Atul Gawande has an excellent article on itchiness in this week’s New Yorker. Money quote:
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.
Gawande claims that an itch is the same thing as a desire to scratch. I’d be interested in reading more about how our desires are bound up with our perceptions. Gawande offers only the following tantalizing preview:
Unlike, say, the nerve fibres for pain, each of which covers a millimetre-size territory, a single itch fibre can pick up an itchy sensation more than three inches away. The fibres also turned out to have extraordinarily low conduction speeds, which explained why itchiness is so slow to build and so slow to subside.
Other researchers traced these fibres to the spinal cord and all the way to the brain. Examining functional PET-scan studies in healthy human subjects who had been given mosquito-bite-like histamine injections, they found a distinct signature of itch activity. Several specific areas of the brain light up: the part of the cortex that tells you where on your body the sensation occurs; the region that governs your emotional responses, reflecting the disagreeable nature of itch; and the limbic and motor areas that process irresistible urges (such as the urge to use drugs, among the addicted, or to overeat, among the obese), reflecting the ferocious impulse to scratch.