stimulus is a stimulus that naturally brings about a particular response without having been learned.

unconditioned stimulus

Explanation:

In Pavlov’s classical conditioning experiment with dogs, the dogs salivated naturally when meat powder was presented to them. The dogs’ salivation is referred to as the unlearned natural behavior (unconditioned response), while the meat powder that elicited the salivation is termed the unconditioned stimulus (UCS). The meat powder is the unconditioned stimulus that brings about a natural response of salivation in the dog, which is unlearned as a reflexive response.

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