A discriminative stimulus is "a stimulus in the presence of which a particular response will be reinforced".- (Malott, 2007, p. 202).
As the intoductory quote states, the discriminative stimulus is ‘a stimulus in the presence of which a particular response will be reinforced’ (Malott, 2007, p. 202).
Before getting into more detail on this, we're going to use an anology to describe the discriminative stimulus (and the stimulus delta).
Within any educational programme there are correct and incorrect answers. For example, consider a letter discrimination programme where printed cards with different letters on them are placed on the desk in front of a student and the teacher then asks the student to “Point to B”.
When the teacher asks the student to point to the card with a “B” on it, then the card with the letter “B” on it is the correct answer while the cards without this letter are the incorrect answers.
Within ABA these cards are regarded as types of stimuli, with each individual card called a stimulus. From this you could then say that in this ABA programme there are correct and incorrect stimuli.
Therefore, a card with a “B” on it is a correct stimulus while a card without a “B” on it is an incorrect stimulus.
When Behaviour Analysts talk about a correct stimulus they would call it a Discriminative Stimulus which is often shortened into just “SD” and is pronounced as “ess-dee”. An incorrect stimulus is called a Stimulus Delta which is shortened into “SΔ” and is pronounced “ess-delta”.
The discriminative stimulus is defined as ‘a stimulus in the presence of which a particular response will be reinforced’ (Malott, 2007, p. 202).
Keeping with the letter discrimination programme, this means that when the teacher says “Point to B” if there is a card on the table with a letter “B” on it, then the student’s response of selecting this card will lead to reinforcement.
This reinforcement could be praise (e.g. "Well done!”) or it could be through delivery of a generalised reinforcer (e.g. a sweet).
An everyday example would be if you went to wash your hands and wanted hot water. The hot water tap (faucet) is the SD in this situation because selecting the hot tap will lead to the delivery of reinforcement (hot water).
The stimulus delta is defined as ‘a stimulus in the presence of which a particular response will not be reinforced’ (Malott, 2007, p. 202).
Again keeping with the letter programme, this means that when the teacher says “Point to B” and there are cards on the table without the letter “B” on them, if the child selects one of these cards then no reinforcement would be delivered.
The basic way to remember this is to consider the stimulus delta (SΔ) as the incorrect answer and so no reinforcement is given when a student gets something wrong.
Using the hand washing example, if you wanted hot water to wash your hands and you selected the cold water tap then your response does not lead to the delivery of reinforcement because you did not get the hot water that you wanted.
This is another discrimination programme where a red card and a blue card are placed on the desk and the student is given another card and told to put it with the same colour.
For the first trial the teacher puts the red and blue cards on the desk and gives the student a red card and says “Put Red with Red”. For the second trial the teacher asks the child to “Put Blue with Blue”.
In each of these cases which of the cards on the desk is the discriminative stimulus (SD) and which is the stimulus delta (SΔ)?
As you can see from the image above, when the teacher says “Put Red with Red” then the red card is the SD (correct answer) and the blue card is the SΔ (incorrect answer).
When the teacher says “Put Blue with Blue” then the blue card is now the SD (correct answer) and the red card is the SΔ (incorrect answer).
If the student pointed to the SD then this correct response would be reinforced but if they pointed to the SΔ then their incorrect response would not be reinforced.
In this programme you are teaching your student Anne to say hello to one of the other students in the school. This other student is called Brian.
This programme is run whenever the opportunity arises throughout the school day. During lunch such an opportunity arises while Anne is in the playground and Brian is there too playing with his friends Toby and Liam.
The teacher on the playground prompts Anne to go and say hello to Brian. In this instance, Brian is now a discriminative stimulus (SD) while Toby and Liam are each a stimulus delta (SΔ).
So if Anne says hello to Brian then this will have been the “correct” response and reinforcement would be delivered by the teacher. If she said hello to either Toby or Liam then this would be the “incorrect” response because she was prompted to say hello to Brian.
A discriminative stimulus does not have to be an educational material that gets put on a desk; it could even be a noise. Consider what happens when the school lunch is over and the bell rings.
The ringing of the bell is now a discriminative stimulus. If Anne responds to it “correctly” by going back inside to her classroom she will be praised by her teacher (reinforcement delivered).
The discriminative stimulus (SD) and stimulus delta (SΔ) are each defined within a discrete trial (but only for some educational programmes). They are typically placed into the “prompt” component when the discrete trial is written out.
The two programmes described above where the child has to discriminate between different stimuli is one situation where the SD and SΔ would be defined. When we say defined, we mean that the numerical amount of each type of stimulus is defined.
We’ll use the letter discrimination programme as an example. Written into the prompt component might be “1 SD and 1 SΔ”. This tells the teacher who will be running the programme to put one correct stimulus (1 SD) and one incorrect stimulus (1 SΔ) on the table before asking the child to “Point to B”.
Typically, there would only be one SD while there could be numerous SΔ. For example, you might see “1 SD and 2 SΔ” or “1 SD and 3 SΔ” or “1 SD and 4 SΔ”. There could be any number of SΔ placed on the desk.
Generally, greater numbers of SΔ mean the programme is more difficult. For this reason, when starting a new programme like those described above, you would only start the programme with “1 SD and 1 SΔ”.
You may even start with a “non-exemplar” which would be like using a blank card as the SΔ while the SD would be the card with the letter “B” on it.
The programme where Anne was being taught to say hello to Brian would typically not have the SD and SΔ defined. This would be due to the fact that it would be difficult to make sure that a specific amount of other students were present every time Anne and Brian would meet.