The Itch

There are a few writers who manage to trigger a contradictory mixture of feelings in me: the joy of reading their prose is fused with the mild anguish of not having written their prose. It's one part status anxiety, a dash of jealousy and a big heaping of aesthetic appreciation. Atul Gawande is one of those writers. His latest article in the New Yorker, on the new science of itching, is a real gem, even by his bejeweled standards. The article opens with the story of a patient M., whose scalp was so itchy she managed to do permanent damage:

For M., the itching was so torturous, and the area so numb, that her scratching began to go through the skin. At a later office visit, her doctor found a silver-dollar-size patch of scalp where skin had been replaced by scab. M. tried bandaging her head, wearing caps to bed. But her fingernails would always find a way to her flesh, especially while she slept.

One morning, after she was awakened by her bedside alarm, she sat up and, she recalled, "this fluid came down my face, this greenish liquid." She pressed a square of gauze to her head and went to see her doctor again. M. showed the doctor the fluid on the dressing. The doctor looked closely at the wound. She shined a light on it and in M.'s eyes. Then she walked out of the room and called an ambulance. Only in the Emergency Department at Massachusetts General Hospital, after the doctors started swarming, and one told her she needed surgery now, did M. learn what had happened. She had scratched through her skull during the night--and all the way into her brain.

Gawande goes on to discuss the surprising connections between chronic itchiness and phantom limb syndrome. Essentially, both conditions occur when the brain is confronted with an unusual form of sensory silence. The cortex, it turns out, requires a constant barrage of inputs from the outside world. When those inputs disappear - when we lose a limb or suffer severe peripheral nerve damage - the mind compensates with vivid and painful hallucinations. We can't stand the quiet and so we invent some awful noise.

The account of perception that's starting to emerge is what we might call the "brain's best guess" theory of perception: perception is the brain's best guess about what is happening in the outside world. The mind integrates scattered, weak, rudimentary signals from a variety of sensory channels, information from past experiences, and hard-wired processes, and produces a sensory experience full of brain-provided color, sound, texture, and meaning. We see a friendly yellow Labrador bounding behind a picket fence not because that is the transmission we receive but because this is the perception our weaver-brain assembles as its best hypothesis of what is out there from the slivers of information we get. Perception is inference.

[SNIP]
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.