An unconditioned stimulus is a stimulus that naturally and automatically triggers a response without any prior learning. It is the biologically significant event at the heart of classical conditioning.

Food in the mouth is an unconditioned stimulus for salivation; a loud noise is an unconditioned stimulus for startle; a mild shock is an unconditioned stimulus for withdrawal or fear. In Pavlov’s experiments the unconditioned stimulus was the meat powder that reliably produced salivation. When a neutral stimulus is repeatedly paired with the unconditioned stimulus, the neutral stimulus becomes a conditioned stimulus capable of producing a similar response on its own.

The effectiveness of an unconditioned stimulus depends on its intensity, the organism’s motivational state, and the species’ evolutionary preparedness. Some stimuli are more readily associated with particular responses than others. Without an unconditioned stimulus, classical conditioning cannot establish a new signal for that response.

The event that already produces a reliable reaction is the unconditioned stimulus.