In classical conditioning, an unconditioned stimulus (UCS) is a stimulus that naturally elicits a response, without any previous conditioning. For example, in Pavlov’s experiment with dogs, the food was the unconditioned stimulus because it naturally elicited the salivatory response in the dogs, without any previous conditioning. An unconditioned stimulus is a stimulus that has an innate, unlearned quality that makes it capable of eliciting a response. It is contrasted with a conditioned stimulus, which is a neutral stimulus that has been associated with the unconditioned stimulus through classical conditioning.
