A discriminative stimulus is a term used in psychology, particularly within the branch of behaviorism and operant conditioning, to refer to a specific type of stimulus that is used to guide behavior.
A discriminative stimulus is a particular type of stimulus that signals whether a reinforcement will occur if someone performs a behavior. That is, it’s a cue that signals whether a certain behavior or response will be rewarded or punished; or, whether the behavior will not lead to any form of reinforcement.
For example, in a classic experiment involving a rat and a lever, the light in the cage might turn on before food is delivered. This light is a discriminative stimulus: it signals to the rat that pressing the lever now will result in food. If the light isn’t on, the lever can still be pressed, but it won’t lead to food.
The key feature of a discriminative stimulus is that it does not necessarily trigger a behavior, but rather, it sets the context for the behavior. The rat in the experiment doesn’t press the lever just because the light is on, but rather because it has learned that the light signals that lever-pressing will result in food.
Discriminative Stimulus Definition
A discriminative stimulus is a stimulus that predicts the delivery of a reinforcer. It is a term often used in Applied Behavior Analysis (ABA) when implementing an intervention to change an individual’s behavior.
Cooper et al. (2020) defined discriminative stimulus as:
“a controlling stimulus that sets the occasion for reinforcement of an operant” (p. 208).
In other words, a discriminative stimulus informs the organism that if a particular behavior is exhibited, a reward is likely.
When referring to a discriminative stimulus that is associated with reinforcement, the symbol SD (pronounced S-dee) is used.
When referring to other stimuli that do not predict reinforcement, the symbol SΔ (pronounced S-delta) is used.
For example, if a child with autism is asked to choose the correct colored card of three options, then the correct card is the SD and the two incorrect cards are the SΔs.
The SD is present in the environment and signals to the student that a certain behavior will be rewarded.
In applied behavioral analysis, SD is considered an antecedent stimulus. When the child sees the SD, it knows that if they engage in a specific behavior, they will be rewarded.
A Hypothetical Example of Discriminative Stimulus
The figure below shows the idealized graph of a hypothetical example of SD and SΔ.
A pigeon placed in a Skinner Box is rewarded when it presses a lever, but only when a red light is on (SD). However, the pigeon is not rewarded when it presses the lever when the green light is on (SΔ).
The red light is a discriminative stimulus (SD) and the green light is the extinction stimulus (SΔ).
The graph depicts the cumulative number of times the pigeon pressed the lever over a 90-minute period.
In the beginning, the number of lever presses is roughly equal. As the pigeon begins to decipher the contingency between reward, lever pressing, and color of light, a distinct pattern of behavior emerges.
After about 60 minutes, lever pressing when the green light is on nearly ceases. The graph shows know cumulative increase in behavior. However, the pigeon continues to press the lever when the red light is on.
Discriminative Stimulus vs Stimulus Discrimination
These two terms are very similar, but are used in different contexts. Discriminative stimulus is used in ABA to refer to when a therapist or teacher should reward a behavior and when they should not.
As with the example above, when a child chooses SD, their behavior is rewarded. However, if they choose SΔ, then their behavior is not rewarded. Discriminative stimulus identifies when a reward is given.
The term stimulus discrimination is used in when referring to one stimulus triggering a response while other stimuli do not.
For example, if a dog has been conditioned to salivate in response to the sound of one specific bell, but it does not salivate in response to all bells, that is stimulus discrimination.
Stimulus discrimination is a phenomenon usually associated with Pavlov’s (1927) theory of learning known as classical conditioning.
Discriminative Stimulus Examples
- Asking for Ice-Cream: Over time, a child has learned that if they ask for an ice-cream when mom is in a good mood (SD), their request will be rewarded. However, when mom has had a bad day (SΔ), asking for an ice-cream will not result in receiving an ice-cream.
- Doing Homework Properly: Sometimes the discriminative stimulus is a behavior exhibited. For instance, when a child studies intently and maintains focus (SD) for an extended period of time, they will be rewarded by their parents. However, if they goof-off (SΔ) a lot while studying and are not serious, they are not rewarded.
- Teaching Assistant A and B: When a student encounters a problem and teaching assistant A is in the room, they ask because teaching assistant A (SD) always provides clear and concise explanations. However, when teaching assistant B is in the room, they don’t ask. Teaching assistant B (SΔ) always provides explanations that are muddled and confusing.
- Which Grandparent Gives Candy: A child quickly learns which grandparent will give candy when asked for it and which won’t. So, when they see one grandparent (SD), they ask for candy. But when they see the other one (SΔ), they do not.
