Classical Conditioning

Classical conditioning (also Pavlovian conditioning or respondent conditioning) is a form of learning in which one stimulus, the conditioned stimulus or CS, comes to signal the occurrence of a second stimulus, the unconditioned stimulus or US. The US is usually a biologically significant stimulus such as food or pain that elicits a response from the start; this is called the unconditioned response or UR. The CS usually produces no particular response at first, but after conditioning it elicits the conditioned response or CR. Classical conditioning differs from operant or instrumental conditioning, in which behavior emitted by the subject is strengthened or weakened by its consequences (i.e. reward or punishment).

Conditioning is usually done by pairing the two stimuli, as in Pavlov’s classic experiments. Pavlov presented dogs with a ringing bell (CS) followed by food (US). The food (US) elicited salivation (UR), and after repeated bell-food pairings the bell also caused the dogs to salivate (CR).

It was originally thought that the process underlying classical conditioning was one where the conditioned stimulus becomes associated with, and eventually elicits, the unconditioned response. But many observations do not support this hypothesis. For example, the conditioned response is often quite different to the unconditioned response. Learning theorists now more commonly suggest that the CS comes to signal or predict the US. Robert A. Rescorla provided a clear summary of this change in thinking, and its consequences, in his 1988 article "Pavlovian conditioning: It's not what you think it is."