Copyright © 2007-2018 Russ Dewey
Stimulus Control
Stimulus control is a term used to describe situations in which a behavior is triggered by the presence or absence of some stimulus. If a person always eats when watching TV, then (in the operant conditioning use of the term) eating behavior is controlled by the stimulus of watching TV.
A student might be talkative with friends but silent in class, the social environment is exerting stimulus control over talking. Stimulus control can either promote or inhibit a behavior.
What is stimulus control? What is the difference between consequences and antecedents?
Teachers of operant conditioning sometimes say behavior is controlled by its consequences. That sums up contingency management, but it is an incomplete description of operant conditioning. In situations of stimulus control, it is antecedents, not consequences, that control behavior.
Antecedents (ant-a-SEED-ince) are things that come before. In operant conditioning, antecedent stimuli are those occurring before a behavior.
In a Skinner Box, a light or buzzer might signal that food reinforcement is available. Behaviors of lab animals are easily brought under control of such stimuli, and that is where the concept of stimulus control originated.
Antecedents, once noticed, function as signals. A red light is antecedent to cars stopping at a traffic light (it signals for them to stop). A green light is antecedent to cars moving forward again.
Much of our day to day behavior is directed by signals. An alarm clock may get us up, a glance at the clock may get us out the door. We might then exchange signals with a car to start it up, follow traffic signals to work or school, and park only where we see an empty spot.
That is why the generalization that "behavior is controlled by its consequences" is incomplete. Moment by moment, a lot of behavior is controlled by signals, which are antecedents to behavior, not consequences.
What statement "sums up much of operant conditioning" and why is it incomplete?
Stimulus control can be ongoing and flexible. When travelling on U.S. interstate highways, drivers tend to maximize their speed by driving at the same speed as surrounding drivers, forming a pack.
The pack speed tends to ratchet up, opportunistically, as traffic permits. Soon the whole group might be driving well over the speed limit.
Then every car slows down at once. This happens if a driver far ahead spots a police car and slows down, flashing brake lights. The slowdown ripples through the group of cars as people see it coming.
A driver would be foolish to pull out from the pack and go faster, at that point. (Bad consequences are likely.) Experienced drivers remain under "stimulus control" of cars around them. Adaptive cruise control automates this process.
Variations of this tactic appear in nature. Hoofed animals form herds. Birds flock. Fish form schools. The rule of thumb imitate those around you minimizes danger, so this tactic emerges repeatedly in group-living animals.
Discriminative Stimuli
Psychologists say an operant behavior is under stimulus control if it is triggered (or suppressed) by certain stimuli. Because an organism must discriminate between controlling stimuli to respond appropriately, they are called discriminative stimuli.
An S+ is a discriminative stimulus that tells an animal reinforcement is available. An S- is a discriminative stimulus that tells an animal reinforcement is not available. Animals quickly learn to approach an S+ and avoid an S-.
A stimulus is made into an S+ by consistently following it with reinforcement. For example, one student told how her father tossed a hula-hoop into a fishpond then tossed food inside the ring.
After a few weeks, fish would swim to the inside of the ring as soon as it hit the water. (She said her father originally intended to use this technique to catch the fish for eating, but he became fond of his trained fish and would not eat them.)
What is an S+? What is an S-? How can you create an S+ for an animal?
Time can become a discriminative stimulus. For example, students may fidget or begin closing books before the end of a class hour.
Certain words can trigger a behavior. A teacher must be careful about uttering the words "In conclusion..." near the end of a classroom hour, because students will start packing up to leave. Those words become an S+ for a negative reinforcement: the end of class.
How do teachers observe the effects of time, or words, as an S+?
A person can be a discriminative stimulus. Be kind to a dog or cat and you become an S+, so the animal will approach you voluntarily. A person who is mean or cruel becomes an S-. The animal will avoid that person.
Discrimination learning can be quite complex. In a classic study Herrnstein, Loveland, and Cable (1976) reinforced pigeons with grain for discriminating pictures containing human beings.
The pigeons were reinforced for key-pecks only when they saw a picture containing a human. The authors describe the stimuli:
Approximately half the photographs contained at least one human being; the remainder contained no human beings–in the experimenter's best judgment. In no other systematic way did the two sets of slides appear to differ.
Many slides contained human beings partly obscured by intervening objects: trees, automobiles, window frames, and so on. The people were distributed throughout the pictures: in the center or to one side or the other, near the top or the bottom, close up or distant.
Some slides contained a single person; others contained groups of various sizes. The people themselves varied in appearance: they were clothed, semi-nude, or nude; adults or children; men or women; sitting, standing, or lying; black, white, or yellow. (p.287)
Despite the variety of people and poses, all five pigeons learned to identify slides that contained humans. Their performance continued to improve over a period of months.
When new slides were used, although the pigeons had never before seen them, the pigeons' responses were accurate. They identified the new slides containing humans.
The pigeons showed generalization by treating different pictures of humans the same way. They showed discrimination by reacting differently to pictures with and without humans. Logically, both processes must be involved in any accurate pattern recognition.
In follow-up studies, the pigeons were taught to recognize things like fish and trees they had never seen, being raised in a laboratory. They were able to do this, showing that pigeons' discrimination abilities did not depend upon prior experiences with the objects they learned to recognize.
What was a classic study by Herrnstein and Loveland with pigeons?
Applied Analysis of Antecedents
As noted above, if a behavior is reliably emitted or suppressed in the presence of a particular stimuli, the behavior is said to be under stimulus control. Humans exploit this tendency by engineering the environment to prompt desired behaviors.
Study skills books used in college courses advise students to set aside a particular time and place for study. The familiar time and location helps to trigger studying behavior.
"Getting started is half the battle" according to motivational self-help books. There is some truth to it. Usually studying (or any other desired activity) is not too painful once one gets started.
Problems occur when a person never gets started or procrastinates or gets distracted by less important tasks. Short-term, small rewards (like texting with a friend) can dominate over long-term, large rewards (like completing a difficult project).
How can you manipulate antecedent stimuli to help study more?
B.F. Skinner
B.F. Skinner, whose research on operant conditioning led to the concepts in the second half of this chapter, used stimulus control to encourage his scholarly work. He followed a fixed daily schedule. At 4 a.m. he got up and ate breakfast, then he wrote for about five hours.
How did B.F. Skinner apply this principle to increase his writing productivity?
Time of day and the stimuli associated with his home office served as discriminative stimuli to get him started on his writing. Around 10 a.m. (after the five hours of writing) Skinner took a walk down to the Harvard campus to meet his morning classes.
Walking probably became a stimulus for mulling over his lectures for the day. In the afternoon he attended meetings and scheduled appointments. With this routine he was always able to put in a few good hours of writing every day during his best part of the day for writing, early morning, while also scheduling adequate time for other activities.
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References:
Herrnstein, R. J., & Loveland, D. H. (1964) Complex visual concept in the pigeon. Science, 146, 549-551.
Herrnstein, R. J., Loveland, D. H., & Cable, C. (1976). Natural concepts in pigeons. Journal of Experimental Psychology: Animal Behavior Processes, 2, 285-302.
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