Which stimulus can evoke a response without prior learning?

• A. Stimulus
• B. Unconditioned Stimulus (UCS)
• C. Conditioned Stimulus
• D. Unconditioned Response

Answer: (B)

In this idea, some stimuli trigger a reflex automatically, without any learning. That automatic trigger is what an unconditioned stimulus does. It naturally and biologically prompts a response because it has inherent significance for the organism. For example, food presented to a hungry animal will cause salivation even if the animal has never learned any association with that situation. That reflexive effect is the unconditioned response, which is the reaction elicited by the UCS.

Others require learning or are the reaction itself rather than the trigger. A stimulus that becomes meaningful only after being paired with something else is the conditioned stimulus, which needs prior learning to evoke a response. The unconditioned response is the actual reflex to the stimulus, not the stimulus itself.