If you want to probe one 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 the feathers against your sole. Now ask a friend to do the same for you. If you are like most people, you will be left stone-faced by one but convulsed in ticklish agony by the other. Why?
Once the domain of childhood curiosity, the question of why we can’t tickle ourselves is now exciting neuroscientists. To understand their interest, consider this: Every time your body moves, it creates sensations that could potentially confuse you in all kinds of ways. Just imagine the chaos if every time one of your hands brushed your leg, you assumed that someone was fondling or attacking you.
Being able to distinguish between your movement and the actions of others is, therefore, a central part of our sense of self and agency, aspects of the psyche that even the smartest robots can’t replicate—yet. Your brain playing tricks on you is just one of the facts you never knew about your own body.
Sarah-Jayne Blakemore, of 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 as the participants attempted to do so themselves.
Turns out, the two brain regions that are activated when we get tickled—the somatosensory cortex, which processes touch, and the anterior cingulate cortex, which processes pleasant sensations—are less active when we tickle ourselves than when we are tickled by someone else.
From the resulting brain activity, Blakemore 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 we would if they had come from someone else, and so we remain calm.
Blakemore suspected there could be ways to fool the process and allow people to tickle themselves. So she designed a machine that allowed her subjects to move a stick that gently stroked a piece of foam over their palm, sometimes instantaneously and at other times with a delay of up to 200 milliseconds. It turned out that the greater the delay, the more ticklish the foam felt, perhaps because the cerebellum’s predictions no longer matched what the person was actually feeling.
Many others have since tried to find ways to trick the brain into tickling itself. For instance, controlling someone’s foot movements with magnetic brain stimulation, so that the hand tickles the foot against the person’s will, seems to do the trick.
But other experiments have produced puzzling results. One study tried to give subjects an out-of-body experience before tickling them, by fitting them with video goggles that let them see from the eyes of the experimenter and by synchronizing their movements. Even with the subjects confused about which body they inhabited, they were largely unmoved when they pressed a button that tickled both bodies simultaneously. Another experiment, in which expert lucid dreamers tried to tickle themselves in their sleep, also failed.
It may seem random, but understanding the self-tickling barrier could answer more practical questions, like why many schizophrenics can tickle themselves or whether robots ever could. “Your inability to tickle yourself suggests neurologically-based definitions of self and other,” writes Robert Provine of the University of Maryland, Baltimore County.
“Developing a similar machine algorithm may lead to ‘ticklish’ robots [that can] distinguish touching from being touched and may provide a [new] construct of machine personhood.” If so, a duster could soon provide a bizarre new test for artificial intelligence: Just aim for the robot’s feet and see if it laughs.