Rewards can exert powerful control over our thoughts and our behavior. It is assumed that the mechanism of these effects is in the brain, and physiological psychologists have been tempted by the possibility that there is as much to be learned about reward from the study of its mechanisms as from the study of its behavioral phenomenology. The mechanisms of reward are not easily exн amined, however, as they are to be found in the tangle of associational systems deep in the brain, rather than in the sensory or motor systems more accessible to the periphery.

Among the most powerful of rewards, however, are two that activate central reward circuitry directly, rather than through the peripheral nerves. These are the rewards of direct electrical stimulation and of habit-forming drugs. Electrical stimulation offers an anatomically selective tool; while it is indiscriminate about what kind of neurons it excites, its potency decreases with the square of the distance from the electrode tip and falls to trivial levels