In classical conditioning, which stimulus can evoke an unconditioned response the first time it is presented?

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

Answer: (B)

In classical conditioning, an unconditioned stimulus is something that naturally and automatically triggers a reflexive response without any learning. Because it doesn’t require training, it can evoke the unconditioned response on the very first presentation. For example, food naturally makes a dog salivate, so the food is the unconditioned stimulus and salivation is the unconditioned response.

The broader term “stimulus” is too vague, since not every stimulus causes an unlearned response. A conditioned stimulus produces a response only after conditioning with the UCS. The unconditioned response is the reaction itself, not the trigger.