramel-like allure of ethyl maltol and the violet scent of allyl alpha-ionone, you get pineapple, which smells wholly unlike its three principal inputs. Still other chemicals have very different structures but produce the same smell, and no one knows why that happens either. The smell of burned almonds can be produced by seventy-five different chemical combinations that have nothing in common beyond how the human nose perceives them. Because of the complexities, we are still very much at the beginning of an understanding of it all. The smell of licorice, for instance, was decoded only in 2016. Many, many other common odors are still to be deciphered.
For decades, it was universally agreed that humans can discriminate about ten thousand different smells, but then someone decided to look into the origin of the claim and found that it was first suggested way back in 1927 by two chemical engineers in Boston who simply guessed at it. In 2014, researchers at the Université Pierre et Marie Curie in Paris and Rockefeller University in New York reported in the journal Science that in fact we can detect vastly more odors than that—at least a trillion and possibly even more than that. At once other scientists in the field called into question the statistical methodology used in the study. “These claims have no basis,” Markus Meister, a professor of biological sciences at Caltech, flatly declared.
An interesting and important curiosity of our sense of smell is that it is the only one of the five basic senses not mediated by the hypothalamus. When we smell something, the information, for reasons unknown, goes straight to the olfactory cortex, which is nestled close to the hippocampus, where memories are shaped, and it is thought by some neuroscientists that that may explain why certain odors are so powerfully evocative of memories for us.
Smell is certainly an intensely personal experience. “I think the single most extraordinary aspect of olfaction is that we all smell the world differently,” Beauchamp says. “Although we all have 350 to 400 types of odor receptor, only about half of them are common to all people. That means that we don’t smell the same things.”
He reached into his desk and pulled out a vial, which he uncapped and passed to me to sniff. I could smell nothing at all.
“It’s a hormone called androsterone,” Beauchamp explained. “About a third of people, like you, can’t smell it. One-third smell something like urine, and one-third smell sandalwood.” His smile broadened. “If you have three people who cannot even agree on whether something is pleasant, revolting, or simply odorless, you begin to see how complicated the science of smell is.”
We are better at detecting odors than most of us realize. In an arresting experiment, researchers at the University of California at Berkeley dragged a chocolate scent around a huge grassy field and had volunteers try to follow the trail as a bloodhound would, on their hands and knees and with their noses to the ground. Amazingly, about two-thirds of the volunteers were able to follow the scent with considerable accuracy. For five of fifteen smells tested, humans actually outperformed dogs. Other tests have shown that people given a selection of T-shirts to sniff can generally identify the one worn by their spouse. Babies and mothers are similarly skillful at identifying each other by odor. Smell, in short, is much more important to us than we appreciate.
Total smell loss is known as anosmia, and partial loss is hyposmia. Somewhere between 2 and 5 percent of people in the world suffer from one or the other, which is a very high proportion. An especially wretched minority experience cacosmia, which is where everything smells like feces, and it is, by all accounts, as horrible as you would imagine. At Monell, they refer to smell loss as “an invisible disability.”
