Behavioral Principles: Stimulus Discrimination and Generalization

As of 11/6/2017



See illustrations under part II – Generalization

A stimulus is defined as anything that acts on the organism. A stimulus acts on the body through sense receptors.

Q-1. That which acts on sense receptors is called a _______.

Q-2 Stimuli can be of many varieties: physical, chemical, biological, or social events which act on the organism. All of them act on the person. They are defined as _______.

Answer: (STIMULI)

The observable activity of the organism, as it moves about, stands still, seizes objects, pushes, pulls, makes sounds, gestures, and so on, is a response

Q-3. That which an organism does is known as _______.
a. a response
b. an effect
c. shaping

Answer: (a)

A child can be taught to respond indiscriminately or we can teach him/her to discriminate, or respond specifically under certain conditions and differently under different conditions. Early in life a child may be shaped to say “da-da” to all males. Eventually only one man must come to control the response if it is to be a useful response. We do this by reinforcing “da-da” only when it occurs in the presence of the real father. When a response is reinforced in the presence of one stimulus and not in the presence of another, the stimulus during which reinforcement occurs is called a discriminative stimulus or SD (pronounced S-Dee). Those conditions in which the response is not reinforced are called S-Delta. A different strength of response will occur between two or more stimulus stiuatuions as a function of differential reinforcement and in the presence of two or more stimulus situations as a function of differential reinforcement. To establish a discrimination, reinforce the response in the presence of a stimulus situation SD and do not reinforce it in the presence of the other situations S.

The response is maintained in the presence of the S-Dee (SD) and decreases in the presence of the S-Delta (S).

Q-4. A child learns that one person will reinforce talking while another will not, and subsequently talks only in the presence of the reinforcing audience. We say the child has made _______.
a. a generalization
b. a discrimination
c. an abstraction

Answer: (b)

Q-5. If two stimuli are intermingled, but responses are only reinforced in the presence of one stimulus, you will produce
a. generalization
b. extrasensory phenomena
c. extinction
d. discrimination

Answer: (d)

Q-6. The experimenter can set up a(n) _______ by presenting food only if the lever is pressed while the light is on rather than off.
a. operant level
b. discrimination
c. generalization

Answer: (b)

Q-7. A stimulus which indicates when a response may be reinforced
a. serves as a Discriminative Stimulus
b. influences the size of the rat’s cage
c. serves as a S-Delta

Answer: (a)

Q-8. When a response is not reinforced it will _______.
a. shape
b. extinguish
c. increase to its upper limit

Answer: (b)

Q-9-10. An organism is said to have developed a discrimination when responding is maintained in the presence of a       (1)        and has undergone some degree of extinction in the presence of       (2)      .
a. SD
b. S-Delta
c. a stimulus

Answer: 1. (a)
2. (b)

Q-11. Bringing an operant behavior under stimulus control involves three events; presenting a stimulus is the occasion upon which a response is followed by _______.


This discriminative stimulus, response, and reinforcer is called a three-term contingency.

Q-12. The following comprise a three-term contingency
a. stimulus, pause, response
b. response, pause, reinforcement
c. discrimitive stimulus, response, reinforcement

Answer: (c)

Q-13. Bringing an operant behavior under stimulus discrimination involves three events: presentation of a _______, which is the occasion under which a _______ is followed by _______.


Q-14. Conditioned discrimination is brought about through_______.
a. selective reinforcement and extinction
b. punishment
c. generalization


The more similar the stimuli which are to be discriminated, the less reliable, or more difficult, the discrimination will be.

Q-15. Of the following, the most difficult conditioned discrimination to establish would be that between metronomes beating _______
a. 92 and 144 times per minute
b. 122 and 144 times per minute
c. 132 and 144 times per minute

Answer: (c)

The process of discrimination has many interesting implications when we think of many of our so-called “abnormal” or inappropriate behaviors as SD problems. If you were to yell in a study room, we would wonder about you. On the other hand, if you did not yell at a football game, people would think you were some kind of a “nut,” “square,” etc. Consider, for example, Richard, who pulls down his pants. If he pulls down his pants in the middle of the street, we frown or arrest him. If he did not pull down his pants under other stimulus conditions, say, at bedtime, or in the bathroom, we would, to say the least, also wonder about him.

For a mnemonic device one could also refer to these three terms of contingencies as the ABC’s of behavior principles:

Antecedent conditions (SD)

Behavior (Responses)

Consequences (Reinforcement)

In the mnemonic device ABC,

Q-16 A refers to events


Q-17. B refers to events


Q-18. C refers to events


Q-19. SD’s are events which precede a behavior or are _______, and when the response occurs in the presence of the SD the behavior is _______.


Q-20. Positive reinforcers which follow a behavior are known as types of _______.



1. Token Economy and Stimulus Control: Narrol (1966) stated about his/her token economy program for alcoholics: “There needs to be within the new system more stimulus control, more of the explicit type of things, such as the red light behind the key for the pigeon, more discrimination. We could do this by a clearer written rule book and purchase list, with a consistent demand that the patient read and follow these or take the consequences of not knowing what is in them. I think it should be part of the independence and responsibility training that the patient read what is there and act on the basis of what he/she himself/herself has found.”

