Recently a group at Queen’s University in Canada investigated why one cannot tickle oneself. Through a clever experiment where a test person would tap her left hand with her right hand, and a motor would transmit the impuls or not, and also add extra taps without self-initiation they were able to conclude that self-induced stimuli are filtered out. The brain tries to predict what will happen and if the prediction and the actual sensation are the same the sensation is attenuated. So, in short: No surprise – no tickle. The team also managed to find the time window during which the prediction is valid, roughly 300µs is the time span during which the brain anticipates the action. (Current Biology, Vol 15, 1125-1128)
This was also discussed by the naked scientists!