Natural rewards, such as water, food, and drugs of abuse may serve as positive reinforcers, increasing the frequency of the response that produces them. To study intravenous drug self-administration, a permanent catheter is implanted in a vein to allow the animal to self-inject a small amount of drug by pressing a lever. The administration of drug constitutes the event that positively reinforces the lever-pressing behavior. Various schedules of reinforcement of drug self-administration behavior have been developed, defined by the response requirements for obtaining each injection.
