A learned response performed by a trained animal to a signal that was previously associated with an event of consequence for that animal. Conditioned reflex (CR) was first used by the Russian physiologist I. P. Pavlov to denote the criterion measure of a behavioral element of learning, that is, a new association between the signal and the consequential event, referred to as the conditioned stimulus (CS) and unconditioned stimulus (US), respectively. In Pavlov's classic experiment, the conditioned stimulus was a bell and the unconditioned stimulus was sour fluid delivered into the mouth of a dog restrained by harness; the conditioned stimulus was followed by the unconditioned stimulus regardless of the dog's response. After training, the conditioned reflex was manifested when the dog salivated to the sound of the bell.