There are two competing theories of how classical conditioning works. The first, stimulus-response theory, suggests that an association to the unconditioned stimulus is made with the conditioned stimulus within the brain, but without involving conscious thought. The second theory stimulus-stimulus theory involves cognitive activity, in which the conditioned stimulus is associated to the concept of the unconditioned stimulus, a subtle but important distinction.

Stimulus-response theory, referred to as S-R theory, is a theoretical model of behavioral psychology that suggests humans and other animals can learn to associate a new stimulus — the conditioned stimulus (CS) — with a pre-existing stimulus — the unconditioned stimulus (US), and can think, feel or respond to the CS as if it were actually the US.

The opposing theory, put forward by cognitive behaviorists, is stimulus-stimulus theory (S-S theory). Stimulus-stimulus theory, referred to as S-S theory, is a theoretical model of classical conditioning that suggests a cognitive component is required to understand classical conditioning and that stimulus-response theory is an inadequate model. It proposes that a cognitive component is at play. S-R theory suggests that an animal can learn to associate a conditioned stimulus (CS) such as a bell, with the impending arrival of food termed the unconditioned stimulus, resulting in an observable behavior such as salivation. Stimulus-stimulus theory suggests that instead the animal salivates to the bell because it is associated with the concept of food, which is a very fine but important distinction.

To test this theory, psychologist Robert Rescorla undertook the following experiment [2]. Rats learned to associate a loud noise as the unconditioned stimulus, and a light as the conditioned stimulus. The response of the rats was to freeze and cease movement. What would happen then if the rats were habituated to the US? S-R theory would suggest that the rats would continue to respond to the CS, but if S-S theory is correct, they would be habituated to the concept of a loud sound (danger), and so would not freeze to the CS. The experimental results suggest that S-S was correct, as the rats no longer froze when exposed to the signal light.[3] His theory still continues and is applied in everyday life.[1]