In behavioral psychology (or applied behavior analysis), stimulus control is a phenomenon in operant conditioning (also called contingency management) that occurs when an organism behaves in one way in the presence of a given stimulus and another way in its absence. A stimulus that modifies behavior in this manner is either a discriminative stimulus (Sd) or stimulus delta (S-delta). Stimulus-based control of behavior occurs when the presence or absence of an Sd or S-delta controls the performance of a particular behavior. For example, the presence of a stop sign (S-delta) at a traffic intersection alerts the driver to stop driving and increases the probability that "braking" behavior will occur. Such behavior is said to be emitted because it does not force the behavior to occur since stimulus control is a direct result of historical reinforcement contingencies, as opposed to reflexive behavior that is said to be elicited through respondent conditioning. Some theorists believe that all behavior is under some form of stimulus control. For example, in the analysis of B. F. Skinner, verbal behavior is a complicated assortment of behaviors with a variety of controlling stimuli. Characteristics The controlling effects of stimuli are seen in quite diverse situations and in many aspects of behavior. For example, a stimulus presented at one time may control responses emitted immediately or at a later time; two stimuli may control the same behavior; a single stimulus may trigger behavior A at one time and behavior B at another; a stimulus may control behavior only in the presence of another stimulus, and so on. These sorts of control are brought about by a variety of methods and they can explain many aspects of behavioral processes. In simple, practical situations, for example if one were training a dog using operant conditioning, optimal stimulus control might be described as follows: The behavior occurs immediately when the discriminative stimulus is given. The behavior never occurs in the absence of the stimulus. The behavior never occurs in response to some other stimulus. No other behavior occurs in response to this stimulus. Establishing stimulus control through operant conditioning Discrimination training Operant stimulus control is typically established by discrimination training. For example, to make a light control a pigeon's pecks on a button, reinforcement only occurs following a peck to the button. Over a series of trials the pecking response becomes more probable in the presence of the light and less