Classical conditioning is learning that occurs when a neutral stimulus is paired with a stimulus that already elicits a response. After repeated pairings the neutral stimulus itself begins to elicit a similar response.

Ivan Pavlov’s experiments with dogs remain the classic illustration. The sound of a metronome, initially irrelevant, was presented just before food. Food naturally produced salivation. After several pairings the metronome alone produced salivation. The food is the unconditioned stimulus; the salivation to food is the unconditioned response; the metronome becomes the conditioned stimulus; the salivation to the metronome is the conditioned response.

The phenomenon is widespread. Emotional reactions to places, people, and advertisements are often classically conditioned. Taste aversions can be acquired in a single pairing of a novel food with later illness. Therapeutic techniques such as systematic desensitization use the same principles in reverse to reduce conditioned fear.

Classical conditioning shows that the nervous system automatically records predictive relationships between events, even when the organism is not trying to learn anything.