In classical conditioning, a stimulus leads to a learned response.
In Ivan Pavlov's experimentations with classical conditioning, a sound was paired with the placement of meat powder in a dog's mouth, and the powder naturally induced salivation. After the powder and the sound had co-occurred a few times, the dog salivated when the sound occurred, even when the meat powder was not administered. Although most research in classical conditioning has involved reflexive behaviors that are typically involuntary, other nonreflexive behaviors have also been classically conditioned. The effects of the conditioned stimulus can vary widely in different circumstances. For example, if the unconditioned stimulus is more intense, the conditioned stimulus will have a greater effect. On the other hand, if the conditioned stimulus does not always occur when the natural, unconditioned stimulus does, the conditioned stimulus will have less effect. Further, if an animal has associated a particular conditioned stimulus with a certain unconditioned stimulus and a new conditioned stimulus is presented, the animal will typically not develop a response to the new conditioned stimulus. Psychologists refer to this lack of a response to the new stimulus as blocking.
The conditioned stimulus seems to exert its effect by providing information to the animal. If the animal has already gained information through an initial conditioned stimulus, the second one will not be very useful. Similarly, if the potential conditioned stimulus does not always occur with the unconditioned stimulus, the information provided by the conditioned stimulus is less useful to the animal. If the conditioned stimulus occurs without the unconditioned stimulus, extinction will occur; that is, the conditioned stimulus will no longer have an effect. The reflex can be conditioned more easily the second time around if the two are again paired. Sometimes, after extinction has taken place, the conditioned stimulus will produce the reflexive behavior without the unconditioned stimulus, a process called spontaneous recovery.
Psychologists have applied knowledge of classical conditioning to human behavior. For example, people with allergies may rely on drugs that have unwanted side effects. Their allergies have been alleviated by pairing a unique odor (the conditioned stimulus) with the drug (the unconditioned stimulus). Over time, presentation of the odor by itself may alleviate the allergic symptoms.