Why you can't tickle yourself

Tickle your own palm and almost nothing happens. Experiments with a pair of robots in London showed why: the brain predicts the touch it is about to cause and turns it down.

Run your fingers lightly over the palm of your hand, then have someone else do exactly the same. Most people find the second far more ticklish. The touch is the same. What differs is who caused it.

In the late 1990s the neuroscientist Sarah-Jayne Blakemore, working with Daniel Wolpert and Chris Frith at University College London, set out to explain why. Their idea came from how the brain controls movement. When it sends a command to the muscles, it also predicts what that movement will feel like, using what researchers call an internal forward model. If the sensation that arrives matches the prediction, the brain can turn it down.

Robots in the way

To test this, they built a setup with two robots. A light touch moved across the palm of a volunteer's right hand. Sometimes a robot produced it on its own. Other times the volunteer moved a robot arm with their left hand, and a second robot passed that movement on to the right palm. Volunteers consistently rated the self-made touch as less tickly, less intense and less pleasant.

Then the team broke the match between movement and touch. They added a delay between the left hand's movement and the touch on the right, or made the touch move in a different direction from the hand. The longer the delay, or the bigger the change in direction, the more ticklish the self-made touch felt. Once the prediction stopped fitting the sensation, the brain stopped cancelling it.

Brain scans pointed to where this happens. In a 1998 fMRI study by the same group, the somatosensory cortex, which processes touch, was more active when the touch came from outside. The cerebellum responded differently when a movement produced a touch than when it didn't, which suggested that it helps predict what a movement will feel like.

The mechanism may matter well beyond tickling. In a 2000 review titled Why can't you tickle yourself?, the researchers pointed to evidence that this prediction works abnormally in people who hear voices, or who feel that their actions are being controlled by someone else.

So the dullness of your own fingers isn't a failure of feeling. It's the brain filtering out what it already expects, which leaves it free to notice what it didn't.