A hungry rat in an operant chamber is reinforced with food after pressing a bar only if a light on the chamber wall is on. If the rat learns to press the bar only when the light is on, this would indicate that the light is a(n) ________

A discriminative stimulus is associated with reinforcement or punishment, that exerts control over a particular form of behavior. The subject discriminates between closely related stimuli and elicits a specific behavior only in the presence of that stimulus. This stimulus increases the probability of a given response because of their historical correlation with the availability of reinforcement for the response.

In this example, the hungry rat gets food if it presses a bar only when the light is on. The rat learns that is most likely to request this food in the future in the presence of this light, because historically, the request has been reinforced in the presence of this light. The light in the example became a discriminative stimulus and it evokes the response.