Ever scratched your arm and felt a scratch over your ribs? How about pinching your leg and feeling a phantom twinge in your back? That sensation is called referred itch or mitempfindung. And here's why scientists think it happens.
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Mitempfindung got its name from Johannes Muller, a German physiology professor who did a lot of work in the mid-1800s, finding out about the anatomy of everything from humans to frogs. It has been recorded all the back in the 1700s, and was probably wondered about well before then. In English, it goes by the name "referred itch," which is less challenging for the tongue. Considering how many people it affects, it's kept a low profile. You've probably heard it spoken of as, "that weird thing where you scratch one place, but feel a scratch in a totally other place."
Referred itch is mostly a subtle thing. If I pinch a spot on my right arm, I feel a twinge on the right side of my rib cage. Some people notice that, when they scratch their back or stomach, they feel something scratching their legs.
Occasionally it gets more dramatic. One man, who had undergone a heart transplant, noticed that when people touched his right ear, he had violent coughing spells. He felt the "tickle" in his throat. For the most part, referred itch does no damage, and so it hasn't been particularly well-studied. One study noted that people who have synesthesia had four times the base level of referred itch, but that study also indicated that only ten percent of the rest of the population had encountered it. Other studies have found that between fifty and ninety percent of people have experienced the phenomenon.
There are quite a few theories on what cause the itch. One is simply that, when people develop as embryos, certain nerves branch out more expansively than others. That would explain why the connected spots on the body are different from person to person. Generally the connected spots are on the arms, legs, and torso, but there are cases of connection between the ear and the throat, or the thumb and the tongue. There's also a theory that damage to the spinocervical pathway, a fast route between the nerves and the brain, might result in hyperactivity of the pathway's neurons. That, though, would probably produce a body-wide sensation, instead of sensation at discrete sites.
In the end, the twin twinges might be the result of the fact that the brain has to pack all the sensations of a large body into a small amount of space. In the cerebral cortex and the thalamus, both brain areas that deal with touch, regions in charge of the torso and the limbs overlap. A jangled nerve may excite a nearby nerve in the brain - but those two neighboring nerves deal with very different regions of the body.
Have you experienced mitempfindung? If so, try this experiment. Scratch one of your twin sites for a good long while. The brain doesn't just play tricks with space, but with time as well. Often you'll feel a scratching sensation in a spot (any spot) well after you stopped. But do you feel a scratching in both? Let us know your mitempfindung stories!
Image: Brain Maps
[Via Cortex, Brain, European Neurology, BMJ.]