What Is Stimulus Control?
Stimulus control occurs when the frequency, latency, duration, or amplitude of a behavior is altered in the presence of an antecedent stimulus. It develops when a behavior is reinforced in one stimulus condition and not reinforced in another. Stimulus control explains why you answer your phone when it rings and not when it is silent, and why a child says "red" when shown a red card but not when shown a blue one.
The Discriminative Stimulus (SD)
A discriminative stimulus (SD, pronounced "ess-dee") is an antecedent stimulus in the presence of which a particular response has been reinforced. Because of that history, the SD signals that reinforcement is available for the response, and the response is more likely when the SD is present.
Examples
- A teacher says "What color is this?" while holding a red block. The question and the block are SDs for saying "red."
- An open sign on a store door is an SD for entering, because entering in that condition produces access to products.
- A green traffic light is an SD for pressing the accelerator.
An SD does not elicit behavior the way a conditioned stimulus does in respondent conditioning. It sets the occasion for behavior because of a history of reinforcement.
The Stimulus Delta (S-delta)
A stimulus delta (SΔ or SD with a delta symbol) is an antecedent stimulus in the presence of which a particular response has not been reinforced. The behavior becomes less likely in the presence of an S-delta.
Examples
- A closed sign on a store door is an S-delta for entering the store.
- When a teacher is on the phone, a child's requests for help go unanswered, so the teacher on the phone is an S-delta for requesting help.
- A blue card is an S-delta for saying "red."
Whether a stimulus is an SD or S-delta depends on the response in question. The same stimulus can be an SD for one behavior and an S-delta for another. A red light is an S-delta for accelerating but an SD for braking.
How Discriminations Develop
A discrimination develops through differential reinforcement: reinforcing the response in the presence of the SD and withholding reinforcement in the presence of the S-delta. Over time the learner responds mainly when the SD is present.
- Present the SD and prompt the correct response.
- Reinforce the correct response in the presence of the SD.
- Present the S-delta and do not reinforce the response (or reinforce a different response).
- Fade prompts so the SD alone evokes the response.
- Test for discrimination with probes that mix SD and S-delta trials.
The resulting pattern is called a three-term contingency under stimulus control: SD, response, reinforcement. In the presence of the S-delta, the same response does not produce reinforcement.
Discrimination Training Procedures
Simple and conditional discriminations
A simple discrimination involves one SD and one response, as in responding only when a bell rings. A conditional discrimination involves a response that is correct only given a certain sample stimulus, as in matching-to-sample. Teaching a learner to select the written word "dog" when the spoken word "dog" is presented is an example of conditional discrimination.
Errorless teaching
Because errors can lead to frustration, many teachers use errorless learning or most-to-least prompting, along with stimulus fading and stimulus shaping to transfer stimulus control from a prompt to the target SD. For example, you might begin with a highly salient correct card and gradually make the incorrect cards more distinct in size or color until the learner responds to the target card alone.
Stimulus class and generalization
Stimulus generalization occurs when behavior reinforced in the presence of one stimulus also occurs in the presence of similar stimuli. Discrimination training narrows generalization to the appropriate stimuli. Both are important: you want the child to say "dog" to many different dogs but not to cats.
SD vs Related Terms
- SD vs motivating operation: An SD signals that reinforcement is available. A motivating operation changes the value of the reinforcer and the frequency of behavior related to it. Being hungry is an MO; seeing an open restaurant sign is an SD.
- SD vs prompt: A prompt is a supplemental antecedent stimulus that evokes the correct response during teaching. It is temporary and is faded.
- SD vs conditioned stimulus: A CS elicits a reflexive response through pairing. An SD evokes operant behavior through a reinforcement history.
- S-delta vs SD for punishment (S^Dp): An S-delta signals that reinforcement is unavailable. An S^Dp signals that punishment is likely, which suppresses behavior.
Clinical Applications
- Safety skills: A stop sign becomes an SD for stopping and a street without a crossing guard becomes an S-delta for crossing without checking traffic.
- Social behavior: A peer who is available and facing the learner is an SD for initiating conversation. A peer wearing headphones may be an S-delta.
- Classroom routines: A visual schedule can become an SD for transitions, and a timer can signal when reinforcement will be available.
- Problem behavior: If a child screams only in the presence of one caregiver who gives in, that caregiver is an SD for screaming, and the other caregiver, who does not respond, is an S-delta.
Exam Strategies
- Identify the response in the question before deciding if a stimulus is an SD or S-delta.
- Ask whether the response is reinforced or not reinforced in the presence of the stimulus.
- Remember that an SD is about availability of reinforcement, not about value of the reinforcer.
- If a question describes a stimulus that evokes behavior due to past punishment, think S^Dp.
Key Takeaways
An SD signals reinforcement is available for a response, and an S-delta signals it is not. Discrimination develops through differential reinforcement. Keep these distinct from motivating operations, prompts, and conditioned stimuli, and always evaluate the relation with reference to a specific response.