The human brain anticipates unimportant sensations, such as your own touch, so it can focus on important input like, say, a tarantula crawling up your neck.
The results might explain why it's hard to tickle yourself, scientists said today.
In the study, 30 people used a finger on their right hand to touch a finger on their left hand by tapping a device place directly over the left finger and could instantly relay the tap. The computer-controlled device could introduce delays of varying length before the left finger was tapped. Researchers used another button to introduce externally generated taps.
Based on the test subjects' reports of what they felt, the sensation in the left finger was less during window of time centered on the instant any self-tapping would have occurred naturally.
Bottom line: When their brains expected a tap and the tap came as expected, the brain noticed it less.
"It lends support to the theory that the brain is constantly predicting what is about to happen, what sensations it's about to receive," said Paul Bays of the Institute of Neurology at University College London.
Why do our minds work this way?
The information we get from our senses is always a little out of date, because it takes time for the electrical signals to travel from the finger, ear or eye to the brain.
"Although this delay is only a fraction of a second, that is long enough to make impossible anything that involves accurate control over our bodies or moving objects," such as catching a ball," Bays told LiveScience. "By combining what our senses are telling us with a prediction of what we expect to be happening we can get an accurate picture of the current state of our bodies and the outside world."
The study is detailed in a recent issue of the journal Current Biology.