Subjects are presented with the blue cross and are cued to anticipate “high pain” (i.e., brief thermal heat temperature that produces high pain sensation) if the color of the cross changes to RED, “low pain” (i.e., brief thermal heat that produces low pain sensation) if the color of the cross changes to GREEN, or “unknown” (i.e., brief thermal heat of either high or low intensity) if the color of the cross changes to YELLOW (50 % probability). Each temperature is delivered for 6 sec.