If you have a brain, and I believe most readers of this blog do, you may want to read an article in this week’s New Yorker by Atul Gawande.
The article begins by describing the symptoms of a patient, M, who had a phantom itch. Ultimately her scratching became so severe that, “She had scratched through her skull during the night—and all the way into her brain.”
There are nerves specifically that convey itch, but after describing them Gawande goes on to discuss the larger phenomenon of phantom itches and phantom pain — the reader who starts to scratch while reading The New Yorker article about itching, the amputee who still feels his arm. Possibly these are, the medical professor suggests, perceptions of the brain and not, as has been long thought, neurological misfires. The brain is far more a part of our perception than we generally think.
The images in our mind are extraordinarily rich. We can tell if something is liquid or solid, heavy or light, dead or alive. But the information we work from is poor—a distorted, two-dimensional transmission with entire spots missing. So the mind fills in most of the picture. You can get a sense of this from brain-anatomy studies. If visual sensations were primarily received rather than constructed by the brain, you’d expect that most of the fibres going to the brain’s primary visual cortex would come from the retina. Instead, scientists have found that only twenty per cent do; eighty per cent come downward from regions of the brain governing functions like memory. Richard Gregory, a prominent British neuropsychologist, estimates that visual perception is more than ninety per cent memory and less than ten per cent sensory nerve signals.
Maybe there is nothing wrong with the engine. Maybe sometimes it’s just the dashboard sensor that’s broken.
[P]erhaps 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. …
Fascinating.
(2006) and this year’s 
.