A learner who says "hi" to the classmate who waves back, but not to the one who never looks up, is showing you something important: behavior that changes depending on what is in front of it. That pattern is stimulus discrimination, and you will recognize it in nearly every program you write.
Why it matters
Almost every skill you teach is a discrimination. Reading, following instructions, choosing the right bathroom door, and knowing when a joke fits the room all depend on responding one way to some stimuli and another way to others. When a program stalls, the problem is often that the learner's behavior is under the wrong stimulus, not that the response itself is missing.
In case work you rarely need the definition alone. You need to decide whether discrimination has occurred, label a stimulus as an SD or an S-delta, and explain why a learner "knows" a skill in one setup and not another.
Key ideas
- Discrimination is seen in the data. You recognize it when a response occurs more often (or differently) in the presence of one stimulus than another. If responding is the same across stimuli, discrimination has not developed.
- It comes from differential reinforcement. In discrimination training, the response is reinforced when one stimulus is present and not reinforced (extinction) when another stimulus is present. Over time, responding tracks the stimuli.
- SDs signal availability; S-deltas signal unavailability. An SD evokes the response because responding in its presence has paid off. An S-delta is not a punisher or a "stop" signal; it simply marks a condition in which the response has not been reinforced.
- Discrimination and generalization sit on a continuum. Responding to many similar stimuli is generalization; responding only to a narrow range is discrimination. A steep generalization gradient means tight discrimination, and a flat one means broad generalization.
- Discriminations can be simple or conditional. In a simple discrimination, one stimulus controls the response on its own: say "ball" when the ball is shown. In a conditional discrimination, which stimulus works as the SD depends on another stimulus, such as the sample in match-to-sample.
- A related signal exists for punishment. A stimulus in whose presence a response has been punished can suppress that response. Keep it separate from the S-delta, which involves no punishment at all.
Definitions
- Stimulus discrimination: a response occurs at a different rate, or in a different form, in the presence of different stimuli, because of a history of differential consequences.
- Discriminative stimulus (SD): a stimulus in whose presence a response has been reinforced (and in whose absence it has not), which makes that response more likely when the stimulus is present.
- S-delta: a stimulus in whose presence a response has not been reinforced, so the response becomes less likely when it is present.
- Discrimination training: reinforcing a response in the presence of one stimulus and withholding reinforcement in the presence of others.
- Conditional discrimination: a discrimination in which the function of one stimulus depends on the presence of another stimulus.
Examples from a session
Recognizing discrimination across settings
- Home: Amara, 8, often asks for screen time when her grandmother is home, because Grandma usually says yes. She almost never asks when her older brother is babysitting, because he never does. Her grandmother is an SD for asking; her brother is an S-delta.
- Clinic: During a tact program, Rafael, 4, says "ball" when shown a ball and stays quiet or says something else when shown a shoe. His responding differs across the pictures, which is the discrimination you want.
- School: Hana, 14, uses her phone freely in the cafeteria and rarely takes it out in chemistry, where the teacher collects it every time. Phone use differs across settings, so this is discrimination too. Notice, though, that chemistry class is linked to punishment (losing the phone), so it is not an S-delta.
- Adult day program: Marcus, 42, waits by the door when the green "van here" card is posted and keeps working when the card is red. Waiting was only ever followed by the van ride during the green card.
- Non-example: Rafael also says "ball" to a beach ball and a soccer ball he has never seen. Responding the same way to new, similar stimuli is generalization, not discrimination.
When you analyze a case, compare responding across conditions. One condition alone tells you nothing about discrimination; you need a difference between two or more stimuli and a history that explains it.
Common mistakes / exam traps
- Calling an S-delta a punisher. An S-delta signals that reinforcement is not available. Nothing aversive has to follow the response in its presence.
- Treating one high-rate condition as proof. Lots of responding when the SD is present does not show discrimination unless responding is lower when the S-delta is present.
- Missing faulty stimulus control. A learner who always picks the card on the left, or watches the therapist's eyes, may score well while discriminating the wrong feature. Rotate positions and keep your own cues neutral.
Exam trap
Discrimination vs. generalization. Ask two questions: Does responding differ across the stimuli? Is there a history of differential consequences (usually differential reinforcement) that explains the difference? If responding is the same across similar stimuli, the label is generalization, not discrimination.
Exam trap
SD vs. motivating operation. An SD changes how likely the response is because it signals that the reinforcer is available. A motivating operation changes how valuable the reinforcer is right now. If a stem shows responding shifting with deprivation or satiation while the stimuli stay the same, the variable is an MO, not an SD.
Quick check
Jun, 30, used to ask both van drivers about the day's schedule. One driver always answers; the other wears headphones and never responds. Jun now asks only the first driver. Which concept best describes this change?