Ever scratch an itch on your stomach and feel the scratch on your elbow? Or scratch your leg and feel a prickly sensation on your neck? You’re not alone. Lots of people report feeling a sensation from a scratch in places far from where they’re scratching, including scratches on the ear that produce tickles in the throat and mixed signals between their thumb and their tongue.
It’s one of those delightfully weird things your body does, and it’s been studied, albeit infrequently, for hundreds of years. English clergyman/scientist Stephen Hales first described the phenomenon in the 1730s, referring to it as “instances of the sympathy of the nerves.” About a century later, physiologist Johannes Muller came along and applied one of those cool words to it that the Germans are really good at coming up with: mitempfindungen. Since then, we’ve just called it “referred itch” in English.
For most people, referred itch is just a weird annoyance. It doesn’t do any damage or have any ill effects, and so it doesn’t get a whole lot of research attention. Scientists haven’t even been able to nail down how common it is. One study found that it only happens in 10 percent of the population. Another said 20 percent. Still others put the figure at anywhere between half and ninety percent of people.
How and why it happens is anybody’s guess, but scientists have a handful of hypotheses to explain what’s going on. One idea is that referred itch is the product of our complex system of nerves being “rather irregularly distributed” throughout the body. British physician Phillip Evans wrote in the 1970s that it was difficult for him to envision “such a diffuse system providing sensations that are so clearly imagined to be in small and easily localised areas of skin.” In other words, in our haphazard map of nerves, some sensations are bound to be confused.
Another explanation in a similar vein is that some branches of a nerve may travel out in a much different path than the others and wind up in some far-flung part of the body. Again, the nervous system gets a little confused and a “stimulus at the end of one branch is interpreted as coming from a point of ending of the other.”
It could also be that the mix up isn’t in the nerves, but the brain. In the parts of the brain that deal with our sense of touch, regions that receive and process information from different parts of the body overlap. “Hand and shoulder areas … overlap the trunk area, and the area for the thumb overlaps that for the upper part of the tongue,” Evans wrote. “There is so much overlapping that it is difficult to see how referred itches arising from different parts of the body can be coherently separated in the cortex.”
Finally, for some people, chronic referred itches could happen in parts of the body “rendered abnormal by damage to the nervous system.”