Smithsonian.com

Photo: Deltamike

At some point, every kid has learned this cold, hard fact of life: it’s impossible to tickle yourself. But why? As the Brain Bank explains, the simple answer is that you’re already expecting the sensation, so you’re less likely to react as you would if being tickle tortured by a big brother or a devious friend.

But expectation alone is not the full answer. Functional magnetic resonance imaging shows that activity in the brain responsible for tickle panic attacks is comparable when people are tickled unexpectedly and when they’re warned that tickling is about to transpire. The distinction, however, is that the brain categorizes self-produced movements as less significant than those initiated by external forces. When people try to tickle themselves, the tell-tale area of the brain shows little if any reaction.

But to every rule there is an exception. Schizophrenia can mean that people with the disorder are able to tickle themselves. Researchers think this might be because neurological changes in the schizophrenic brian disable the person’s ability to differentiate self-initiated actions. Schizophrenic patients are aware of their own intentions, but while they can process that movement has occurred, they can’t link the resulting tickling sensation with the fact that they’re responsible for the tickling themselves. They may also experience self-induced phantom tickling.

More from Smithsonian.com:

Humans Don’t Have the Last, or Only, Laugh