Classical conditioning exhibits some patterns that can be associated with the initial establishment of a response to stimuli or to the disappearance of a response to stimuli. These patterns -- often referred to as the phenomena of classical conditioning -- include: Acquisition, extinction, spontaneous recovery, stimulus generalization, and discrimination. Acquisition. In the initial stage of learning, a response is established through continued association with the presentation of a stimulus. The salivation of Pavlov's dog in response to the sound of a bell is the classical example of acquisition. Prior to the training period, the bell itself has no association with the act of feeding the dog. However, over time, as the sound of the bell is paired with the presentation of food, the dog begins to anticipate the arrival of food upon first hearing the bell ring. Thus, the acquisition phase has been established, the stimulus and the response have been conditioned, and the association may be further strengthened (or at least made more persistent) through repetition and by varying the schedule of reinforcement.

Extinction. When a conditioned stimulus stops being paired with an unconditioned stimulus,...

For the classic pairing of dog food and whistle in Pavlov's early demonstrations of the phenomenon, the conditioned stimulus is the sound of a bell, the unconditioned stimulus is the food, and the conditioned response is the salivation of the dog in response to hearing the whistle.
Spontaneous recovery. A phenomenon of classical conditioning that receives less attention that the other four patterns is spontaneous recovery. Occasionally, a conditioned response will reappear or seem to reestablish itself in a behavioral chain, even after some time has passed or the conditioned response last occurred at a very diminished level. If the association between the conditioned stimulus and the unconditioned stimulus is no longer present (i.e., even though the dog has salivated, food does not appear after the sound of the bell), then extinction is likely to rapidly follow the period of spontaneous recovery.

Stimulus generalization. A conditioned stimulus will occasionally evoke responses that have not been conditioned. Generally this occurs if a stimulus that is similar to the conditioned stimulus presents in…