Within such a framework several kinds of stimuli have been distinguished.

In the theory of classical conditioning, unconditioned stimulus (US) is a stimulus that unconditionally triggers an unconditioned response (UR), while conditioned stimulus (CS) is an originally irrelevant stimulus that triggers a conditioned response (CR). Ivan Pavlov's dog experiment is a well-known experiment that illustrates these terms. The unconditioned stimulus is the dog's food that would naturally cause salivation,