Another experimental method that is used to study the biological basis of drug addiction involves surgically implanting stimulating electrodes in brain regions where electrical stimulation is directly reinforcing. Animals are trained to press levers to receive brief pulses of electrical stimulation to certain brain areas. Addictive drugs enhance the reinforcing impact of such electrical stimulation. Brain stimulation reward (BSR) offers a convenient method of screening large numbers of new compounds and of determining the time course of drug action. Like CPP studies, the interaction of a drug with BSR does not directly measure drug reinforcement, but there is excellent concordance between drugs that facilitate BSR and drugs that are addictive in humans.