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

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

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

Tickler Warning

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

The mechanism once protected us against predators by distinguishing between stimuli that were created ourselves and those generated externally.

But the system can be fooled.

When the robot used by the volunteers to tickle themselves delayed the action by a fraction of a second, the tickling sensation was there.

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