RACHEL MARTIN, HOST:
Now, let's talk about something deeply philosophical - tickling; more specifically, why you can tickle someone else, but you can't tickle yourself.
JAKOB HOHWY: It's a very basic kind of phenomenon that every child knows.
MARTIN: This is Jakob Hohwy. He's a professor of philosophy at Monash University, in Australia. He says the reason you can't tickle yourself is simple: You can't outsmart your body. It knows the tickle is coming. Hohwy and his colleagues, studying perception and the brain, wondered: If you thought you were someone else, could you tickle yourself then? - which led to their recent experiment.
HOHWY: So we wanted to swap people into other people's bodies.
MARTIN: Not literally. But turns out, it is possible to trick people into thinking they are outside their own body. Hohwy says you can even do this at home.
HOHWY: So you have to sit at a table, put your hand up on the table, and cover the hand behind a tray or a towel - or something like that. And then visible to you, you put a rubber hand. It might be stuffed-up dishwashing glove or a real rubber hand or a mannequin hand, or something like that. So you can see that. And then you ask another person sitting across from you to tap the rubber hand and the hidden hand in synchrony - so at the same time. So what you feel on your hidden real hand is a touch. And you can see a rubber hand - which is clearly, not yours - being touched as well, in synchrony. And then after a while, you get this illusion that the touch you can feel is located on the rubber hand.
MARTIN: Does that mean that our sense of sight trumps our sense of feeling?
HOHWY: Exactly.
MARTIN: In his tickling experiment, Hohwy used his own version of the rubber hand trick. He made his subjects wear a pair of video goggles hooked up to a camera on another person's head. And with good, old-fashioned synchrony, he got them to feel, as if they were actually the person sitting across the table. And in that moment, he had them try to tickle their palm.
HOHWY: And then we ask, how ticklish is it? And it turns out that when they do it themselves, they still can't tickle themselves.
MARTIN: It seems to be even more impossible than we ever thought to tickle yourself. Except...
HOHWY: When you ask people with schizophrenia to tickle themselves, then they can.
MARTIN: Really?
HOHWY: So yeah. It's quite striking. So one theory there is that people with schizophrenia are relatively poor at predicting what the sensory consequences will be of their own movement.
MARTIN: Are you saying that the brain is always making predictions about what's going to happen, or how your body's going to respond to something?
HOHWY: Yep. So both for the body itself but also for, you know, things out in the world. So when you turn your head around and you get new sensory input, the brain's constantly trying to be a little ahead of yourself and predicting what's going to happen there.
MARTIN: So when we're surprised, then that's crazy. That means the brain has been totally duped.
HOHWY: Yep, exactly.
MARTIN: That surprise? That is a tickle. And this is NPR News. Transcript provided by NPR, Copyright NPR.