Atul Gwande, a third generation doctor and author, writes in the New Yorker about a woman with an uncontrollable itch on her scalp. Uncontrollable to the point that she scratched herself clear into brain tissue and had to be institutionalized for a time.

Doctors had no answers.

When she took a biopsy of the itchy skin, it showed that ninety-six per cent of the nerve fibres were gone. So why was the itch so intense?

Oaklander came up with two theories. The first was that those few remaining nerve fibres were itch fibres and, with no other fibres around to offer competing signals, they had become constantly active. The second theory was the opposite. The nerves were dead, but perhaps the itch system in M.’s brain had gone haywire, running on a loop all its own.

The second theory seemed less likely. If the nerves to her scalp were dead, how would you explain the relief she got from scratching, or from the local anesthetic? Indeed, how could you explain the itch in the first place? An itch without nerve endings didn’t make sense. The neurosurgeons stuck with the first theory; they offered to cut the main sensory nerve to the front of M.’s scalp and abolish the itching permanently. Oaklander, however, thought that the second theory was the right one—that this was a brain problem, not a nerve problem—and that cutting the nerve would do more harm than good. She argued with the neurosurgeons, and she advised M. not to let them do any cutting.

M did have the nerve cut and it worked for a while, but then the itch returned, just as intense.

A new scientific understanding of perception has emerged in the past few decades, and it has overturned classical, centuries-long beliefs about how our brains work—though it has apparently not penetrated the medical world yet. The old understanding of perception is what neuroscientists call “the naïve view,” and it is the view that most people, in or out of medicine, still have. We’re inclined to think that people normally perceive things in the world directly. We believe that the hardness of a rock, the coldness of an ice cube, the itchiness of a sweater are picked up by our nerve endings, transmitted through the spinal cord like a message through a wire, and decoded by the brain.

In a 1710 “Treatise Concerning the Principles of Human Knowledge,” the Irish philosopher George Berkeley objected to this view. We do not know the world of objects, he argued; we know only our mental ideas of objects. “Light and colours, heat and cold, extension and figures—in a word, the things we see and feel—what are they but so many sensations, notions, ideas?” Indeed, he concluded, the objects of the world are likely just inventions of the mind, put in there by God. To which Samuel Johnson famously responded by kicking a large stone and declaring, “I refute it thus!”

Still, Berkeley had recognized some serious flaws in the direct-perception theory—in the notion that when we see, hear, or feel we are just taking in the sights, sounds, and textures of the world. For one thing, it cannot explain how we experience things that seem physically real but aren’t: sensations of itching that arise from nothing more than itchy thoughts; dreams that can seem indistinguishable from reality; phantom sensations that amputees have in their missing limbs. And, the more we examine the actual nerve transmissions we receive from the world outside, the more inadequate they seem.

Our assumption had been that the sensory data we receive from our eyes, ears, nose, fingers, and so on contain all the information that we need for perception, and that perception must work something like a radio. It’s hard to conceive that a Boston Symphony Orchestra concert is in a radio wave. But it is. So you might think that it’s the same with the signals we receive—that if you hooked up someone’s nerves to a monitor you could watch what the person is experiencing as if it were a television show.

Yet, as scientists set about analyzing the signals, they found them to be radically impoverished. Suppose someone is viewing a tree in a clearing. Given simply the transmissions along the optic nerve from the light entering the eye, one would not be able to reconstruct the three-dimensionality, or the distance, or the detail of the bark—attributes that we perceive instantly.

So the new theory posits that the brain takes some sensory input data, and then based on prior experiences, creates a perception. Most of the time, it works. But with such things as phantom limb syndrome — where an amputee feels pain in a missing arm or leg — the brain gets it wrong.

Could a similar thing be causing the itch?

The article mentions Dr. V. S. Ramachandran, a neuroscientist at UC San Diego, has done trailblazing research on how the brain maps the body. His book, Phantoms in the Brain, is a fascinating read.

And here’s a video.