Why can't you tickle yourself?

Someone did a really clever experiment on this recently. They had special apparatus where they could tap your finger with this automatic device, and it made you think you were doing it yourself. They recorded to see how much they thought they were being tapped when they were actually doing it themselves and when the machine did it for them. What they found was that when the brain programmes a movement to make a sensation such as a tickle or a scratch, what it actually does is sends an inhibitory signal to the part of the brain that would normally be tickled. So in other words there is a kill joy area in the brain that says if you are tickled in this area, you will ignore the tickle. As your brain is sending the message itself, you ignore the tickle. However, when someone else tickles you, your brain doesn't know it's going to happen and turns off the kill joy signal.