Tickling yourself is impossible even in another's shoes

Life 20 November 2013

Can you tickle yourself if you are fooled into thinking that someone else is tickling you? A new experiment says no, challenging a widely accepted theory about how our brains work.

It is well known that we can’t tickle ourselves. In 2000, Sarah-Jayne Blakemore of University College London (UCL) and colleagues came up with a possible explanation. When we intend to move, the brain sends commands to the muscles, but also predicts the sensory consequences of the impending movement. When the prediction matches the actual sensations that arise, the brain dampens down its response to those sensations. This prevents us from tickling ourselves (NeuroReport, DOI: 10.1097/00001756-200008030-00002).

Jakob Hohwy and George Van Doorn of Monash University in Clayton, Australia, and colleagues decided to do a tickle test while simultaneously subjecting people to a body swap illusion.

In this illusion, the volunteer and experimenter sat facing each other. The subject wore goggles that displayed the feed from a head-mounted camera. In some cases the camera was mounted on the subject’s head, so that they saw things from their own perspective, while in others it was mounted on the experimenter’s head, providing the subject with the experimenter’s perspective.


Using their right hands, both the subject and the experimenter held on to opposite ends of a wooden rod, which had a piece of foam attached to each end. The subject and experimenter placed their left palms against the foam at their end. Next, the subject or the experimenter took turns to move the rod with their right hand, causing the piece of foam to tickle both of their left palms.

Mind bending

In situations where the subjects saw the scene from the experimenter’s perspective, they experienced a body-swap illusion, which resulted in them feeling like they owned the experimenter’s body. The subjects agreed with statements like “I had lost control of my real hand” and “The experimenter’s hand began to resemble my own”.

If the experimenter tickled the subject while they were under the illusion, the subject could feel the tickle on their palm. But if the subject initiated the action while still under the illusion – attempting to tickle themselves – the subject didn’t feel the tickle on their left palm, even though it felt as though the experimenter was moving the rod. In other words, they couldn’t tickle themselves, even under the influence of the illusion.

This goes against the ideas of Blakemore and colleagues. If the brain makes fine-grained predictions based on actual motor commands to figure out when to dampen the tickle response, then those predictions should fail during the illusion, since the command to move doesn’t result in the sight of your own hand moving – enabling one to tickle oneself.

Hohwy thinks that the work lends support to an idea called “active inference”, developed by Karl Friston, also at UCL, and his colleagues, which argues that the brain attenuates incoming sensations anytime we make a movement. “[So] you should not be able to tickle yourself, no matter what the context,” says Hohwy.

Anil Seth, a cognitive neuroscientist at the University of Sussex in Brighton, UK, agrees that the work is consistent with the active inference hypothesis. “It is indeed fascinating that sensory attenuation can still occur in this highly unusual and surprising context [of body swapping],” he says.

Journal reference: Consciousness and Cognition, in press