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For centuries, scientists and philosophers alike have tried to understand why human beings consistently make choices that are not in our best interests. For example, the serious health risks from smoking are well known, but that does not stop millions of Americans from indulging in cigarettes every day. Why?

The obvious answer is that engaging in behaviors that are bad for us feels good. Whether we are enjoying an after-dinner cigar or hang-gliding in the mountains, we are trading short-term pleasure for potential long-term risks. Some trade-offs allow us to experience extraordinary joy or success. But problems arise when pleasure-seeking behaviors take over a person's life, as in drug addiction or obesity. An increased understanding about how the brain produces pleasure is leading the way to more effective interventions for when pleasure brings pain.

The Uncontrollable Id

Freud postulated that pleasure-seeking behaviors are driven by a subconscious entity called the "id," which operates on what he called "The Pleasure Principle": satisfying urges for food, drink, and sex. Normally, id's desires are kept in check by the more rational "ego" and societal mores. However, sometimes the id can take over, leading a person to make selfish and even self-destructive choices.

While in the contemporary perspective, many of Freud's ideas have been scrutinized, one aspect is considered to sure: specialized areas of the brain produce pleasure. That understanding has provided insight into the complex nature of human motivation.

A Search for Balance

Homeostasis, the term used to describe the constancy of the body's internal environment, is also used to explain a concept of motivation. Temperature, weight, and sexual behavior are all regulated around fairly fixed balance points, and living organisms, including humans, usually behave in ways that maintain these balances. When the balance shifts for some reason, we change our behavior to regain homeostasis. For example, if you eat more than usual over a holiday weekend, you will probably find your appetite is diminished during the next few days. But the drive to maintain equilibrium cannot explain all pleasure-seeking behavior. We overeat, drink too much, and many of us rely on drugs like caffeine and tobacco to make it through the day. So, what motivates us to go beyond simple satisfaction of biological needs to indulgence?

Looking Inside the Brain

One of the most influential discoveries related to reward mechanisms in the brain and how they are related to motivation and pleasure came in 1954, when researchers James Olds and Peter Milner implanted rats with electrodes that could deliver a tiny stimulation to a given brain area. They then trained the rats to press a lever in order to activate the electrodes. Rats seemed to enjoy the electrical brain stimulation because they quickly began pressing the lever, and continued pressing until exhaustion.

Focus on Dopamine

Scientists probed further to see if they could link the brain's reward systems to a particular set of chemical mechanisms. They soon found that blocking the neurotransmitter dopamine seems to reduce an animal's pleasure-seeking drives. For example, depleting dopamine in the brain causes animals to become oblivious to food and other normally pleasurable rewards. Rats with such lesions will starve to death unless force-fed. They retain the ability to find the food, chew, and swallow, but they seem to lose all desire to eat, even when the food is placed right under their noses. Scientists also found that the rat hypothalamus produced more dopamine when the rats were engaged in pleasurable activities, suggesting that dopamine was indeed closely linked to pleasure and reward.

The mechanics of how dopamine might signal pleasure in the brain, however, is still a matter of scientific controversy. The neurons that produce dopamine in response to pleasure often seem to activate just before the pleasurable activity occurs, which suggests that the relationship between pleasure and dopamine may be complex. The data on the timing of dopamine release are somewhat contradictory, but the theory emerging is that the dopamine reward signal acts as a kind of teaching tool. In this model, our brains release a certain amount of dopamine as a predictor of how pleasurable some activity is going to be. The dopamine motivates us, increasing our energy and drive and compelling us to engage in the pleasurable activity. If everything is as nice as the brain predicted, dopamine levels remain elevated. If things turn out even better than the brain hoped, dopamine levels are increased; we engage in the pleasurable activity even more vigorously. If, on the other hand, the activity is less pleasurable than we thought it would be, dopamine levels plummet.

Interestingly, learning can cause dopamine responses to transfer from primary rewards (such as tuna fish to your cat) to reward-predicting stimuli (such as the sound of the can opener). This suggests that reward in general, and the dopamine hypothesis in particular, may play a central role in how and why we are able to learn.

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