A moving story

To control our movements, the brain predicts what input it should receive. This has important implications for tickling…

Our brain is constantly bombarded with stimuli from the outside world and from inside the body, most of which we don't need to know about. We don't, for example, need to notice the movements of our tongue when we eat or feel the soles of our feet while walking.

To save its attention for more important unexpected signals, like stubbing your toe, the brain filters the cacophony of sensations picked up by our senses.

This reflects the way the brain controls movement. When planning a move (picking up a doughnut, for example), it calculates what sensory input it should experience during an arm motion and when the doughnut has been grasped. When it receives the expected input, these responses are dampened down; if you miss the doughnut and hit the table, they are not.

This mechanism explains why we can't tickle ourselves: the brain anticipates the effects of our action and dulls the sensation. Using a 'tickling robot' and a brain scanner, researchers have shown that a touch normally lights up parts of the somatosensory cortex, but if a person tickles themselves, these areas light up far less brightly.

All this depends on the cerebellum. It monitors movements, interpreting signals from, for example, our muscles and inner ear (ensuring that we don't keep falling over).