Sully has a painful experience at the dentist's. The next time his mother brings him to the dentist's, Sully gets tense and anxious. Sully's mother then takes him to a new dentist, Dr. Theron, who makes sure that Sully's visits are painless. Sully is anxious at first, but after a few visits, he no longer resists going to the new dentist. Sully's change in behavior can probably be best explained in terms of which of the following