Applied Behavior Analysis (ABA) is a scientific method focused on understanding and improving socially meaningful human behavior. The field uses principles of learning to help individuals acquire new skills and reduce challenging behaviors. A foundational concept is the Discriminative Stimulus (SD). This environmental cue guides an individual’s actions, making learning systematic and predictable. Understanding the function of the SD is the first step in recognizing how ABA programs are designed to teach skills.
Defining the Discriminative Stimulus
The Discriminative Stimulus (SD) is a specific antecedent, an event that occurs immediately before a behavior. The SD acts as a signal indicating that a specific response will be followed by reinforcement. This means the SD tells a person that if they perform a certain action now, they are highly likely to receive a desired consequence.
A common example of an SD in everyday life is a traffic light turning green. The green light signals that if the driver presses the gas pedal, they will be reinforced by proceeding through the intersection. In an educational setting, the therapist saying, “Touch your nose,” serves as the SD. This cue signals to the learner that the correct motor response will be met with praise, a preferred item, or another form of reinforcement.
The SD does not force a behavior to occur; rather, it increases the probability of the behavior happening because of a past history of reinforcement. Consequently, the SD is not a command but a reliable predictor of reinforcement availability. The cue is defined by the history of what has followed it, creating an association between the signal and the subsequent reward.
The SD’s Role in the Three-Term Contingency
The SD operates within the fundamental structure of Applied Behavior Analysis known as the three-term contingency (A-B-C). This framework analyzes behavior by examining the relationship between an Antecedent (A), the resulting Behavior (B), and the Consequence (C). The SD functions as the antecedent component.
The sequence begins with the SD, which is presented to the learner to prompt a specific behavior. If the learner performs the correct behavior, it is immediately followed by a consequence, typically a positive reinforcer. This consequence increases the likelihood that the learner will perform that same behavior the next time the SD is presented. For instance, a therapist presents a picture of a dog and asks, “What is this?” (SD), the child says “dog” (Behavior), and the therapist says “That’s right!” (Reinforcement).
This functional relationship establishes that the SD is a predictor that a consequence is available, but only if the correct response is performed. The SD makes the reinforcement contingent upon the behavior. If the child were to say “cat” instead, the consequence would not be a reinforcer.
The strength of the SD lies in this consistent pairing, which results in the learner understanding the precise conditions under which a behavior is appropriate. Over time, the cue becomes a powerful tool for teaching complex skills. Therapists plan the SDs to ensure they are clear, consistent, and directly related to the skill being taught, making the learning process efficient.
Establishing Effective Stimulus Control
The goal of pairing the SD with a reinforced response is to achieve Stimulus Control. This occurs when a behavior reliably happens immediately after the SD is presented, and only in its presence. Stimulus control means the learner has successfully discriminated between the relevant cue and all other stimuli in the environment.
To establish this precise relationship, ABA therapists use discrimination training, which involves introducing the S-delta ($S^\Delta$). The S-delta is a stimulus that signals that reinforcement is not available, even if the target behavior is performed. For example, if a child is learning to identify a ball, the therapist might ask, “Touch the ball” (SD) while presenting a ball and a block ($S^\Delta$).
Reinforcement is delivered only when the child touches the ball in the presence of the SD. If the child touches the block ($S^\Delta$), no reinforcement is provided. This differential reinforcement teaches the learner to distinguish between the two stimuli. By reinforcing responses only in the presence of the SD and withholding reinforcement in the presence of the $S^\Delta$, the behavior comes under the control of the correct stimulus.
Therapists also employ techniques like prompt fading to ensure the SD alone gains control over the behavior. Initially, a prompt (extra help, like a physical guide) may be needed to ensure the learner responds correctly to the SD and earns reinforcement. This assistance is gradually reduced, or “faded,” until the SD is the only cue needed to evoke the desired action. This process transfers control from the artificial prompt to the intended environmental cue, promoting independence and skill generalization.