Where We Meet the World: The Story of the Senses by Ashley Ward; Basic Books, 320 pp., $30
As an ambitious young scientist at Cambridge University, Isaac Newton famously trained a glass prism on a narrow beam of sunlight to divvy the white light into its constituents, a dazzling rainbow of colors. There were no demarcations between one component color and the next, no clear way to count just how many basic hues were on view, but Newton was a systematizer with an appreciation for the number seven. After all, seven was synonymous with luck, and it showed up everywhere in Western culture: there were seven days in a week, seven notes in the musical scale, seven wonders of the world, seven virtues, and seven vices. Why not seven colors in the rainbow? And so, Newton specified as the primary ingredients of white light the septet captured by the old childhood mnemonic ROY G BIV: red, orange, yellow, green, blue, indigo, and violet. Some insist that the list should lose the penultimate letter. “It has never seemed to me that indigo is worth the dignity of being considered a separate color,” Isaac Asimov wrote. But even the electromagnetic spectrum cannot be decoupled from culture, precedent, and the imprimatur of one of history’s greatest geniuses. Roy’s surname remains intact, and many insist they can see the indigo band just fine.
As Ashley Ward elucidates in his jaunty, reader-friendly tour of the senses, the pathway from an external stimulus to our interpretation of that stimulus is long, loopy, and seeded with booby traps. Our senses are brilliant. Our senses contradict one another. Our senses are easily fooled. The old line, “Who are you going to believe, me or your own lying eyes?” is supposed to be funny, the pitch of a con artist. But eyes, ears, nose, tongue, and skin do lie and are lied to in turn.
Consider a few examples. When people watch a video of somebody mouthing the words “ba ba ba” while the accompanying soundtrack says “da da da,” viewers will swear they heard “ba ba ba.” Take two glasses of good white wine and secretly add a few drops of red food coloring to one of them, and serious oenophiles will ascribe to an undoctored glass conventional white winey virtues like “crisp,” “floral,” and “lemon” while finding in the dyed wine “intense” notes of “cherry” and “raspberry”—terms normally reserved for bold reds. Place your index and ring fingers on two coins that have just spent 15 minutes in the freezer and then let your middle finger slowly descend onto a room temperature coin, and all three fingers will conclude they are touching circles of ice. In 1965, when BBC programmers prankishly told television viewers they had figured out a way to transmit through the airwaves aromas like a “pleasant, country smell” or the odor of an onion being chopped, viewers called in to complain of hay fever or teary eyes.
Our reflexive faith in the prowess of our senses can have more serious consequences. In the courtroom, eyewitness testimony has proved disturbingly unreliable, with studies revealing that the wrong people are identified anywhere from 25 to 73 percent of the time. Yet the quirks and mistakes of our sensory systems reveal a fundamental truth about the nature of perception: from moment to moment, we are surrounded by such a vast amount of sensory information that we have no choice but to ignore most of it, and to take shortcuts and fill-in-the-blanks with the rest. “[H]ow does the brain manage to keep up with it all? The answer is that it doesn’t. It filters and winnows the information in its perpetual quest for what’s important,” Ward writes, in particular “novelty and change.” The feeling of your clothes lying smoothly against your skin is normal and thus safely disregarded; the feeling of a loose thread quivering against your leg is not normal and must be attended to, lest it prove to be not an unraveling seam but something with legs of its own.
In the case of the cold coin trick, it’s normal for the fingers of your hand to sense heat or cold as a unit, so if two fingers have already touched something cold, the brain preemptively decides the third digit will, too. “Signals from the outside world are interpreted and layered with biases, prior expectations and emotions,” Ward writes. “[T]he convincing perception of reality that we each enjoy is actually a complex but brilliant illusion.”
Ward, a professor of animal behavior at the University of Sydney, defines a sense as “a faculty that detects a specific stimulus by means of a receptor dedicated to that stimulus.” He presents the senses in a fairly standard sequence, from the primacy of vision, which “consumes more of the brain’s resources than all the rest of our senses combined,” through the other skull-based sensory systems of hearing, smell, and taste, and on to touch, our most distributed sense. He considers each sense physiologically, evolutionarily, historically, linguistically, emotionally. At times he drifts into the speculative realms of evolutionary psychology, emphasizing minor sex differences in sensory acuity and wondering why “we’re suckers for a pretty face.” Could it have something to do with facial symmetry as a signifier of good genes? Maybe. Or maybe it’s an incidental byproduct of our “sensory skew towards sight.”
The eye is a magnificent organ, starting with the black pupil in the middle, a kind of camera shutter named for the way it reflects to viewers a tiny image of themselves—pupilla is Latin for “little doll.” The pupils expand and contract in response to ambient light. They also dilate when we’re excited or aroused, which is why poker players wear sunglasses to hide how they feel about their cards. Through the joint efforts of the cornea, which sits atop the eye, and the lens, stationed just beneath the iris, incoming light is bent and focused onto the all-important retina, a delicate half-dollar-size disk of tissue at the back of each eye. In the retina, tens of millions of rod cells and three types of cone cells translate light into electrical signals fit for the brain. The rods are simple yes-no light detectors, while the cones put color in our world, each type tuned to a different segment of visible light and their signals mixed and matched to convey a sense of red, blue, yellow, green, maybe a million colors or more. As diurnal apes, we have far better color vision than most mammals, whose eyes remain better suited for the nightlife they adopted to sidestep the dinosaurs. But our color sense pales in comparison with that of birds, who have four cone types to our three, or the mantis shrimp, a carnivorous crustacean whose retinas, for reasons unknown, are stocked with at least a dozen varieties of cone receptors.
Other species claim astonishing auditory powers, too. The giant head of the sperm whale, for example, serves as an acoustic lens, a component of the echolocation system that allows the whale to detect and focus bounce-back signals and thus to map out every feature of its surroundings. Recent research suggests that plants, too, have the equivalent of ears. Pea plants will grow toward the sound of running water even in the absence of added moisture in the soil, while evening primroses can distinguish between the sound of a bee, its preferred pollinator, and that of other flying insects, and will sweeten its nectar only when cued by a buzz. Yet human hearing remains impressive. Compared with other species, Ward says, “we can detect a massive spectrum of sound,” and we’re geniuses at detecting and decoding sounds of 1 to 6 kHz, the frequencies associated with the human voice. The tiny ossicle bones in the middle ear, the pea-size spiraling cochlea of the inner ear, and the hair cells that transform pressure waves into electric signals, all are optimized to make the most of human conversation.
Even the evolutionarily ancient sense of touch, through which primordial cells mapped out their boundaries, advanced, and retreated, has been finessed to magnificence in the form of the human hand. The tips of our fingers hold a dense weave of specialized cells and corpuscles so sensitive, they can detect bumps and notches a micron wide—the size of a bacterium. There are receptors for pain, receptors for pressure, receptors for hot and cold. All nerve fibers considered, the skin at our fingertips is some 27 times more sensitive than the skin of our torso. “Nowhere on our bodies gets anywhere near the sensory richness of the hand,” Ward writes. And the more we put our fingers to use, the more attention the brain pays and the greater grows the neural real estate devoted to keeping track of our every move. In brain scans of professional violinists, the cortical representation of the left hand, which controls the notes played, is significantly larger than that of a nonmusician.
Similar changes are showing up in brain images of young people addicted to their cellphones. In this case, it’s not a matter of left hand or right. From the look of their scans, the kids are all thumbs.
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