"The leading theory holds that tickling provokes laughter thanks to a prediction error by the brain. An unpredictable touch confuses it, sending it into a mini frenzy. Self-touch is always predictable … so, no frenzy.
But Brecht thinks it’s not really about prediction. Instead, he suggests that as a person touches themselves, the brain sends out a body-wide message that inhibits touch sensitivity. “We think what is happening is the brain has a trick to know: As soon as you touch yourself, don’t listen,” he says. If it didn’t, he argues, we’d all be constantly tickling ourselves every time we scratched an armpit or touched our toes."
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The question was mildly interesting but I'm pretty grumpy about reading long, long, loooong articles that are supposed to answer N questions, where the answers are buried deep in some random location/s... :-p I guess this one is just around 4/5 pages or so, but the answer fits in just two paragraphs.