Someone else can tickle your fancy, but you can't tickle your own. Analysis has shown that areas of the brain linked to touch and pleasurable sensations respond much more powerfully when tickled by another than if you try to tickle yourself.
There is little surprising in the fact that tickling yourself isn't much of a laugh, but scientists were curious about what was happening in the brain to make this so.
"We were interested in how the brain filters information . . . how it gets important information and gets rid of the information of less importance," said Ms SarahJayne Blakemore of the Well come department of cognitive neurology at University College London.
This was a crucial skill, she said, because our reactiveness to external stimuli could become a matter of life or death. It was biologically important for survival to be able to know instantly whether a touch represented self or external stimulation.
The distinction between the two could be lost in certain conditions, she said. Schizophrenics who suffer "auditory hallucinations" could not distinguish between external and self-generated speech, and people with "passivity phenomenon" claimed others were controlling their movements and said they were being touched when this was not the case.
The research group produced a "tickle robot" which could deliver a consistent tickle to the palm, whether self-administered or given by the experimenter. The subjects' brains were scanned to study any differences in brain activity while the tickles were being felt.
All subjects thought the external tickle was both more ticklish and more pleasurable. Two areas of the fore brain were linked to the sensation: the somatosensory cortex, responsible for touch, and the anterior singulat cortex, associated with pleasant feelings, Ms Blakemore said.
More activity was noted when the external tickle occurred, but all tickles caused some reaction. The group believed that the self-tickle response was being damped down by the cerebellum, at the rear of the brain.
"It is very important in movement control and in predicting the consequences of movement," she said. They believed the cerebellum was sending a message saying the touch was self and of no importance, causing less of a reaction in the two front areas of the brain.
They further tested this by using the robot tickler to cause momentary delays between the subject's self-tickling movement and the delivery of the tickle. This short delay of several hundred microseconds was enough to cause subjects to respond more strongly even when the subject was doing the tickling.
