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I once had the good fortune to gaze into the eyes of one primate relative, a six-year-old chimp named Jack. For years I had heard about how similar we are to the chimpanzee in evolutionary terms, how much DNA we share, how close we are in anatomy and physiology. But nothing brought home the deep-down kinship like sitting face to face with the sensitive, intelligent, and
funny
Jack. There were differences, of course: Jack had a smaller head and bigger ears. His legs were short, his feet had thumbs, and he used his hands for walking. He didn't pray or sing nursery rhymes or tattle on his peers, at least not in a way that I could understand. But I found it startling and very moving to look into his eyes—darker, perhaps, but clearly akin to my own.

Jack loved nothing so much as the grapes and other small fruit snacks he received as rewards for his training. These he would balance on his lower lip thrust out as far as it would go, then slowly roll backward and flick into his mouth.

It's true that our eyes are the eyes of our predatory ancestors, insofar as they are set in front of the head in order to track prey with binocular vision. But human eyes, like chimp eyes, are also the eyes of the picky frugivores and leaf eaters that came before us, which may help to explain our idiosyncratic brand of color vision.

What allows me to see my clothes in a range of hues—scarlet, burgundy, turquoise, olive—is the interplay of three kinds of cone cells in the retina, each with a pigment especially responsive to light from a different portion of the visual spectrum: red, green, and blue. With this trichromatic, three-cone system, we humans can distinguish some 2.3 million gradations of color. So remarkably sensitive to the red/ green segments of the spectrum is our tribe that we can tell the difference between the colors of light in this part of the spectrum with only a r percent difference in wavelength.

Our early mammalian ancestors saw the world in dichromatic vision, as most mammals do now, without the red part of the spectrum. Then, thirty or forty million years ago, the monkeys and apes of Africa—among them the primate ancestors of humans—experienced a mutation in a gene for a light-receptor protein that shifted its sensitivity from green light to red. It was a small change, but one that some scientists suspect gave our arboreal primate ancestors a clear advantage in the search for food, for picking out ripe fruits and tender red leaves against a background of green foliage. (This enhanced color vision may also have been useful for distinguishing other important objects from the surrounding foliage: brightly colored venomous snakes, for instance.)

New research hints at individual variations in this red-spectrum vision. When scientists recently analyzed a single gene that codes for a red-sensing protein in 236 people around the globe, they found 85 variants—about three times more variety than one might expect to see in other genes. This variation likely gives us each a unique view of hues.

Some percentage of women may experience even more distinctive color vision because they possess an extra red photopigment. If the visual cortex processes the additional input from this different class of red-sensitive cells, these women may be able to distinguish colors that look identical to the rest of us, allowing them to see a subtle color world forever unavailable to most of humanity.

It could be argued, then, that behind the simple act of everyday color perceptions—picking out a shirt from your closet, reading a traffic light, admiring a Rothko painting—lies a visual apparatus fine-tuned to locating red leaves and fruit, and an answer to the old philosophical dilemma, Is my red your red?

The answer is probably no. My experience of a tomato likely differs from yours, both its exuberant red hue and its tartly acidic tang. As the great psychologist William James said, the mind works on the data it receives "very much as a sculptor works on his block of stone. Other sculptors, other statues from the same stone!"

If we owe our brilliant color vision to our ape ancestors, we may be obliged to another creature for our fine sense of hearing. As you get dressed or make lunch or tidy up before you leave for the office, you listen with one ear to the morning news while the other is tuned to a volley of conversation from your spouse settling plans for the evening, or to your children searching for school books, or to the annoying, persistent barking of your dog from the far end of the yard. How in the world do your ears pick up the subtle vibrations of sounds, faint and furious, and sort the cacophony into intelligible, sensible parcels?

Detecting the source of sound and interpreting it—a Bach sonata or your teenage daughter's plea for a matching sock—may seem like an easy feat, but it's exquisitely complex. When we hear someone call our name and turn toward the sound, we are relying on the brain's ability to calculate direction from the interaural time difference, or ITD—the difference in the time it takes for the sound to reach our two ears. "Incredibly, we can detect ITDs of only a few microseconds, allowing us to distinguish between sounds separated by only a few degrees in space," writes the neurobiologist George Pollak.

