Reward System

dopamine by Serge Bloch

A reward is a appetitive stimulus given to a human or some other animal to alter its behavior. Rewards typically serve as a reinforcers, something that, when presented after a behavior, causes the probability of that behavior’s occurrence to increase.

Note that just because something is labelled a reward does not necessitate it as a reinforcer. A reward can only be said to be a reinforcer if its delivery has increased the probability of a behavior. Certain neural structures, called the reward system, are critically involved mediating the effects of reinforcement.

Reward or reinforcement is an objective way to describe the positive value an individual ascribes to an object, behavioral act or an internal physical state. Primary rewards include those that are necessary for the survival of species, such as food, sexual contact, or successful aggression. Secondary rewards derive their value from primary rewards. Money is a good example. They can be produced experimentally by pairing a neutral stimulus with a known reward. Things such as pleasurable touch and beautiful music are often said to be secondary rewards, but such claims are questionable. For example, there is a good deal of evidence that physical contact, as in cuddling and grooming, is an unlearned or primary reward. Rewards are generally considered more desirable than punishment in modifying behavior.

In a fundamental discovery made in 1954, researchers James Olds and Peter Milner found that low-voltage electrical stimulation of certain regions of the brain of the rat acted as a reward in teaching the animals to run mazes and solve problems. It seemed that stimulation of those parts of the brain gave the animals pleasure, and in later work humans reported pleasurable sensations from such stimulation. When rats were tested in Skinner boxes where they could stimulate the reward system by pressing a lever, the rats pressed for hours at a rate up to 2000 times per hour.

Research in the next two decades established that dopamine is one of the main chemicals aiding neural signaling in these regions, and dopamine was suggested to be the brain’s ‘pleasure chemical.’ Almost all drugs causing drug addiction increase the dopamine release in the mesolimbic pathway (e.g. opioids, nicotine, amphetamine, ethanol). After prolonged use, psychological drug tolerance arises, as does sensitization, an increase in the user’s sensitivity to the effects of the substance. The hypersensitivity that it causes is thought to be responsible for the intense cravings associated with drug addiction, and is often extended to even the peripheral cues of drug use, such as related behaviors or the sight of drug paraphernalia.

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