A previously neutral stimulus that has come, through learning, to predict an unconditioned stimulus and produce a response on its own.

The feed bin rattle is a conditioned stimulus once the animal has learned it predicts food. The sound of a clicker is a conditioned stimulus once it has been paired enough times with food. The sight of the leash, the sound of the kettle, the appearance of a particular handler, the smell of a particular environment, can all become conditioned stimuli depending on what they have been paired with.

Conditioned stimuli are central to classical conditioning and are how the predictive relationships that organise an animal’s experience are built. In welfare contexts, conditioned stimuli can also be aversive. The sight of the trailer becomes a conditioned stimulus for stress if the trailer has predicted unpleasant journeys in the past. The sound of a particular treatment room door becomes a conditioned stimulus for anxiety if that room has predicted painful procedures.

The process by which a neutral stimulus becomes a conditioned stimulus is called acquisition. Acquisition typically requires multiple pairings between the conditioned stimulus and the unconditioned stimulus, though the number of pairings needed varies with the strength of the unconditioned stimulus, the salience of the conditioned stimulus, and the consistency of the pairing.

Conditioned stimuli can lose their predictive power through extinction, where the conditioned stimulus is presented repeatedly without the unconditioned stimulus following. They can also be modified through counterconditioning, where a previously aversive conditioned stimulus is paired with a positive unconditioned stimulus until the response changes.