In a two-phase study, participants were randomly assigned to either group A or group B. All participants were subjected to a series of irritating air puffs directed at the face. In the first phase of the study, participants in group A pressed a button that stopped the air puffs. Participants in group B pressed a button that had no effect on the air puffs. In the second phase of the study, both groups had buttons that, if pressed, would stop the air puffs. The study most directly explores the concept of .