Playful punch can be big pain

Detroit Free Press - July 11, 2003

When you give a friend a playful punch, he or she might feel real pain.

A recent study shows that it is because you may really not be aware of the force you are applying and because of the way your brain is wired, British researchers said Thursday.

"The key message is our findings show we are not as aware of our actions as we think," said Daniel Wolpert of University College London, who led the study.

To try to understand what, neurologically, takes place during such force escalation, the researchers conducted a tit-for-tat experiment using six pairs of young adults. The left forefinger of each person was hooked up to a small force transducer to measure the amount of force applied. One member of each pair applied pressure to the other's left index finger. Each person was then instructed to return the favor, using the same force that was given. The participants went back and forth and responses quickly escalated.

When the subjects were asked to apply force via a joystick -- pushing up for more pressure and down for less -- the pressure given and received matched each other much more accurately.

What seems to be happening is that, just before you make a movement, your body sends a warning signal to the brain. That signal causes you to apply more force than you think you are using.

This also explains why you can't tickle yourself, because your brain is warned in advance.

The response probably has an evolutionary advantage. "It's protective," says Dr. Eric Braverman, director of PATH Medical in New York. "It's designed as a survival instinct: 'I've got to kill you before you kill me.' "

Wolpert said the findings could also be a warning to parents who spank their children.

"It may well be when parents smack their children they don't realize how hard they are hitting," he said.

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