The observation that animals would work in order to receive electrical stimulation to discrete brain areas was first described by Olds and Milner (1954). In this paper, they stated, "It is clear that electrical stimulation in certain parts of the brain, particularly the septal area, produces acquisition and extinction curves which compare favorably with those produced by conventional primary reward." This phenomenon is usually referred to as brain-stimulation reward (BSR), intracranial self-stimulation (ICSS), or intracranial stimulation (ICS).

Most abused substances increase the rate of response (lever pressing) for rewarding ICS, and this has been interpreted as an increase in the reward value of the ICS. Because changes in rate of response could also be a function of the effects of the drug on motor performance, a number of methods have been developed that control for the confounding nonspecific effects of the drugs under...