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My sister's friend Heather Sellers, a professor of English at Hope College and a deeply gifted writer, can't recognize or recall the faces of friends or even family members. Each time they reappear in her life, they seem new and strange. Heather has severe prosopagnosia, a bewildering syndrome only partly understood, which disrupts the brain's ability to recognize and remember features of the human face. "When I see a face, I assume I see exactly what you see," she told me. "Faces aren't blurry, foggy, altered in any way. But what I remember about them, what I keep—that's what's different."

Heather believes that she and other prosopagnosics have trouble recalling faces because they generalize them in the same way that non-foresters generalize trees and non-chicken experts generalize chickens; they don't see and retain the details necessary to categorize them into subtypes. "I can't describe lips, noses, face-bone structure, foreheads, chins, even eyes," she says. "When I think of someone I know well, like your sister," she told me, "I see her hair and feel her warmth, her energy. I see her in a beige linen blouse and can conjure gold earrings. I know she has a face, but I can't tell you a single thing about it."

Like other sufferers of prosopagnosia, Heather uses alternative strategies for recognizing people—nonfacial "handles" such as gait, hairstyle, body outline, mannerisms, and tone of voice—but these often fail. "Winter is harder than summer," she says. "People are bundled up, and their padding screws up their gait and their outline; sometimes all that shows is their face. In these situations, I can't recognize even my closest friends." Not surprisingly, the prospect of casual social contact fills her with dread. "The worst situation is a party with ten people I know fairly well," she says. "I know that I'm not going to recognize them. So I'll be a nervous wreck and have to work incredibly hard to identify them and to hold it all together, to manage the anxiety." She tries to avoid parties altogether, or if she can't, to take with her what she calls a "seeing-eye human" who will whisper to her the identity of friends and colleagues: "There's the provost, Jim. Coming in from the left is John'S. from Psychology. There's Dede in the brown dress and the bangles. Talking to us right now is Lynn."

Oddly enough, Heather didn't know she had prosopagnosia until she was forty. (If you don't know what it's like to recognize a face, she told me, you don't necessarily know that you aren't doing what others do.) Then she stumbled on descriptions of the disorder while researching schizophrenia for one of her fictional characters. When she read the accounts, she was astonished by how accurately they described her own experience. She signed up for a study at Harvard, where she was officially diagnosed in 2005. "I was relieved and elated," she says. "I felt I had a great excuse for all the horrible social encounters I've had. It was the best test I ever failed."

Some cases of prosopagnosia result from stroke or damage to a blueberry-size patch of cortex in the right brain just behind the ear, known as the fusiform gyrus. Imaging studies show that neural activity in this small patch surges when normal people view faces. People who have suffered lesions in the area neither recognize familiar faces nor remember new ones. Most cases of the disorder, however, are a mystery, perhaps rooted in subtle developmental or genetic problems affecting this and other brain regions. Surveys suggest that as many as 2 percent of people have some degree of face blindness.

"I've learned that facial recognition is an enormously complicated process," Heather told me, "involving not only the ability to 'read' the topography of faces, but also memory, sensation, and emotion. To me, it's not so weird that I can't read a face," she says. "It's stunning that you can."

For years scientists have argued about where facial recognition occurs in the brain and how it normally works. Do our brains have specialized face-recognition modules? In a recent experiment with monkeys, Doris Tsao of the University of Bremen found that 97 percent of the cells in the fusiform gyrus respond almost exclusively to faces—evidence that this brain region may be just such a module.

Do the millions of neurons involved in the process work together, orchestrating the myriad bits of information about shape of nose, size of eyes, symmetry of lips into a single familiar visage? Or do individual neurons have the ability to respond selectively to a given face?

