Blood flow in the brain was measured using positron emission tomography to test the hypothesis that the flow increased in particular areas of the brain when subjects transitioned from a relaxed state into a state of high vigilance. Blood flow in the areas of interest was measured in 10 subjects and shown to be significantly higher when the subjects underwent a transition in state. The authors concluded that increased blood flow in the brain areas studied caused (in the but-for sense) arousal and vigilance in human beings.