Our refined ability to parse sounds in time and locate them spatially we may owe to the dinosaurs, which forced our earliest mammalian ancestors to retreat to a nighttime niche. For millions of years, our shrew-like forebears carried out their lives under cover of darkness, where sound held sway over sight. Over the eons, they evolved a highly sophisticated auditory system incorporating the time dimension. Now our ears can perceive sounds lasting only a fraction of a second in their correct order and locate them in space.

Sounds arrive at the ear as waves, which your eardrum, or tympanic membrane, converts into energy that rattles the three delicate little bones of the middle ear. This causes pressure changes within the cochlea, the coiled, fluid-filled tube at the heart of the ear, which in turn translates the energy into chemical and nerve signals that are sent to the brain.

The cochlea is no passive spiral cavity, as once believed, but, in neuroscientist Jim Hudspeth's words, "a three-dimensional inertial-guidance system, an acoustical amplifier, and a frequency analyzer compacted into the volume of a child's marble." Our ability to hear depends on "hair" cells, arranged in the cochlea in a zigzag pattern. These cells are relatively scarce—only sixteen thousand to an ear—fewer cells, Hudspeth notes, than you have on a hangnail or a flake of dry skin—which accounts for the vulnerability of our acoustical system. Hair cells damaged by infection, drugs, aging, or excessive exposure to Deep Purple are lost for good.

If I put a small microphone to my sleeping husband's ear, I might well hear his hair cells hard at work. In a quiet environment, the hair cells in most normal human ears are turned up to amplify softer sounds—turned up so far that they themselves generate faint but constant tones of sound, like the feedback noise from an electronic amplifier. In a loud environment, a thunderstorm or rock concert, the hair cells adjust, turning down their amplifiers. It is thanks to these mini-amps that we can follow ten to twenty distinct sounds per second, distinguish pitch, and hear noises that last only a few thousandths of a second.

We rarely notice the sound made by our hair cells because the brain filters it out. Likewise, when we speak or sing or vocalize in any way, the brain halts the firing of our auditory neurons so that we won't be swamped by our own song. So, too, the brain allows us to suppress a whitewater of auditory stimuli—the buzzing, banging, humming, thumping background noise of our typical morning routine—so that we may hear only what interests us; the rest fades into a kind of muted roar that we hear with just "one" ear at first, then with no ear at all.

This is one example of desensitization, the same phenomenon that makes the aroma of bacon or reeking garbage fade from perception, that helps our eyes adjust to bright light, that allows us to forget the rub and weight of clothes on skin, and that attenuates the nervous jolt initially provided by coffee. Desensitization can take place over seconds (light), minutes (smells), or days (caffeine).

At any given moment, we tune in to what's important in our world by turning off stimuli. We also fill in what we are missing. Think of conversing over the ambient noise of the morning radio. Often you hear only part of the conversation (the rest being masked with radio chatter), but nevertheless grasp the gist of it by filtering out the irrelevant noise and filling in the missing sounds.

Something similar happens in the brain when you sing a song inside your head. In 2005, scientists scanned the brains of subjects while they listened to the soundtracks of familiar songs (the Rolling Stones's "Satisfaction," for example, and the theme from
The Pink Panther)
in which silent gaps had been inserted. The auditory cortex of the subjects continued to show the same pattern of activity even when there was silence on the soundtrack and the subjects were just "singing" in their heads. The ear wasn't hearing the song, but the brain was.

Sensing isn't what we thought it was. It's a far more sophisticated endeavor shaped by our genetic makeup, our creative powers of filtering and filling in—and, quite possibly, some significant crosstalk between the senses. I've been discussing the morning as if we sensed its elements separately, one facet at a time, but in fact the brain is always uniting the different qualities of individual objects so that we don't associate the color of one thing with the movement of another—so, for instance, we can see a cat as black, shaped like a cat, and meowing, and a yellow dog as dog-shaped, yellow, and howling. Scientists are still searching for the "glue" that binds together different sensory aspects; some theorize it may be the synchronous firing of the neurons from different areas of the brain involved in perception.

