In 1954, James Olds and Peter Milner discovered that a rat would press a bar to receive a brief impulse of electricity through an electrode implanted in certain areas of the brain. Although it was known that such stimulation in other areas of the brain could produce motivated behaviors of eating, drinking, sexual behavior, or aggression (and that lesions of the brain could produce the converse behaviors), it now appeared that psychologists had discovered a “brain reward” system. The ESB was serving as a reinforcer. Rats bar pressed at rapid rates for 15 to 20 hours until exhausted in order to receive the stimulation. During the process, they ignored food or water, and rat mothers ignored their pups. Jose Delgado, in 1955, demonstrated that rats would also learn to bar press in order to turn off stimulation to the hippocampus, a neural system not involved in pain transmission. Other researchers found that the positive and negative ESB sites (stimulation of which induced animals to obtain or avoid receiving ESB) appeared to be concentrated in the limbic system.
Humans who have had electrodes implanted in their brains to alleviate symptoms of Parkinson's disease have described the experience as mildly pleasurable and satisfying. Although ESB has been found to result in the release of neurotransmitters such as dopamine and norepinephrine, current research strategies have focused on the use of drugs rather than ESB to regulate production of these neurotransmitters.