HOW COME? It's a ticklish subject

How come it doesn't tickle when you tickle yourself? asks Gabriella Niederauer, a student in Brookville, NY.If your little brother tickles your ribs, you might jump, squirm away, and giggle. But waggle your fingers in your own ribs, and . . . crickets.

Whether you find being tickled fun or unpleasant, scientists say that the tickling effect depends on surprise and uncertainty. Even if you see your brother approaching, fingers waggling, you don't know exactly where the tickle will land -- or when. Making tickling a bit like having a spider suddenly drop on your arm from above.

The fact that tickling can seem like a threat, even when it's playful, and that we can't tickle ourselves into a fit of giggles, are related, scientists say. Experiments show that our brains are experts at screening out sensations of what we ourselves are doing. In fact, if the brain didn't do this "sensory attenuation," we couldn't make sense of the information (and possible threats) coming from the world around us.

Experiments show that when we try to tickle ourselves, the brain anticipates the coming touch, and its impact is minimized. In fact, the brain deliberately reduces our perception of touch.

Researchers say that the cerebellum, in the back of the brain, monitors movement. So the cerebellum knows what's coming when you try to tickle your own ribs. But it can't precisely predict sensations and timing when it's someone else's fingers that will be doing the tickling.

Try to tickle yourself, and the cerebellum orders the response to be muted. And the parts of the brain that process touch obediently ramp down. (On an MRI, scientists say, a self-tickle causes little activity in the brain's "somatosensory cortex," which lights up when the tickling is being done by someone else.)

Why is it a good thing that we can't tickle ourselves? All day long, our sensory system is overwhelmed by information coming from the world around us. So touch your own arm, and it won't seem startling.

But when something outside of you unexpectedly touches your arm -- like that stealthily dropping spider -- the brain wants you to react. Sensations coming from "outside" are deliberately amplified: a tree branch brushing your head, a bug crawling across your cheek, a stranger touching your arm on the subway.

So try to tickle yourself, even on your extra-ticklish feet, and it's a ho-hum situation, the brain's way of saying, "No worries -- it's only me."