The last time anyone saw my smile was the night of February 1, 2002. I’d been out for Korean barbecue with old friends, and had been complaining about a headache that I’d had for five days. I’d likened it to a spike driven through the top of my head and down to the back of my throat. “We’ll send you heavy-duty pliers,” someone joked as we said good night.

When I woke up the next morning, my face felt slack; I thought I’d slept too long in one position. Then I looked in the bathroom mirror. There were no creases on the left side of my forehead, and the laugh fold to the left of my nose had almost disappeared. I could raise the right eyebrow but not the left. The right eye blinked; the other was frozen open. I couldn’t pucker or retract my lips. My head was still throbbing.

At the emergency room, the doctor was unimpressed. “It’s Bell’s palsy,” she said, as if describing a case of flu. She told me that it was a common, temporary paralysis brought on by the inflammation of the seventh cranial nerve, which activates the facial muscles. The condition is named for the Scottish physician Charles Bell, who called it by its then familiar name, “blight,” in cranial-nerve studies that he conducted in the eighteen-twenties. (Bell also considered the problem “not formidable.”) Bell’s palsy afflicts about one in sixty-five people at some point in their lifetime; most recuperate within three weeks, but about fifteen per cent never recover fully. The exact cause is unknown.

That was Saturday. I was sent home with a prescription for a strong pain reliever for the headache and an eye patch to protect my cornea until I could blink again. By Monday, I couldn’t keep food or water down; I returned to the hospital and was told that my kidneys had failed. By Tuesday, I was constantly dizzy, even while lying in bed. The inflammation had disturbed the eighth cranial nerve, which regulates the inner ear. It and the seventh cranial nerve pass through a pair of tiny canals, one on each side of the skull. The nerves had likely been squeezed up against the ragged internal walls of the left canal and damaged.

After several days, my kidneys started working again. (Their failure turned out to have been a side effect of the pain medication; my headache dissipated on its own during the following week.) Six months of physical therapy restored my sense of balance. The eighth cranial nerve didn’t heal, but with effort the eyes can be trained to take over the work of the inner ear, providing information about balance to the brain. The seventh cranial nerve was able to heal, but only in part. Like a utility trunk line, it is a bundle of thin strands that attach to an array of facial muscles. When the strands are “insulted,” as my doctor put it, the ones leading to the upper face often heal first and best; those serving the lower face can take longer and sometimes reattach sloppily to the wrong neuronal partners.

And so I regained the ability to blink and wink on both sides, to frown, and to look absorbed. But the nerve that once connected to my left zygomaticus major, the muscle that traverses the cheek and pulls it up for a smile, had attached itself to my left platysma, the muscle that extends into the neck and pulls down the mouth and chin. Doctors call this condition synkinesis. For the past thirteen years, my smile has been an incoherent tug-of-war between a grin on one side and a frown on the other: an expression of joy spliced to an expression of horror.

Smiles are our most important form of nonverbal communication. They express warmth and familiarity; they signal receptiveness, openness, alliance, approval, arousal, mirth, and pleasure. They’re also pleas for attention; tools of ingratiation, seduction, appeasement; flags of disapproval, contempt, embarrassment. Some people wield them parsimoniously; others dole them out willy-nilly. The spontaneously joyful smile is the facial expression most easily recognized from a distance—as far as a hundred metres, researchers say.

If a stranger approaches me smiling and I try to return the greeting, I watch the person’s face fade into apprehension and wariness. I teach theatre at Hunter College, and, when I first enter the classroom each semester, new students have no idea what to make of me, because my face doesn’t corroborate my claim of being accessible and eager to work with them. A confusing message is the last thing I want to send, so I try to produce the most normal-looking expression possible, which in my case is usually a flat, tight-lipped half smile that comes off as ungenerous, patronizing, or insincere. If something tickles or pleases me, I try to smile, and my left cheek is yanked down as if by a rubber band.

I have a photograph in my office of Bill Clinton and me, taken about ten years ago, when he visited campus. Clinton, the old pro, is beaming. The camera has caught me leaning slightly away, trying to show pleasure with my eyes while holding my mouth flat to avoid looking deformed. Most people who see the photo chuckle and assume that I couldn’t stand Clinton. Actually, I have always rather liked him and am sorry the record seems to show otherwise.

The scientist who first explained the separate actions and effects of the human facial muscles was an eccentric nineteenth-century Frenchman named G. B. Duchenne de Boulogne. His 1862 compendium, “The Mechanism of Human Facial Expression,” established the “orthography” of humanity’s “language of the emotions,” which Duchenne presumed to be “universal and immutable.” (Charles Darwin, Paul Ekman, and others later provided evidence for the claim of universality.) Duchenne’s was the first scientific work to illustrate its findings with photographs. The book contains more than a hundred remarkable plates, some as misshapen as anything a Bell’s-palsy patient is likely to see in the mirror.

Duchenne cast aside the pseudoscientific literature on physiognomy—reading character and morality in people’s faces—that had been piling up since the seventeenth century. Using an apparatus he’d invented, Duchenne stimulated volunteers’ facial nerves at the points where the nerves entered each facial muscle. He then photographed the resulting contractions, compiling a catalogue of discrete muscle actions that correspond to particular emotions.

Among the experimental subjects were a mustachioed young actor with the rare ability to move many of his facial muscles independently; an attractive, nearly blind young woman; and a prematurely wizened middle-aged man so “stupefied by the abuse of alcohol” that he died of the D.T.s ten days after being photographed. I found my own condition in photo No. 58. One of the regular subjects, a toothless old man with nerve damage so severe that he could not feel the electrodes, is having his left platysma muscle “electrized.” His neck is grotesquely strained on the left, and his lower lip is drawn sharply down and sideways. He looks irritated, baffled, and attentive all at once. Duchenne called the platysma “the muscle of fright.”

Duchenne was the first to observe that a spontaneously joyful smile cannot be faked, because it results from the simultaneous contraction of two muscles, only one of which is ordinarily under conscious control. Most people can voluntarily lift the corners of the mouth, but authentic joy lives in the eyes. It requires contractions of the orbicularis oculi, the sphincter muscle surrounding the eye socket, which, Duchenne wrote, “is only put in play by the sweet emotions of the soul.” The effect of this muscle is unmistakable: it subtly lifts the lower eyelids and pushes the skin around the eyes inward, and the eyes seem to sparkle.