Drugs with increased addiction liability generally decrease the stimulation threshold for ICSS responding, while drugs with aversive properties generally increase the stimulation threshold to achieve ICSS responding. These studies provide insight into the specific neurochemical mechanisms involved in the facilitation of BSR, and how reward perception can be modulated by pharmacologically altering the activity of specific neurotransmitter systems. Pharmacological manipulation of these systems can have either a direct or indirect effect on the activity of the reward circuitry. Understanding drug-specific effects on ICSS response thresholds has helped elucidate how different neurotransmitter systems influence the reward circuitry by either potentiating or suppressing sensitivity to rewarding stimulation and influencing motivation to perform reward-associated behaviors.