a phenomenon that occurs during an operant performance test when a conditioned response to a positive stimulus is reduced by another stimulus that is associated with an aversive stimulus. For example, a rat may be trained to press a lever to receive food. During this procedure, the rat is occasionally exposed to a series of brief electric shocks that are preceded by a tone (the conditioned stimulus). As a result, when the rat subsequently hears the tone alone, its rate of lever pressing is reduced. Conditioned suppression is also used to study classical conditioning.