Why Self-Tickling is Impossible

L O N D O N,   Sept. 11, 2000 -- Scientists may have unraveled a mystery which has puzzled them and millions of children foryears — why is it impossible to tickle yourself?

The Daily Telegraph said today the secret lies in thecerebellum, a region at the back of the brain which predicts thesensory consequences of movements and sends signals to the restof the brain instructing it to ignore the resulting sensation.

Sarah-Jayne Blakemore of the University College of Londonexamined six volunteers using magnetic resonance imaging to scantheir brains as their palms were tickled by a machine. The scanwas repeated while they tickled their own palms.

Tickler Warning

In the first case the machine succeeded in tickling thevolunteer because the cerebellum cannot warn the rest of thebrain when the stimulus is external, even if the brain knows itis about to be tickled.

The mechanism once protected us against predators bydistinguishing between stimuli that were created ourselves andthose generated externally.

But the system can be fooled.

When the robot used by the volunteers to tickle themselvesdelayed the action by a fraction of a second, the ticklingsensation was there.

“So it is possible to tickle yourself, but only by usingrobots,” Blakemore said.