The theory—and a theory is all it is right now—has begun to make sense of some bewildering phenomena. Among them is an experiment that Ramachandran performed with volunteers who had phantom pain in an amputated arm. They put their surviving arm through a hole in the side of a box with a mirror inside, so that, peering through the open top, they would see their arm and its mirror image, as if they had two arms. Ramachandran then asked them to move both their intact arm and, in their mind, their phantom arm—to pretend that they were conducting an orchestra, say. The patients had the sense that they had two arms again. Even though they knew it was an illusion, it provided immediate relief. People who for years had been unable to unclench their phantom fist suddenly felt their hand open; phantom arms in painfully contorted positions could relax. With daily use of the mirror box over weeks, patients sensed their phantom limbs actually shrink into their stumps and, in several instances, completely vanish. Researchers at Walter Reed Army Medical Center recently published the results of a randomized trial of mirror therapy for soldiers with phantom-limb pain, showing dramatic success.

A lot about this phenomenon remains murky, but here’s what the new theory suggests is going on: when your arm is amputated, nerve transmissions are shut off, and the brain’s best guess often seems to be that the arm is still there, but paralyzed, or clenched, or beginning to cramp up. Things can stay like this for years. The mirror box, however, provides the brain with new visual input—however illusory—suggesting motion in the absent arm. The brain has to incorporate the new information into its sensory map of what’s happening. Therefore, it guesses again, and the pain goes away.

The new theory may also explain what was going on with M.’s itch. The shingles destroyed most of the nerves in her scalp. And, for whatever reason, her brain surmised from what little input it had that something horribly itchy was going on—that perhaps a whole army of ants were crawling back and forth over just that patch of skin. There wasn’t any such thing, of course. But M.’s brain has received no contrary signals that would shift its assumptions. So she itches.

Not long ago, I met a man who made me wonder whether such phantom sensations are more common than we realize. H. was forty-eight, in good health, an officer at a Boston financial-services company living with his wife in a western suburb, when he made passing mention of an odd pain to his internist. For at least twenty years, he said, he’d had a mild tingling running along his left arm and down the left side of his body, and, if he tilted his neck forward at a particular angle, it became a pronounced, electrical jolt. The internist recognized this as Lhermitte’s sign, a classic symptom that can indicate multiple sclerosis, Vitamin B12 deficiency, or spinal-cord compression from a tumor or a herniated disk. An MRI revealed a cavernous hemangioma, a pea-size mass of dilated blood vessels, pressing into the spinal cord in his neck. A week later, while the doctors were still contemplating what to do, it ruptured.

“I was raking leaves out in the yard and, all of a sudden, there was an explosion of pain and my left arm wasn’t responding to my brain,” H. said when I visited him at home. Once the swelling subsided, a neurosurgeon performed a tricky operation to remove the tumor from the spinal cord. The operation was successful, but afterward H. began experiencing a constellation of strange sensations. His left hand felt cartoonishly large—at least twice its actual size. He developed a constant burning pain along an inch-wide ribbon extending from the left side of his neck all the way down his arm. And an itch crept up and down along the same band, which no amount of scratching would relieve.

H. has not accepted that these sensations are here to stay—the prospect is too depressing—but they’ve persisted for eleven years now. Although the burning is often tolerable during the day, the slightest thing can trigger an excruciating flareup—a cool breeze across the skin, the brush of a shirtsleeve or a bedsheet. “Sometimes I feel that my skin has been flayed and my flesh is exposed, and any touch is just very painful,” he told me. “Sometimes I feel that there’s an ice pick or a wasp sting. Sometimes I feel that I’ve been splattered with hot cooking oil.”

For all that, the itch has been harder to endure. H. has developed calluses from the incessant scratching. “I find I am choosing itch relief over the pain that I am provoking by satisfying the itch,” he said.

He has tried all sorts of treatments—medications, acupuncture, herbal remedies, lidocaine injections, electrical-stimulation therapy. But nothing really worked, and the condition forced him to retire in 2001. He now avoids leaving the house. He gives himself projects. Last year, he built a three-foot stone wall around his yard, slowly placing the stones by hand. But he spends much of his day, after his wife has left for work, alone in the house with their three cats, his shirt off and the heat turned up, trying to prevent a flareup.

His neurologist introduced him to me, with his permission, as an example of someone with severe itching from a central rather than a peripheral cause. So one morning we sat in his living room trying to puzzle out what was going on. The sun streamed in through a big bay window. One of his cats, a scraggly brown tabby, curled up beside me on the couch. H. sat in an armchair in a baggy purple T-shirt he’d put on for my visit. He told me that he thought his problem was basically a “bad switch” in his neck where the tumor had been, a kind of loose wire sending false signals to his brain. But I told him about the increasing evidence that our sensory experiences are not sent to the brain but originate in it. When I got to the example of phantom-limb sensations, he perked up. The experiences of phantom-limb patients sounded familiar to him. When I mentioned that he might want to try the mirror-box treatment, he agreed. “I have a mirror upstairs,” he said.

