Ever tried to tickle yourself? It doesn't work, and now scientists know why.

The short answer: You know what you're up to, and the body adjusts ever so slightly to prepare. Anticipating your own touch wrecks the do-it-yourself tickle effect.

It's well known that you can't tickle yourself, researcher J. Randall Flanagan, PhD, says in a news release.

One explanation is that since all the sensations are completely predictable, we do 'sensory attenuation' which reduces our touch perception, continues Flanagan, who works at the Center for Neurosciences at Queen's University in Canada.

In other words, no surprise, no tickle.

Ticklish Topic

Flanagan and colleagues didn't ask people to tickle themselves. Instead, they studied 20 people who gave themselves finger taps. The results appear in Public Library of Science -- Biology.

Participants didn't directly tap on their own fingers. Instead, they tapped a machine, which in turn tapped one of their other fingers. Participants then rated the strength of the machine's taps.

Picture the machine as a stapler without staples, moving downward when touched by a finger to hit another finger below, where the paper would be in a real stapler.

Sometimes, the machine briefly delayed its tap. Participants rated those taps as being harder, but the taps weren't any different. The delay likely surprised participants, the researchers note.

In other trials, the scientists secretly removed a sensor that triggered the machine's taps. When participants tapped the machine, they never touched the sensor -- but they didn't know it.

In those cases, the researchers controlled the machine remotely, making it tap at the correct time. Participants didn't rate those taps differently, probably because they were expecting those taps, the researchers note.

Anticipation Is Key

Whether it's a tap or a tickle, people try to predict and prepare for touch, the study shows.

That skill comes in handy, Flanagan notes. If we try to deal with all the sensory information directed at us at any given time, it's overwhelming, he says.

Anticipation may also let people mentally rehearse movements before carrying them out, the researchers write.

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