Defensive conditioning, in Pavlovian (classical) conditioning, happens when the unconditioned stimulus is noxious, unwanted or aversive to the subject. Understandably, this unwanted stimulus usually causes an unwanted response like fear, anxiety or revulsion as the usual unconditioned response.
In classical conditioning, unconditioned stimuli are naturally occurring stimuli that elicit a response from a subject, called the unconditioned response. Ivan Pavlov first discovered that a neutral stimulus can elicit the same response from the subject if that neutral stimulus is paired with the unconditioned stimulus. The neutral stimulus is always presented first, followed by the unconditioned stimulus which then elicits the unconditioned response. If this process is done often enough, the subject eventually comes to associate the neutral stimulus with the response so that the response is elicited even when only the neutral stimulus is presented. At this point the neutral stimulus becomes a conditioned stimulus and the response becomes a conditioned response.
Since the unconditioned response is unwanted but the stimuli can’t always be removed, behavioral therapy often focuses on pairing the unwanted stimulus with another stimulus that elicits a more favorable or calm response from the subject. The new stimulus is presented first as in classical conditioning with the unconditioned stimuli being presented gradually until the subject no longer associates the fear or anxiety with the unwanted stimulus. An example of this type of therapy is systematic desensitization, also known as graduated exposure therapy, developed by African psychiatrist, Joseph Wolpe. Another example is counterconditioning (also called stimulus substitution) used to great effect by Mary Cover Jones when she treated a young boy from his fear of rabbits.