IF YOU are ever sitting out on the ocean in a small boat and a bad tempered great white shark pops its head out of the water, then 鈥渒nismesis鈥� is the life-saving word that should spring to mind. Knismesis is a light tickle. Simply cup your hand over the great white鈥檚 snout and gently tickle. Peter Benchley, the author of Jaws, saw it done off the South African coast and describes the result in Shark! his very latest book. The great white, he says, 鈥渞ose out of the water, gaped for several seconds as if hypnotised, then slipped backward, down and away, in what I can only describe as a swoon鈥�.
What is it about tickling that keeps even a shark happy? Knismesis produces pleasurable sensations, but tickling isn鈥檛 always pleasurable. Gargalesis is a harder kind of tickle which mixes pleasure and pain and produces squealing in infant humans, and something rather similar in chimpanzees. Both words were coined by psychologists Stanley Hall and Arthur Allin back in 1897.
Has there been progress in understanding tickling since then? A little. The pleasure of knismesis seem be related to low levels of stimulation to highly sensitive areas of the body. Take the great white. Sharks track prey using a concentration of specialised sensory organs around their snout. This includes nostrils that can detect a drop of blood in an Olympic-sized pool and gel-filled canals that can detect bioelectrical impulses from potential prey. Low levels of stimulation here are pleasurable. A high level is very painful 鈥� hence the advice to hapless scuba divers to kick a shark on the nose if all else fails.
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Gargalesis is more difficult to explain, as it mixes pleasant and unpleasant. It is certain that you cannot tickle yourself. Brain scanner studies at University College London show that when you are tickled, parts of your sensory cortex light up. But if you tickle yourself the message is blocked, probably because the cerebellum, which coordinates voluntary movement, sends out a signal saying what incoming messages to expect.
Interestingly, the message is not blocked in people suffering from schizophrenia. Their sensory cortex lights up no matter who does the tickling. This helps explain why schizophrenics can have difficulty distinguishing between internally and externally generated stimuli.
But just why do we have such mixed reactions to gargalesis? By laughing we encourage the tickler even though the experience can be torture. One theory is that the laughter keeps the game going while discomfort motivates children to develop their defensive reactions. In other words, gargalesis has evolved to help children develop in rough and tumble play. So next time you tickle your friends, tell them you are doing them a favour by honing their senses for action.