Fig. 1. Hierarchical model of opioid–hypnotic drug interaction. Painful stimulus is first attenuated by the action of opioids. The potency of the opioids in attenuating pain decreases (e.g. , shifts to the right) with increasingly painful stimulation. The attenuated signal then projects to the cortex, where hypnotics act to modulate the probability of response. Similar to opioids, the ability of the hypnotic to attenuate the response is shifted rightward with increasing stimulus.