Temporal conditioning: The unconditioned stimulus (US) is presented at regularly timed intervals, and CR acquisition is dependent upon correct timing of the interval between unconditioned stimulus (US) presentations. The background, or context, can serve as the conditioned stimulus (CS) in this example. Unpaired conditioning: The conditioned stimulus (CS) and unconditioned stimulus (US) are not presented together. Usually they are presented as independent trials that are separated by a variable, or pseudo-random, interval. This procedure is used to study non-associative behavioral responses, such as sensitization.