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would make rats learn faster. In the course of this work, they noticed that some of the rats that were stimulated for a short time quickly returned to the place in the cage where they experienced the stimulation. The rats returned to the place where they were stimulated again and again, as though they wanted more stimulation! This suggested that there was something positive, rewarding, and reinforcing about stimulating the part of the brain containing the electrodes.
    A big surprise was that in the animals behaving this way, the electrodes were not in the reticular formation at all, but rather in another area called the septum. A mistake had been made in calculating where the electrodes should have been placed. Wow! It was an accidental discovery in more ways than one. In subsequent experiments, when the rats were given control over their own stimulation, when they were allowed to press a lever to self-stimulate their brains, they did. This was so striking to Olds and Milner, who realized they were on the verge of a discovery, that they dropped their original plannedexperiments and decided to study the rewarding and reinforcing properties of the electrical stimulation. They implanted electrodes in various places in the brain to see whether the various brain regions would support lever pressing for electrical stimulation (see Figure 3-1 ).
    Figure 3-1. This figure depicts a rat rotating a wheel that results in an electrical stimulation of a specific part of the brain. The wheel that rotates produces the same stimulation as a lever that is pressed, and either can be used. The part of the brain that is stimulated is selected by placing an electrode during sterile surgery with anesthesia. In practice, the electrical stimulator is attached to the electrode into the brain by a loose spring so that the animals can move freely about the cage. By systematically exploring brain regions, a map of the “pleasure” centers, for example, sites where animals will self-stimulate, in the brain has been produced. (From Roberts, A.J., and G.F. Koob. “The Neurobiology of Addiction: An Overview.”
Alcohol Health & Research World
, 21(2): 101–106, 1997. Updated: October 2000.)

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    James Olds’ Own Words
    “I applied a brief train of 60 cycle sine-wave electrical current whenever an animal entered one corner of the enclosure. The animal did not stay away from that corner, but rather came back quickly after a brief sortie, which followed the first stimulation and came back even more quickly after a briefer sortie, which followed the second stimulation. By the time the third electrical stimulus had been applied, the animal seemed to be ‘coming back for more.’” (Valenstein, E.S.,
Brain Stimulation and Motivation
, 1st edition, © 1973. Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ.)
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    By carefully mapping the sites of the electrodes in the brain 1 , they found that there were several regions in the brain that produced this repetitive, reinforcing self-stimulating behavior, and they sometimes referred to these areas as the pleasure center(s). High rates of electrical self-stimulation were found in the lateral hypothalamus, the medial forebrain bundle, and other areas. These anatomical mapping studies have revealed much about the brain regions involved, but for our purposes, we don’t need to go into that level of anatomical detail. These regions contain many components that are likely stimulated at the same time, and any one of them can be the contributor to the electrical self-stimulation. Later experiments showed that at least one major component of the medial forebrain bundle supporting electrical self-stimulation was the nerve cells, or neurons, that contain the neurotransmitter dopamine. Electrical self-stimulation of the medial forebrain bundle caused a release of dopamine, and chemicals that blocked dopamine blocked electrical self-stimulation. Drugs like cocaine also cause an increase in