Oddly enough, the escalation of force in this experiment seems to be related to your inability to tickle yourself. When you perform an action, your brain automatically anticipates the sensory consequences of the action and uses that information to damp down the sensory effects of the action. As a result, self-produced sensations are less salient than sensations produced by another person. (I know what you’re thinking, and, yes, that’s correct.)
Aha.. :) best moment reading this book! Yes, I was thinking the same.
