The vivid sensation of a missing limb is experienced by almost everyone who has had a limb amputated. Lord Nelson, the naval commander who lost his right arm in battle, declared that the phantom sensations in his missing right arm were proof of the existence of a soul. Today we needn’t take recourse to the mystical, since we know that the brain holds maps of the body that can be independent of the body parts it represents.

In his book Phantoms in the Brain, the neuroscientist Dr V S Ramachandran describes a woman born without arms who reported having phantom hands, which she used to gesticulate when she spoke. Other parts of the body have been known to come in phantom versions too: women who have undergone mastectomies report phantom breasts; people whose bladders have been removed still feel the strong urge to go; men who have undergone penectomies report phantom erections. One night, years after his leg was gone, Stephen woke up at 4am, swung his phantom leg off the bed, and crashed to the floor on his stump, necessitating a bloody trip to hospital. Even today, ten years after his amputation, he can feel a sore he had on his heel from a cycling shoe.

Phantom limbs might be a strange, even occasionally reassuring, phenomenon if they didn’t hurt so much. Somewhere between 50 and 80 per cent of all amputees complain of pain in their phantom limbs. In the past, a few doctors believed phantom limbs were a form of wishful thinking on the part of amputees, and that pain in the missing limb had to be psychological in origin. But most believed the pain was caused by damaged nerves near the stump. So they tried to treat phantom pain by shortening the stump, which sometimes gave relief, but seldom for long.

Then, in the early 1990s, Dr Ramachandran and his colleagues at the University of California, San Diego, conducted simple experiments with amputees that changed the understanding of phantom limbs and sensations. When they stroked the left side of the face of a young man who had recently lost his left arm, he felt sensations not only on his face but also on his phantom hand.

They already knew that the brain’s cortex has superimposed on it a virtual map of the body corresponding to sensory inputs from different parts – and that that the face’s representation on this map is adjacent to the hand’s. Could the young man’s phantom sensations be the result of sensory inputs from his face ‘invading’ the now-deprived region of the cortex that mapped to his missing hand?

Brain imaging confirmed this was the case. Other researchers found that these rewired inputs might be activating neural pain pathways for the missing hand, or at least generating ‘junk’ signals that were interpreted by the brain as a range of sensations – including pain.

It was possible, too, that when signals sent to move the missing hand didn’t lead to any corresponding visual or sensory confirmation of the movement, this dissonance was perceived as pain. The brain is known to emphasise visual feedback over other types – which may also be the reason why passengers get carsick more often than drivers. (When a passenger reads in a car moving along a curvy road, the balance sensors in the inner ear report motion that differs from what the eyes are seeing, and the dissonance is thought to be expressed as nausea. Whereas for the driver, the balance sensors in the ears, the spatial sensors in the body and what the eyes report are all in reassuring agreement.)

Dr Ramachandran and his group wondered if visual feedback of the phantom limb’s movement might help relieve pain in it. They put together what they called the ‘mirror box’ – a simple but ingenious contraption that hid the stump while allowing a reflection of the intact limb to be superimposed over the phantom limb. Now, if the amputee moved the intact and phantom limbs in sync, the brain could ‘see’ the phantom limb move.

The first amputee to try the mirror box reported being able to move his phantom limb for the first time in over a decade, and he felt immediate relief from pain. Subsequent users too found they could manage their phantom pain using the box.

Surgery and medication have been found to be only slightly or not at all effective when dealing with phantom pain. Stephen knew this, and he tried to will his phantom pain away: “Optimism. Mind over matter. I thought I could beat it.” But it kept coming back, and it kept getting worse. “Then I tried to drink it to death, which was costly and messy in every conceivable sense, plus totally ineffective.”

In 2008, Stephen was working as a property manager in south Baja, Mexico, when he had a particularly agonising bout of phantom pain. “I was not presentable for 72 hours,” he says. He was aware of mirror therapy from having looked online for treatments, and he decided to give it a try. He got into his truck and drove two-and-a-half hours to the nearest Home Depot to buy a mirror. He tried it right there in the parking lot, and in five minutes the pain was gone.

Stephen used the mirror for two weeks, then stopped because the pain had not returned. About a year and a half later, he felt the pain again, and this time he stayed the course for the full five weeks. He hasn’t had phantom pain for over four years. “It’s gone now,” he says. “It’s gone because I treated myself with a mirror.”