Differential Reinforcement and Discrimination

When an organism makes a response in one situation but not in another, we say that the animal shows a discrimination between the situations. The simplest way to train a differential response or discrimination is to reinforce an operant in one situation and withhold reinforcement in the other.

Figure 8.1 shows the development of a differential response to a single key that is alternately illuminated red and green for 5 min. The graph shows the cumulative number of responses over a 90-min session. Pecks to the red light by a pigeon are intermittently reinforced with food. Responses emitted in the presence of the green light are extinguished, never reinforced.

As you can see in this idealized experiment, the pigeon begins by emitting about the same number of responses to the red and green stimuli. After about 20 min, the cumulative response curves start to separate. This indicates that the bird is pecking more in the presence of red than in the presence of green. At about 60 min, the pigeon seldom responds when the key is green, and this is shown by the leveling off of the curve for this stimulus. Notice, however, that the cumulative curve for pecking the red key continues to rise. Because the bird pecks in the presence of red but does not respond when the key is green, we may say that the pigeon discriminates between these two stimuli. At this point, it is possible to label


FIG. 8.1. Development of a differential response in the presence of red and green stimuli. Cumulative number of responses over a 90-min session in which responses in the presence of red are reinforced and responses in the presence of green are on extinction.

the red and green stimuli in terms of their functions. The red light is called a discriminative stimulus, or SD, and the green color is an SA, or extinction stimulus.

Suppose that the bird is returned to its home cage after 90 min of such differential reinforcement. On the next day, the pigeon is again placed in the operant chamber and the key is illuminated with the red light. During this test session, reinforcement is not given for pecking in the presence of either red or green. Because of its previous training, a high probability exists that the bird will strike the red key. Over a 60-s period, the bird may emit many responses when the SD is present. After 60 s, the key light is changed from red to green. When the green light comes on, the probability of response declines and the bird makes few pecks to the green key. By continuing to alternate between red and green, the researcher can show the stimulus control exerted by the respective stimuli.

Stimulus control refers to a change in behavior that occurs when either an sd or an

SA is presented. When an SD is presented, the probability of response increases; when an SA is presented, the probability of response decreases. The stimuli that commonly control human behavior occur across all sensory dimensions. Stopping when you hear a police siren, coming to dinner when you smell food, expressing gratitude following a pat on the back, elaborating an answer because the student looks puzzled, and adding salt to your soup because it tastes bland are instances of stimulus control in human behavior.

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