The latter concept, known as the grandmother neuron theory, used to seem laughable to some: So you have a single cell devoted to Grandma? One to Hillary Clinton? And another to Mick Jagger? Indeed, the idea appeared to be pretty far-fetched until 2005, when a team of scientists, including Christof Koch and Itzhak Fried, a neurosurgeon at UCLA, showed that individual neurons are in fact surprisingly adept at "face-spotting." In a study of eight patients implanted with electrodes, the team found that a single neuron fired selectively in response to assorted pictures of the same celebrity. In one patient, the same neuron was triggered by seven different pictures of the actress Jennifer Aniston. "This neuron looks for all the world like a 'Jennifer Aniston' cell," one neuroscientist remarked. The researchers are quick to say that these are not literally grandmother neurons but cells that are wired to fire in reaction to something specific and familiar, such as a well-known face. Their response may be more memory-related than visual. "I suspect," Koch explains, "that if this patient were to lose these cells, he would still recognize Jennifer Aniston as a female face but might not know that it was the Jennifer Aniston who had that TV show and who used to be married to Brad Pitt."

How would single cells "encode" specific faces? Doris Tsao's work suggests that each face-recognizing neuron is "tuned" to a set of facial characteristics; each acts as its own set of "face-specific rulers," she says, "measuring faces along multiple distinct dimensions," such as size and shape of individual features—iris size, for instance, or distance between eyes. By combining the measurements of all these little rulers, Tsao proposes, individual face cells may accomplish the miraculous task of reconstructing a face in the brain.

In scoping out the party crowd, does one face catch your eye? The Tierra del Fuegans have an expression,
mamihlapinatapei,
which is listed in the
Guinness Book of Records
as the world's most succinct word. It refers to the act of "looking into each other's eyes, each hoping that the other will initiate what both want to do but neither chooses to commence."

What draws two people together? Scientists have found that both face and gaze send a profusion of visual signals about mutual interest, health, even good genes. Though we've been taught not to judge a book by its cover, Shakespeare was right: One reads in a face many strange matters—identity, expression, even intent. We all do it, probably hundreds of times a day.

Take gaze. We're alone among animals in possessing eyes that signal where we are looking. The whites of our eyes, which highlight the iris, allow us to make eye contact and tell us instantly the direction of someone's gaze. This enhances "gaze signaling," a key cue for communication and cooperative behavior. A team at University College London found that a direct gaze from an unfamiliar attractive face enhances its appeal and activates the dopamine circuits in our brain that are dedicated to predicting reward. By contrast, if that same face looks away from us, the activity in this area diminishes, lire heightened dopamine activity is not rooted in the attractiveness of the gazer per se, but in the potential for interaction signaled by eye contact,
mamihlapinatapei.

Whether a meaningful glance leads to some more intimate interaction depends in large part on snap judgments we make without knowing it. The sense of who we find attractive, says the latest research, may lie in heartless formulas for seeking healthy partners with good genes. We carry these formulas buried in our minds and respond to the signals that promise to fulfill them.

So, what are we looking for?

Facial symmetry for starters. Most of us prefer faces with neat bilateral symmetry, which may signal a strong immune system and the absence of genetic problems. (Asymmetries often arise during fetal development from biological stresses such as poor nutrition, disease, parasites, or inbreeding.)

The masculine or feminine quality of a face is another such beacon. A team of Scottish and Japanese scientists recently showed that both men and women are attracted to more feminized faces of the opposite sex. In the sculpting of our faces in utero and throughout life, testosterone helps to carve the more chiseled masculine facial features of men; estrogen helps to shape the softer, rounder features of women. The researchers manipulated photographs of faces by enhancing or diminishing differences between the sexes. Subjects rated as more honest and cooperative the male and female faces that had been feminized—rounded, with smaller jaws. Feminized male faces, in particular, seemed to convey to women a "good father" signal. The scientists speculate that this preference may actually have limited the extent of human sexual dimorphism in facial appearance.

