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Motivation for specific tasks derives significantly from environmental feedback. For instance, if performance of an action results in immediate sensation of pain, I am likely to avoid repeating it; however, if the action results in pleasurable feedback, I will look for an opportunity to do it again. Thus, one learns through trial and error to perform actions that are positively reinforced and avoid those which result in negative reinforcement. The cognitive and even cellular bases for these psychological phenomena have begun to be unravelled by combining psychological studies with positron emission tomography (PET), functional magnetic resonance imaging (fMRI), electrophysiology, and biological chemistry. Rats may be trained to repeat complex actions, such as traversing a maze or operating a lever, if the performance of this action results in a food reward. In ] 954, aids and Milner announced that the food reward could be effectively substituted by electrical stimulation of specific regions of the brain; in fact, the rats would learn to stimulate themselves. In ensuing years, the basal forebrain as well as some other brain regions have been identified not only as part of a brain-stimulation reward pathway, but also as the pathway activated by natural reward. The natural reward system involves many regions of the brain and many neurotransmitter systems. However, a substantia! role is played by the dopaminergic mesocorticolimbic pathway which impinges on the basal forebrain. More simply put, neurons that secrete or respond to the transmitter dopamine, playa very important role in the sensation of reward, which critically underlies the reinforcement of particular behaviours. Food, success, and sex activate dopaminergic transmission in the !?;r!.al forebrain and behaviours directed towards these comme>dities are normally reinforced in the brain. Thus, healthy animals (those selected for in the wild) are motivated to achieve social and sexual success. A subset of animals in any species, for instance rogue lions and chimpanzees among non-human animals, or saints, scholars, and cocaine addicts in human society, do not play by the same rules as others. Behaviours that stimulate their reward pathways are presumably different in some way. This has been demonstrated recently by fMRI scanning studies on cocaine addicts where brain activities were measured itl vivo during the anticipation, rush, high, and low phases that occur at different stages of cocaine reception (Breiter et at 1997). Cocaine, which blocks the re-uptake of dopamine following its release, prolongs dopaminergic transmission in parts of the brain that constitute the reward pathway. Thus, by directly activating an acute sensation of reward, cocaine seeking behaviour is very strongly reinforced. It is probably not a coincidence that it is those with fewer rewards from "normal life" that are more frequently attracted to an easy pharmacological solution. The sensitivity of the reward pathway to different environments is also influenced by genetic rather than epigenetic phenomena (Rocha et at ] 998). Like all mammalian systems, the brain shows a remarkable tendency towards homeostasis. For this reason, chronic use of cocaine probably results in adaptation of the reward system, such that the "set point" for hedonism (intense sensation of reward) is altered. The reduced sensitivity of this adapted reward system may now increase craving for cocaine stimulation (for a more rigorous and detailed review read Koob and Moal 1997). While reward-seeking is a major source of motivation, fear, the desire to avoid negative reinforcement in the form of pain, is also a strong source of motivation. Like the sensation of reward, fear may be induced in rats by direct electrical stimulation of a specific anatomical region of the brain-the central nucleus of the amygdala. In classical conditioning experiments, a conditioned stimulus such as a high pitched tone can be associated with a negative (unconditioned) stimulus such as electric shocks applied to the feet of a rat (footshock). Recent studies have shown that while a tone produces only a small response in the amygdala of an untrained rat, it produces a greatly enhanced response in the amygdalar.