Images of axial sections obtained with PET to measure the numbers of DA D2 receptors in subjects who reported the effects of the stimulant drug methylphenidate as pleasant versus those that reported its effects as unpleasant. Subjects with high numbers of DA D2 receptors tended to report the effects of methylphenidate as unpleasant, whereas subjects with low numbers of DA D2 receptors tended to report it as pleasant. The rainbow scale was used to code the PET images; radiotracer concentration is displayed from higher to lower as red > yellow > green > blue. Adapted with permission from ref. 53.