He brought a cheval glass down to the living room, and I had him stand with his chest against the side of it, so that his troublesome left arm was behind it and his normal right arm was in front. He tipped his head so that when he looked into the mirror the image of his right arm seemed to occupy the same position as his left arm. Then I had him wave his arms, his actual arms, as if he were conducting an orchestra.

The first thing he expressed was disappointment. “It isn’t quite like looking at my left hand,” he said. But then suddenly it was.

“Wow!” he said. “Now, this is odd.”

After a moment or two, I noticed that he had stopped moving his left arm. Yet he reported that he still felt as if it were moving. What’s more, the sensations in it had changed dramatically. For the first time in eleven years, he felt his left hand “snap” back to normal size. He felt the burning pain in his arm diminish. And the itch, too, was dulled.

“This is positively bizarre,” he said.

He still felt the pain and the itch in his neck and shoulder, where the image in the mirror cut off. And, when he came away from the mirror, the aberrant sensations in his left arm returned. He began using the mirror a few times a day, for fifteen minutes or so at a stretch, and I checked in with him periodically.

“What’s most dramatic is the change in the size of my hand,” he says. After a couple of weeks, his hand returned to feeling normal in size all day long.

The mirror also provided the first effective treatment he has had for the flares of itch and pain that sporadically seize him. Where once he could do nothing but sit and wait for the torment to subside—it sometimes took an hour or more—he now just pulls out the mirror. “I’ve never had anything like this before,” he said. “It’s my magic mirror.”

There have been other, isolated successes with mirror treatment. In Bath, England, several patients suffering from what is called complex regional pain syndrome—severe, disabling limb sensations of unknown cause—were reported to have experienced complete resolution after six weeks of mirror therapy. In California, mirror therapy helped stroke patients recover from a condition known as hemineglect, which produces something like the opposite of a phantom limb—these patients have a part of the body they no longer realize is theirs.

Such findings open up a fascinating prospect: perhaps many patients whom doctors treat as having a nerve injury or a disease have, instead, what might be called sensor syndromes. When your car’s dashboard warning light keeps telling you that there is an engine failure, but the mechanics can’t find anything wrong, the sensor itself may be the problem. This is no less true for human beings. Our sensations of pain, itch, nausea, and fatigue are normally protective. Unmoored from physical reality, however, they can become a nightmare: M., with her intractable itching, and H., with his constellation of strange symptoms—but perhaps also the hundreds of thousands of people in the United States alone who suffer from conditions like chronic back pain, fibromyalgia, chronic pelvic pain, tinnitus, temporomandibular joint disorder, or repetitive strain injury, where, typically, no amount of imaging, nerve testing, or surgery manages to uncover an anatomical explanation. Doctors have persisted in treating these conditions as nerve or tissue problems—engine failures, as it were. We get under the hood and remove this, replace that, snip some wires. Yet still the sensor keeps going off.

So we get frustrated. “There’s nothing wrong,” we’ll insist. And, the next thing you know, we’re treating the driver instead of the problem. We prescribe tranquillizers, antidepressants, escalating doses of narcotics. And the drugs often do make it easier for people to ignore the sensors, even if they are wired right into the brain. The mirror treatment, by contrast, targets the deranged sensor system itself. It essentially takes a misfiring sensor—a warning system functioning under an illusion that something is terribly wrong out in the world it monitors—and feeds it an alternate set of signals that calm it down. The new signals may even reset the sensor.

This may help explain, for example, the success of the advice that back specialists now commonly give. Work through the pain, they tell many of their patients, and, surprisingly often, the pain goes away. It had been a mystifying phenomenon. But the picture now seems clearer. Most chronic back pain starts as an acute back pain—say, after a fall. Usually, the pain subsides as the injury heals. But in some cases the pain sensors continue to light up long after the tissue damage is gone. In such instances, working through the pain may offer the brain contradictory feedback—a signal that ordinary activity does not, in fact, cause physical harm. And so the sensor resets.

This understanding of sensation points to an entire new array of potential treatments—based not on drugs or surgery but, instead, on the careful manipulation of our perceptions. Researchers at the University of Manchester, in England, have gone a step beyond mirrors and fashioned an immersive virtual-reality system for treating patients with phantom-limb pain. Detectors transpose movement of real limbs into a virtual world where patients feel they are actually moving, stretching, even playing a ballgame. So far, five patients have tried the system, and they have all experienced a reduction in pain. Whether those results will last has yet to be established. But the approach raises the possibility of designing similar systems to help patients with other sensor syndromes. How, one wonders, would someone with chronic back pain fare in a virtual world? The Manchester study suggests that there may be many ways to fight our phantoms.