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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 these stimuli or "tell them apart" in order to respond to them in different ways, these are called discriminative stimuli.

What is an S+? What is an S-? How can you create an S+ for an animal?

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.)

How do teachers observe the effects of time, or words, as an S+?

Time of day 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 this behavior. A teacher must be careful about uttering the words "In conclusion..." near the end of a classroom hour, or books slam shut. Those words become an S+ for the negative reinforcer of finishing class.

A person can be a discriminative stimulus. A person who is kind to a dog becomes an S+; the dog will approach the person. A person who is mean or cruel to a dog becomes an S-; the dog will avoid the person.

Discrimination learning can be quite complex. In a classic study, Herrnstein , Loveland, and Cable (1964) 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.

What was a classic study by Herrnstein and Loveland with pigeons?

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 which 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. In follow-up studies, the pigeons were taught to recognize things like fish and trees that they had never seen, being raised in a laboratory. This showed that the pigeon's discrimination abilities did not require prior experiences with the objects they learned to recognize.

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