Why Can't You Tickle Yourself? - BBC Future

It’s almost impossible to get a laugh by self-tickling, says David Robson, and the reason why tells us surprising things about the brain and consciousness.

If you want to probe some of the great mysteries of the human mind, all you need is a duster and your feet. Sit back, take your shoes and socks off, and gently stroke its feathers against your sole. Now ask a friend, parent or child to do the same for you. If you are like most people, you will be left stony-faced by one, but convulsed in a pleasurable agony by the other. How come?

Once the domain of childhood curiosity, the question of why we can’t tickle ourselves is exciting neuroscientists. “It leads to these bigger questions of consciousness and self-awareness, who we are,” says George Van Doorn at Monash University in Australia. For this reason, they are now going to some – often extreme – lengths to overcome the brain’s barriers and to get people to tickle themselves in the lab.

To understand their interest, consider this: every time your body moves, it creates potentially confusing sensations that could lead you astray in all kinds of ways. Just imagine the chaos if you assumed that someone was fondling or attacking you, every time one of your hands brushed your leg, for example. Being able to differentiate between your movements, and the actions of other people, is therefore a central part of our sense of self and agency – aspects of the psyche that even the most sophisticated robots can’t replicate. And examining these kinds of traits, you want to find an example that is easily replicated in the lab. “Tickling is a nice example because the contrast between ticklish sensations produced by others and the inability to tickle oneself is so clear,” says Jennifer Windt at the Johannes Gutenberg University of Mainz in Germany.

Sarah-Jayne Blakemore, at University College London, was one of the first to investigate the way the brain makes these lightning-fast decisions about the self and others. She scanned subjects’ brains as her colleagues tickled the palms of their hands, and when they attempted to do so themselves. From the resulting brain activity, she concluded that whenever we move our limbs, the brain’s cerebellum produces precise predictions of the body’s movements, and then sends a second shadow signal that damps down activity in the somatosensory cortex – where tactile feelings are processed. The result is that when we tickle ourselves, we don’t feel the sensations with the same intensity as if they had come from someone else, and so we remain calm rather than writhing with that familiar mix of discomfort and pleasure that comes when someone else tickles us.