"The visual neurons are still intact, and they're firing off, telling the brain one thing," Tsao said. "The propriaceptive neurons are firing off, telling the brain something else. ...My thinking is that there is some sort of center in the brain that coordinates these signals. ... Somehow, this mismatched feedback is what's generating the sensation that the limb is frozen or in pain."
The Exploratorium in SF used to have an exhibit consisting of two tightly interwoven coils, about the right size to grasp. Push one button, and tepid water would flow through one coil. Push the other button, and very cold water would flow through the other. The amazing thing was that putting one hand on the coils while pushing both buttons with the other wouldn't result in a cool sensation, midway between the two, but instead a very hot, burning sensation.
Apparently the brain gets one set of signals encoding (heat,mild) but another set that encodes (cold,extreme) and resolves them by generating a (heat,extreme) sensation.
One might argue that we're only set up to pass the evolutionarily-run unit tests; neither steep temperature gradients nor limb loss are common enough to warrant implementing accurate responses.
-Dave
