The discriminative stimulus (SD) is a fundamental concept within the framework of operant conditioning, a key component of behaviorism pioneered by B.F. Skinner. In operant conditioning, behaviors are modified through reinforcement or punishment based on their consequences. The discriminative stimulus plays a crucial role in signaling when a particular behavior is likely to be reinforced or punished. This stimulus essentially discriminates between situations where a behavior will lead to a consequence and situations where it will not. But the discriminative stimulus does more than discriminate. It elicits particular responses to which it has been conditioned through the action of reinforcement. This conditioning happens when a reinforcer occurs after a response that has occurred in the presence of a stimulus, or stimulus complex.
The function of the discriminative stimulus can be understood through its ability to set the occasion for a specific behavior to occur. It informs the organism about the availability of reinforcement or punishment contingent upon engaging in a particular behavior in a given context. Essentially, the discriminative stimulus serves as a cue that influences the likelihood of a behavior occurring under certain conditions. It makes a distinction between responses in a person’s response repertoire. We can think about a response as connected to a particular stimulus and stimulus complex. The person responding may or may not be aware of the discriminative stimulus and the fact that it is setting the occasion for a particular response. This knowledge by the responder is not necessary for the discriminative stimulus to exert its function of controlling a response.
To illustrate this concept, let’s consider an example involving a laboratory rat learning to press a lever to receive food. In this scenario, the discriminative stimulus might be a light that is illuminated above the lever. When the light is on (the discriminative stimulus), pressing the lever is likely to result in food being dispensed (reinforcement). However, when the light is off, pressing the lever will not lead to any food being dispensed. Here, the presence or absence of the light serves as a discriminative stimulus that signals whether the behavior of lever-pressing will be reinforced. This situation was developed for controlled scientific research on the operant. In the real world, discriminative stimuli are seldom this simple. That’s why one should always think of a discriminative stimulus as a complex of multi-sensory stimuli. Only very simple behaviors are controlled by a single stimulus. For example, a knee-jerk reflex is controlled by a single tap on the knee cap.
Similarly, in a classroom setting, consider a student who raises their hand to answer a question. The teacher’s acknowledgment of raised hands (the discriminative stimulus) by calling on students to answer reinforces the behavior of raising hands. However, if the teacher ignores raised hands and instead selects a student who calls out without raising their hand, the discriminative stimulus for raising hands is not present, and the behavior is less likely to be reinforced. In this example, the response of raising hands is not presented by the student who calls out. However, other students may be raining their hands. Thus, the discriminative stimulus that experiences reinforcement is the teacher asking a question. The response that gets reinforced is calling out. This is a different operant from a discriminative stimulus of teacher asking question, response of student raising hand, and reinforcer of student allowed to answer question. For the students who raised their hands, their responses were not reinforced when the teacher allowed the student to call out. Because their response of raising their hands was not reinforced in the presence of the discriminative stimulus of the teacher asking a question, their responses would be less likely to occur when the teacher asked the class other questions.
The discriminative stimulus can also be used to differentiate between different responses within the same situation. For example, imagine a pigeon trained to peck a green disc for food reinforcement but not a red disc. In this case, the color of the disc serves as the discriminative stimulus. When presented with the green disc, pecking behavior is reinforced with food. However, when presented with the red disc, pecking behavior is not reinforced. Here, the color of the disc functions as a discriminative stimulus, signaling when the behavior of pecking will be reinforced. While organisms such as pigeons can respond to discriminative stimuli, the stimulus complexes to which they respond are much simpler than those to which humans can respond.
Furthermore, discriminative stimuli can be complex and multi-dimensional. They can involve not only physical stimuli such as lights, sounds, or colors but also temporal cues, social cues, or even internal states of the organism. For instance, a dog may learn that the presence of its owner (a social cue) signals the likelihood of going for a walk, whereas the absence of the owner does not. While this description is accurate, it is not meant to suggest that an absence of anything is a discriminative stimulus. When a particular stimulus, such as a teacher asking a question, does not occur, other stimuli do. Remember, a discriminative stimulus occurs within a complex of other stimuli. For example, a teacher asking the class a question may also be accompanied by the teacher standing in front of the classroom with instructional material written on the white board. The teacher standing in front of the classroom with instructional material written on the white board but not asking a question is a different discriminative stimulus from the teacher standing in front of the classroom with instructional material written on the white board and asking a question. Both are a discriminative stimulus complex and the second complex described will not gain control over students raising their hands.
In conclusion, the function of the discriminative stimulus in operant conditioning is to signal when a behavior is likely to be reinforced or punished. By providing cues that discriminate between situations where a behavior will or will not be reinforced, discriminative stimuli play a crucial role in shaping behavior in various contexts. Whether it’s in a controlled laboratory setting or in everyday life situations, the discriminative stimulus influences the probability of specific behaviors occurring based on the consequences that follow the behaviors.