The process of closely associating a neutral stimulus with one that evokes a reflexive response so that eventually the neutral stimulus alone will evoke the same response.
Classical conditioning was pioneered by the Russian physiologist Ivan Pavlov (1849-1936) in the 1890s in the course of experiments on the digestive systems of dogs (work which won him the Nobel Prize in 1904). Noticing that the dogs salivated at the mere sight of the person who fed them, Pavlov formulated a theory about the relationship between stimuli and responses that he believed could be applied to humans as well as to other animals. He called the dogs' salivation in response to the actual taste and smell of meat an unconditioned response because it occurred through a natural reflex without any prior training (the meat itself was referred to as an unconditioned stimulus). A normally neutral act, such as the appearance of a lab assistant in a white coat or the ringing of a bell, could become associated with the appearance of food, thus producing salivation as a conditioned response (in response to a conditioned stimulus). Pavlov believed that the conditioned reflex had a physiological basis in the creation of new pathways in the cortex of the brain by the conditioning process. In further research early in the 20th century, Pavlov found that in order for the conditioned response to be maintained, it had to be paired periodically with the unconditioned stimulus or the learned association would be forgotten (a process known as extinction). However, it could quickly be relearned if necessary.
In humans, classical conditioning can account for such complex phenomena as a person's emotional reaction to a particular song or perfume based on a past experience with which it is associated. Classical (sometimes called Pavlovian) conditioning is also the basis for many different types of fears or phobias, which can occur through a process called stimulus generalization (a child
Gormezano, Isidore, William F. Prokasy, and Richard F. Thompson. Classical Conditioning, 3rd ed. Hillsdale, NJ: L. Erlbaum, 1987.
Lieberman, David A. Learning: Behavior and Cognition. Belmont, CA: Wadsworth Publishing Co., 1990.
Mackintosh, N.J. Conditioning and Associative Learning. New York: Oxford University, 1983.