Conditioning is a way of establishing new behaviors by providing either a stimulus or a reward for the desired behavior. It has widespread applications inpsychology. There is also growing evidence that conditioning might be usefulto boost the body's immunity against disease, or to suppress the immune system's tendency to reject transplanted organs.

There are two main types of conditioning: classical and operant.

The best-known examples of classical conditioning are the experiments conducted in the late 1800s and early 1900s by the Russian physiologist Ivan Pavlov.Initially, Pavlov was interested in the functioning of the digestive system(he received the 1904 Nobel Prize in Medicine and Physiology for that work).In experiments with dogs, he developed an experimental apparatus for measuring their saliva formation as they ate, using tubes that redirected secretionsfrom the animals' salivary ducts. While conducting these experiments, Pavlovnoticed that some dogs started salivating before they were fed.

New dogs did not salivate in this way, only those that had previously participated in similar laboratory sessions. Pavlov showed that this salivation wasinitiated by some stimulus associated with the earlier feeding, perhaps the ringing of a bell or just the appearing of the lab worker who fed the animals.

Thus, in classical conditioning, a stimulus not typically associated with a particular response (i.e. the ringing of the bell) is repeatedly presented around the same time as a stimulus that causes the response naturally (in the case of Pavlov's dogs, food), until the initial stimulus evokes the response onits own. This is known as a conditioned reflex.

Continuing to study this phenomenon for more than three decades, Pavlov concluded that a great deal of animal and human behavior resulted from classical conditioning. Although his view of the importance of conditioning was probablyexaggerated, subsequent research has uncovered dozens of reflexes that respond to classical conditioning, including blinking of the eyes, the knee-jerk response, and stimulation of the heart, liver, kidneys, and stomach.

More recently, researchers have become interested in the effects of classicalconditioning on the immune system, which creates antibodies that fight infection. Studies in mice and humans have demonstrated that when the immune-strengthening drug interferon is administered a few times combined with the odor of camphor (an aromatic compound that normally has no effect on the immune system), later exposure to camphor alone increased activity of the body's natural killer cells, which act against tumors and viruses. Similarly, animal research has suggested that classical conditioning might be useful for suppressingthe body's immune response that rejects transplanted organs.

Other medical applications of classical conditioning are for combating alcohol and tobacco abuse, and to teach children to stop bedwetting.

Operant conditioning, on the other hand, does not use a stimulus to prompt behavior. Rather, it employs a `reward' after the desired behavior, or sometimes a punishment to discourage inappropriate behavior. Examples of operant conditioning are the `puzzle box' experiments conducted by the American psychologist Edward Thorndike in the late 1800s. Thorndike placed a hungry cator other animal in a box that could only be opened if the animal pulled on astring or a latch, or performed a combination of similar actions. The first time the animal was placed in the box, it usually took some time to escape andenjoy some nearby food. But over time, the animal learned to escape within seconds. In these experiments, behavior was modified through the rewards of escape and food.

Operant conditioning can be used, among other things, to shape the behavior of cocaine users or patients with severe behavioral problems.

Many (perhaps doubtful) stories are told of how college and university classes use operant conditioning to shape the behaviors of their professors. In onesuch story, students in an introductory psychology course agreed to `reinforce' their professor for each move to the left by nodding approval and payingattention, ignoring him and stopping their note-taking if he moved to the right. According to the story, recounted in a popular textbook about the psychology of learning, the professor got midway through the lecture and then fell off the left side of the stage.

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