News of reproductive status may also figure into facial attraction, at least for men. Craig Roberts and his team at the University of Newcastle reported that men find especially appealing the faces of women who are ovulating. It had long been thought that women didn't reveal when they were ovulating with any kind of visual signal. While most animal species advertise their fertility through ruddied rumps or splashy scent, we humans seemed to hide ours. But Roberts's studies hint at the captivating possibility that our faces are a giveaway. The team showed that men judged photographs of women's faces taken in their fertile phase to be more alluring than photos of the same women taken in the luteal, or non-ovulating, phase.

"This increase in facial attractiveness is subtle," says Roberts. It involves variations in lip color and size, pupil dilation, and skin color and tone. But, he says, in evolutionary terms, even such understated effects can have a substantial impact on reproductive success by raising a woman's profile at a time in her cycle when the probability of conception is highest.

A direct gaze, a feminine, symmetrical face, full lips, and dilated pupils; throw in a smile (a potent signal that, if sufficiently broad, may be read accurately at a distance of a few hundred feet) and you have the sum of visual signals we may read in that face across the room or by our side.

But there's something else going on here. Well below our visual radar and beneath the screen of consciousness are other sorts of messages—chemical signals that convey far more than we ever imagined.

As you wander among the party guests, consider what you're taking in as social cues. It may seem all talk and visual clues. But a growing body of evidence suggests that in the matter of social evaluation and attraction, smell may be at least an equal partner. "In my lectures, I ask whether the ladies in the audience are turned on by the smell of certain men," says Mel Rosenberg of Tel Aviv University. "Invariably, I receive a positive response." To determine whether attractiveness of the opposite sex was influenced by smell, a team of British scientists asked thirty-two young women to rate male faces on aspects of attractiveness, then exposed them to a dab of male underarm sweat and asked them to rate the faces again. After the whiff, the women found the men significantly more appealing.

Though we have fewer olfactory receptors than an animal such as a mouse or a dog, which sniffs its way to food and sex, this doesn't mean we aren't swayed by the subtle powers of odor. As we now know, our olfactory system is exquisitely sensitive, capable of distinguishing tens of thousands of odorants in vanishingly small amounts. Women are better at the task than men, say scientists at the Monell Chemical Senses Center in Philadelphia—at least women of reproductive age. This boost in sensitivity may result from female sex hormones that kick in at puberty, and likely serves to help women detect poisons in food while pregnant and to bond with children and mates.

We've also come to realize the impressive nature of our own odors. According to D. Michael Stoddart, a zoologist at the University of Tasmania, we humans are among the most highly scented of the apes. Odor glands abound in our face, scalp, upper lip, eyelids, ear canals, nipples, penis, scrotum, and pubis. But most of our normal, healthy body odor, a musky scent, issues from sebaceous and apocrine glands clustered in our armpits, or axillae, which start to function only at puberty. Apocrine glands secrete an oily substance that is odorless until the vast populations of microorganisms living in and around the underarm hair follicles and shafts break it down to produce musky-smelling compounds. (Another example of our microbial partners shaping our ways.) These molecules are wicked out into the world by underarm hair, says Charles Wysocki of the Monell Center. Removing the bacterial habitat and the smell "antennae" by shaving may result in a reduction of odor. But inevitably the axillary jungle returns, and with it the full aroma of those fragrant molecules—among them, fatty-acid compounds much like those that serve as sex signals in other animals.

It has long been suspected that axillary glands produce a scent attractive to the opposite sex. In his book
The Scented Ape,
Stoddart quotes folk stories of "a young man who would woo a peasant girl by placing his handkerchief in his axilla during a dance. When the young girl perspired, he chivalrously produced his handkerchief to wipe the sweat from her face. The power and allure of his axillary scent was such that she immediately succumbed to his wishes." In rural Austria, it was formerly a practice for girls to keep a slice of apple in their armpits during dances, writes Stoddart. At the end of the dance the girl would present the apple to the swain of her choice, who would—gallantly or readily—eat it.

Indeed, "one of the reasons that dancing is so appealing is that it's an opportunity for people to smell one another up close," adds Mel Rosenberg, who met his own wife on the dance floor.