What if we did process only one sense at a time, if you took in only the sight of your child's face and not the timbre of her voice, or if you could only smell your morning juice but not see it? Would it taste the same?

Probably not. What you see changes what you taste. When a French scientist, Gil Morrot, gave a panel of fifty-four tasters a white wine artificially colored red, the group—experts and nonexperts alike—described its odor and taste as that of a red wine.

Likewise, what you see affects what you hear and feel. In one study, researchers placed monkeys in the center of a semicircle of speakers and trained them to look in different directions while they listened. Then the team monitored the signals arriving in the part of the monkeys' brain that transmits information from the ears to the auditory cortex. To their surprise, the cells in this region fired at a different rate depending on where the monkeys' eyes were directed.

Similarly, scientists have found that when people look at a spot on their body where they're being touched, their brains show greater activity in their somatosensory cortex—the touch region of the brain—than if they don't see the touch. The reverse is true as well. Giving people both a touch and a visual stimulus at the same time and on the same side of the body enhances activity in the visual cortex.

So vision is never just seeing, touch is never just touch. We spot objects more easily if we hear a relevant sound simultaneously. When we see a banana or a crimson shirt, we're also feeling it with our mind's "hand."

This crosstalk occurs in sensory memories, too. Jay Gottfried and his colleagues have found that a memory cue in one sense reactivates other sensory memories. Most of us know this from experience—the smell of coconut oil elicits the intertwinkling facets of that white stretch of beach and light slap of waves; the scent of smelts evokes my grandfather's kitchen, his cigar smoke, and his smile with its flashing gold tooth.

Our senses, then, are hardly the simple, discrete instruments we once imagined, but picky, idiosyncratic tools that interact with subtlety and speed to grant us out of mere electrical disturbance our own distinctive view of ... of what?

Whatever we're paying attention to at a given moment—say, the traffic during our morning drive to work.

3. WIT

Y
OU'RE OUT THE DOOR
and on your way, cruising along at forty or fifty miles per hour, your mind—well, your mind is not entirely on the highway along which your two-ton steel bullet is hurtling. You may think you're absorbing the scene in all its detail, four-lane road, swerving Subaru, white morning glare, but the impression you have of seeing everything is an illusion. Though your senses are taking in some ten million bits of information a second, you're consciously processing only about seven to forty bits. Even fewer if your thoughts are really elsewhere—on your upcoming meeting, say, or replaying that morning family spat or perhaps trying to settle it now by cell phone as you drive.

"We actually only see those aspects which we are currently visually 'manipulating,'" says the psychologist J. Kevin O'Regan, and we visually manipulate only the things we're paying close attention to.

This came home to me one cold winter morning a few years ago when I stood with my ten-year-old daughter, Zoë, watching the waking of whooper swans from the bank of a calderic lake in Hokkaido, Japan. The lake was ringed with blue volcanic hills and fed by hot springs; on the shore near us was an outdoor Japanese bath. My eyes were all for the swans—pale white mounds of feathers, heads tucked beneath wings—so as to note the pattern and behavior of their waking. One by one the necks of the birds before us unfurled and their heads popped free of their wings. But what was that low, furtive shape slinking along on the ice behind them? A dog? A fox? So focused was I on the furry form that I failed to notice another dark figure not ten yards from us, a man walking buck-naked to the baths.

Zoë saw him, all right.

My failure to spot the obvious is an example of "inattentional blindness." When the brain is attending to its surroundings, it is aroused, ensuring full awareness and efficient activity. But when it's distracted, it's capable of missing what's most obvious. This is the phenomenon demonstrated in those "man in the gorilla suit" studies showing that people told to engage in a simple task, such as counting the number of shots taken during a basketball game, will completely miss the man running across the court dressed in a gorilla suit. It also underlies what my family calls refrigerator vision, when you miss what you are looking for—the obvious jar of mayonnaise or leftover lasagna—because someone is asking for the ketchup.