Learning has been of central interest to psychologists since the emergence of the field in the late 1800's. Learning refers to changes in behavior that result from experiences. The term "behavior" includes all actions of an organism, both those that are directly observable, such as typing at a keyboard, and those that are unobservable, such as thinking about how to solve a problem. Psychologists studying learning work with a variety of species, including humans, rodents, and birds. Nonhuman species are studied for a variety of reasons. First, scientists are interested in fundamental principles of learning that have cross-species generality. Second, the degree of experimental control that can be obtained with nonhumans is much higher than with humans. These controlled conditions make it more likely that any effect that is found results from the experimental manipulations, rather than some uncontrolled variable. Third, studying the learning of nonhumans can be helpful to animals. For example, a scientist might need to know the best way to raise an endangered giant condor to maximize its chances of survival when introduced to the wild.

There are two major types of learning. Classical conditioning (also called Pavlovian conditioning, after Russian physiologist Ivan Pavlov) involves transfer of control of reflexes to new environmental stimuli. For example, a glaucoma test at an optometrist's office used to involve a puff of air being delivered into the patient's eyes, which elicited blinking. After this experience, putting one's head into the machine would elicit blinking; the glaucoma-testing machine would elicit the reflex of blinking, before the air puff was delivered.

Operant conditioning, also called instrumental conditioning, involves the regulation of nonreflexive behavior by its consequences. American psychologist Edward Thorndike was a pioneer in the study of operant conditioning, publishing his work about cats escaping from puzzle boxes in 1898. Thorndike observed that over successive trials, movements that released a latch, allowing the animal to get out of the box and get some food, became more frequent. Movements not resulting in escape became less frequent. Thorndike called this the Law of Effect: responses followed by satisfaction would be strengthened, while responses followed by discomfort would be weakened. The study of operant conditioning was greatly extended by American behaviorist B. F. Skinner, starting in the 1930's.

Beginning in the 1960's, American psychologists Martin Seligman, Steven Maier, J. Bruce Overmier, and their colleagues discovered that the controllability of events has a large impact on future learning. Dogs exposed to inescapable electric shock became passive and failed to learn to escape shock in later situations in which escape was possible. Seligman and colleagues called this phenomenon "learned helplessness" because the dogs had learned that escape was not possible and gave up. The laboratory phenomenon of learned helplessness has been applied to the understanding and treatment of human depression and related conditions.

In the 1970's, some psychologists thought the use of rewards (such as praise or tangible items) was harmful to motivation, interest, and creativity. Beginning in the 1990's, however, American Robert Eisenberger and Canadian Judy Cameron, conducting research and analyzing previous studies, found that rewards generally have beneficial impacts. Rewards appear to have detrimental effects only when they are given regardless of how the person or animal does. Furthermore, the work of Allen Neuringer and colleagues has shown that, contrary to previous thinking, both people and animals can learn to behave in random, unpredictable ways.

The changes in behavior produced by learning are accompanied by changes in physiological makeup. Learning is associated with changes in the strength of connections between neurons (nerve cells in the brain), some quite long-lasting. Eric R. Kandel and his colleagues have documented the changes in physiology underlying relatively simple learning in giant sea snails, progressing to more complex behaviors in mammals. Similar physiological changes accompany learning in a variety of organisms, highlighting the continuity of learning across different species.

Classical Conditioning

Classical conditioning was first systematically investigated by Ivan Pavlov in the late 1800's and early 1900's. Classical conditioning involves the transfer of control of an elicited response from one stimulus to another, previously neutral, stimulus. Pavlov discovered classical conditioning accidentally while investigating digestion in dogs. A dog was given meat powder in its mouth to elicit salivation. After this process had been repeated a number of times, the dog would start salivating before the meat powder was put in its mouth. When it saw the laboratory assistant, it would start to salivate, although it had not initially salivated at the sight. Pavlov devoted the rest of his long career to the phenomenon of classical conditioning.

In classical conditioning, a response is initially elicited by an unconditioned stimulus (US). The US is a stimulus that elicits a response without any prior experience. For example, the loud sound of a balloon bursting naturally causes people to blink their eyes and withdraw from the noise. The response that is naturally elicited is called the unconditioned response (UR). If some stimulus reliably precedes the US, then over time it, too, will come to elicit a response. For example, the sight of an overfull balloon initially does not elicit blinking of the eyes, but if the sight of the balloon reliably precedes the loud noise that comes when it bursts, people eventually come to blink and recoil at the sight of an overfull balloon. The stimulus with the new power to elicit the response is called the conditioned stimulus (CS), and the response elicited by the CS is called the conditioned response (CR).

Classical conditioning occurs with a variety of behaviors and situations. For example, a person who was stung by a wasp in a woodshed may now experience fear on approaching the woodshed. In this case, the building becomes a CS eliciting the CR of fear because the wasp's sting (the US) elicited pain and fear (the UR) in that place. To overcome the classical conditioning, the person would need to enter the woodshed repeatedly without incident. If the woodshed was no longer paired with the painful sting of the wasp, over time the CR would extinguish.

Many phobias are thought to arise through classical conditioning. One common successful treatment is systematic desensitization, in which the person, through progressive steps, gradually faces the feared object or situation until the fear CR extinguishes. Classical conditioning has been recognized as the culprit in food aversions developed by people receiving chemotherapy treatments for cancer. In this case, the food becomes a CS for illness (the CR) by being paired with the chemotherapy treatment (the US) that later elicits illness (the UR). Using more advanced principles of classical conditioning learned through research with nonhumans, people are now able to reduce the degree of aversion that occurs to regular meals, thus preventing the person from developing revulsions to food, which would further complicate the treatment of the cancer by introducing potential nutritional problems.

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