- Behaving in Class: Primary school students have learned which teachers they can defy and which ones they must obey. As soon as one walks into the room, the students all sit quietly. However, when the other teacher enters the room, the students completely ignore them and continue playing. One teacher is an SD for play, and the other is an SΔ for being serious.
- A Barking Dog: A dog can sense which people their owners like and which they do not. So, a dog will only bark when some friends knock at the door (SD), but not others (SΔ). In this example, the dog’s reward might not be easily observed, but most dogs can sense when their owner is pleased.
- Red Light, Green Light: Because of previous learning, drivers go when they see a green light (SD). Driving in response to the green light has been rewarded. However, when they see a red light (SΔ), they stop. Driving has not been rewarded in response to a red light.
- Passing the Basketball: When deciding to whom the ball should be passed to, the point guard knows that passing to player X (SD) usually results in a score, while passing to player Y (SΔ), usually results in a turnover.
- Locating Books on the Shelf: Searching for a book in the library involves looking at the call numbers posted on the books and moving on until the correct book is located. In this example, all of the wrong call numbers are SΔs, and the one correct call number is the SD.
- Time for Dinner: The aroma that fills the house while someone is cooking indicates that dinner is almost ready (SΔ), but not yet. However, the sounds of dinner plates and glasses being set on the table means the meal is ready (SD).
Applications of Discriminative Stimulus
1. In Applied Behavior Analysis (ABA)
Applied behavior analysis (ABA) is a behavior modification strategy that utilizes the principles of classical and operant conditioning to decrease undesirable behavior and increase desirable behavior (Madden, 2012).
It is often utilized with students or adults that are suffering from a learning disability or behavioral disorder.
A psychologist or teacher incorporates a discriminative stimulus to teach the individual a new behavior or reward existing behaviors.
For example, a child can be taught to say “please” when they want to play with a toy by being prompted with a card which depicts one person politely asking for something.
If the child says “please” when being shown the card (SD), then they are rewarded with the toy.
After the child has been taught this new strategy for getting what they want, in a positive and constructive manner, they can then generalize that learned behavior to other situations and say “please” when they want other objects.
Types of Stimulus Discrimination In ABA
There are several types of discriminative stimuli that are commonly used in ABA.
Visual SDPictures or symbols that depict or represent the desired target behavior are frequently used as discriminative stimuli. A picture of a pencil can be used to prompt a child to write, or a picture of a coat, hat, and boots can let the child know that it is time to put on the clothes they wear to go outside.
Spoken words or short phrases can help a child know when they should engage in certain behavior. For example, saying “time to clean-up” can serve as a verbal cue for the child to put away their toys or clean their desks.
Certain objects in a classroom can serve as discriminative stimuli. This can include a single object or maybe an entire classroom setting. For example, entering a library can trigger thoughts about being quiet and signify that it is time to go find a book to read.
Not all children will respond to verbal or visual cues, so the therapist or teacher may need to use a tactile SD such as a physical touch. Lightly touching the child on the shoulder can serve as a prompt for the target behavior. Tactile discriminative stimuli are especially effective with kinesthetic learners.
Creating a Stimulus Discrimination Hierarchy
Helping a child learn a target behavior can take time. Typically, the teacher will start by creating a hierarchy of prompts (i.e., discriminative stimuli) that are arranged from very intrusive to nearly non-existent. The goal is that eventually the SD will not be needed at all.
However, the first SD may involve physically guiding a child’s hand or adjusting their torso. Once the child needs very little assistance in this regard, then the teacher may move on to a slightly less intrusive prompt such as a physical gesture or verbal instruction.
This process is sometimes referred to as fading; use of the SD is gradually diminished.
Fading is an important part of the ABA process. An over-reliance on prompting with the SD can lead to prompt dependency. Fading plays an important role in achieving the eventual goal of the child engaging the target behavior without prompting.
The key to utilizing a discriminative stimulus effectively is to know the student’s learner profile and their particular learning style. This takes time, but when teachers work with others that interact with child frequently it can be accomplished.
A discriminative stimulus is a stimulus that informs a person that a certain behavior will be rewarded.
People’s lives are full of discriminative stimuli that tell us when and when not to engage in a certain action. For example, seeing a green light when driving tells us to keep going.
Students may learn that some teaching assistants will provide better explanations than others, while basketball players will understand who to pass the ball to if they want to score.
Discriminative stimuli are frequently used in ABA to help children learn constructive behavior. A teacher or therapist might show a card, provide a light touch on the shoulder, or say a short phrase that will let the child know that if they engage in a certain action, they will be rewarded.
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Dr. Chris Drew is the founder of the Helpful Professor. He holds a PhD in education and has published over 20 articles in scholarly journals. He is the former editor of the Journal of Learning Development in Higher Education. [Image Descriptor: Photo of Chris]