Why you can't tickle yourself

If cells that respond to touch and vision are so exquisitely sensitive, why can’t you tickle yourself? You can wiggle all ten of your fingers and drawn them close to your belly, yet you won’t feel a flutter of anticipation or mirthful desire to pull away. You’re a zombie when it comes to self-inflicted tickling.

The reason has to do with the fact that your brain is in the business of predicting your interactions with the world. When someone else tickles you, there is a sense of surprise and panic. Those other fingers feel like creepy crawlies, making you howl in protest. But they aren’t dangerous creepy crawlies, and so you laugh with relief.

When your own fingers are involved, though, there is no surprise. You are in control. Your touch is familiar. Your brain predicts the force, location, and speed of your movements and, this being ho-hum, cancels out or attenuates the sensation of self-touch

Your brain carries out this feat by generating a carbon copy of your movements along with the actual motor command. The duplicate, called on efference copy, (en.wikipedia.org/wiki/Efference_copy) predicts the effects of your action, such as bringing your wiggly fingers toward your body. The predicted effect of the efference copy and the actual sensation from your motor command are compared. If there’s a mis-match, you know the sensation came from the outside world (someone else’s fingers). But if there is no mismatch, your prediction is deemed accurate. You do not feel a tickling sensation.

Incidentally, the reason you can’t tickle yourself also explains why shoving matches tend to escalate into fistfights. In a study called “Two Eyes for an Eye: The Neuroscience of Force Escalation,” people held out their left index finger which was given a slight push by a motor. Then they were asked to make the force by pressing a force transducer that delivered a second push to the same finger.

People consistently pushed back harder with the transducer. Just as when you try to tickle yourself, your brain predicts the consequences of your movements and reduces the sensation. So when somebody hits you, you will hit back with greater force because your brain overestimates how hard you were hit. This is one way that street brawls spiral out of control. Or children take up the refrain: But she hit me harder! ~ Page 118 (The Body Has a Mind of Its Own)