Dr. Culpepper is helping a client who has developed a fear of clowns. After talking with the client, Dr. Culpepper says, "It seems when you were younger, you were at a circus and just when you heard a loud noise, you also saw a clown. The loud noise frightened you but because you were watching a clown at the same time, you associated the clown with being scared." Dr. Culpepper's explanation represents what theory on learning?