Applied Behavior Analysis (ABA) is a systematic, data-driven methodology dedicated to understanding and improving socially meaningful behavior. ABA focuses on how the environment influences actions and how new skills can be taught effectively across various contexts. A central concept in this framework, and a fundamental building block of skill acquisition, is the Discriminative Stimulus (SD).
Defining the Discriminative Stimulus (SD)
The Discriminative Stimulus (SD) is an environmental cue that signals that a specific behavior will likely result in a favorable outcome or consequence. It acts as a predictor, letting the learner know that reinforcement is available for a particular action at that moment. Consider a ringing phone: this auditory cue is the SD for the behavior of answering it, which is typically reinforced by connecting with the caller. Without the ringing sound, answering the phone is unlikely to be reinforced.
This concept is best understood within the three-term contingency, which breaks down learning into three parts: Antecedent, Behavior, and Consequence (A-B-C). The SD is placed squarely in the Antecedent position, as it occurs immediately before the behavior. For example, when a teacher says, “Touch your nose,” this verbal instruction is the Antecedent (SD) that sets the occasion for the Behavior (touching the nose), which is then followed by a positive Consequence (praise or a token).
The SD increases the probability that a specific response will occur because, in the past, that response has been reinforced under similar conditions. It makes the behavior more probable by signaling a window of opportunity for reward. Everyday life is filled with SDs, such as a green traffic light signaling drivers to press the gas pedal, or a stop sign signaling the behavior of braking. The effective use of the SD allows practitioners to structure learning environments and teach new responses reliably.
The Importance of the S-Delta
To understand the function of the SD, it must be contrasted with the S-Delta (\(S^\Delta\)), which is the stimulus that signals the absence of reinforcement for a particular behavior. The \(S^\Delta\) tells the learner that performing the specific action will not result in the desired outcome. Learning to respond only to the SD and not to other stimuli, including the \(S^\Delta\), is known as discrimination training.
This differentiation process ensures that the learned skill is precise and appropriate to the context. For instance, if a child is taught to clap when the therapist says “Clap hands” (SD), the therapist must also ensure the child does not clap when the therapist says “Hands down” (\(S^\Delta\)). In the presence of the \(S^\Delta\), the behavior is not reinforced, which helps the learner distinguish between when an action is appropriate and when it is not.
A common real-world illustration involves a teacher asking a question (SD) versus the teacher talking to a colleague (\(S^\Delta\)). Raising one’s hand and answering is reinforced when the teacher asks the class a question, but that same behavior is not reinforced when the teacher is engaged in a private conversation.
Implementing SDs Prompting and Fading
When a learner is first introduced to a new SD, they may not know the correct response, requiring assistance to ensure success. Prompting involves giving supplementary cues or help to increase the likelihood of the correct response occurring immediately after the SD is presented. These prompts can take various forms, such as a physical guide for a motor task, a verbal hint, a gestural cue, or a visual aid.
To achieve independence, control of the behavior must shift from the temporary prompt to the natural SD itself through a systematic process called fading. Fading involves the gradual reduction of the prompt’s intensity or intrusiveness. The goal is to make the prompt less noticeable over successive trials until the learner responds only to the SD, such as the verbal instruction or the visual context.
For example, when teaching a child to respond to the SD “What color is this?” while holding a red object, a therapist might initially use a full verbal prompt by saying “Red” immediately after the SD. Through fading, the therapist might next use only the initial sound (“R-“), then a cough, and finally remove the prompt entirely. This methodical removal prevents the learner from becoming dependent on the prompt, ensuring the SD gains control over the response.
Achieving Stimulus Control and Generalization
The establishment of stimulus control occurs when a specific behavior is consistently and reliably performed only in the presence of the SD. Strong stimulus control is evidenced by the learner responding immediately and accurately when the SD is presented and withholding the response when the SD is absent. This signals that the individual’s behavior is now regulated by the relevant environmental cue.
Once stimulus control is achieved in the initial teaching environment, the next necessary step is generalization. Generalization is the ability of the learner to perform the acquired skill across different, but related, stimuli, people, or settings. For instance, a child who learns to respond to the SD “Clap hands” from one therapist should be able to respond correctly to the same instruction given by a parent or a teacher in a different room.
Generalization ensures that the skills learned in a structured setting are functional and adaptable in the real world. To promote this outcome, practitioners program for generalization by varying non-essential aspects of the SD, using multiple people to deliver the instruction, or practicing the skill in various locations.