Narrol contends: “Another way of supplying more stimulus control would be supplying bank books in which the patient might keep his/her own accounts the same as we have to, so they might hand in a weekly report. We might dole out currency instead of reporting their point earning to them. This would leave more leeway for the exercise of individual responsibility, i.e. taking care of one’s own money, making sure that one didn’t allow himself/herself to be manipulated by a fellow alcoholic, loaning it unwisely, and that kind of thing. Narrol states: “Disagreements might be staged between the patients and staff as to how much he/she had really earned. We give him/her practice in worthwhile acceptable argumentative behavior in support of his/her own valid claims, something which he/she may have had no occasion to do in the past. If he/she kept his own record and he/she brings this as required on a given week to pick up his/her pay, we check his/her records against our records, and our records say that he/she earned 2,000 points less than his/her record and he/she would say, ‘Well look here and I got so and so for that and I worked here.’ The staff reply might be, ‘That looks right, you have earned 2,000 more points than we have got you down for, are you sure you did?’ ‘Yes, I am sure, here it is right here in the book.’ ‘Paul/Paula come over here, look at so and so’s book, is this right? We have it written down in our book.’ Paul/Paula could admit that he/she has made a mistake.”

This could be a big help for many individuals, this coming up against the opportunity to assert oneself and be reinforced for it by finding out that one was right in doing so.

2. Examples Presented in the Film “Behavior Theory in Practice”:
a. Pigeon: the written words “peck,” “turn,” and corresponding responses to these visual stimuli.
b. Dog: visual, hand signals
c. Horse: tactual SD in riding
d. People: various SD’s recorded in and around a city, e.g. turn, stop, flashing red light, etc.


1. Light Intensity Changes Sex Appeal: A bathroom mirror recently came on the market with different light intensity corresponding to the light intensity of the place to be seen on that evening’s date. The advertisement was aimed at females and is built on the concept that women should put on her make-up in the same light as the light existing where she will be part of the stimulus situation, i.e. the dinner theater, the night club, and so on.

2. SD and Social Imitation: By imitation we mean the tendency for a person to perform the activities exhibited by a model. The modeling or social imitation notion of self-control has been an area investigated by Albert Bandura. Bandura & Walters (1963) cite various cross-cultural observations to support the notion that a child learns habits from the parents by imitation. It appears that the parent’s behavior could serve as the discriminative stimulus (SD) in the operant paradigm. (See Skinner, 1954, p. 119-120.) If a child’s parents have many religious responses in their repertoire, most likely they will reinforce similar behavior in the child. Bandura & Kupers (1963) found that when an adult model required a high level of performance before self-reinforcement, all the children who had watched the model also tended to require a high level of performance before self- reinforcement.

New behaviors are often acquired through a process of observing another person’s behavior and the consequences of his/her action. This process is commonly referred to by a variety of terms, all relatively equivalent in meaning: “modeling,” “imitation,” and “identification.”

The act of imitation, like other behaviors, can be reinforced. After reinforcement of imitation has occurred in different situations, a person will show many such imitative behaviors even though only a small proportion of them are reinforced directly.



In everyday life examples of generalization are very common. A small child may be reinforced with attention for saying “cow” when he sees a four legged farm animal of considerable size, well known for producing milk. Later he may see another four legged animal also present on farms, large enough for a cowboy to ride, wearing a saddle, and still say “cow.” He was reinforced for responding in a certain way in a certain situation. He will respond in the same way although the situation has been altered somewhat, unless we teach him that this new animal has a different shape, and is commonly called a horse.

A stimulus present when a response is emitted and reinforced becomes a discriminative stimulus. For example, the letters “C-O-W” are a discriminative stimulus for saying cow. If the child is reinforced for saying cow, the response is relatively likely to be emitted when this stimulus is present. Other stimuli having something in common with the cow are also effective, but produce a lower response frequency. When this occurs, we say generalization has occurred.

The belief that the study in mathematics and Latin “trains the mind” is known as the theory of formal discipline. Earlier this was a common belief. A statement more frequently stated in the past was that “the study of Latin trains the reason, the powers of observation, comparison, and synthesis.” This has been largely discredited by experiments but these concepts are forerunners of our current concepts of generalization.

Q-1. The doctrine of formal discipline:
a. receives considerable support from contemporary psychological research
b. has been largely discredited by experiments
c. has proven that certain disciplines, such as Latin, mathematics, and physics, strengthen the green area in the back of the mind
d. reveals that certain courses train the ability to get big muscles in the brain

Answer: (b)

A three-year-old is exposed to a new name, Roxanne, who is a new baby-sitter. In the next two appearances as a baby-sitter, she is first called Stone-anne, and the second time Sox-anne.


Stimulus generalization is defined as an increase in responding to other stimulus situations after a response has been conditioned to a specific stimulus. When a child who has been frightened by one dog becomes frightened of all animals that look like a dog, we say the dog has generalized. Another example of stimulus generalization is when a dog who has been struck by a postman is frightened by any man in a uniform. If a person or animal responds to a stimulus with the same behavior with which it responded to the original stimulus, it is called stimulus generalization.

Q-2. A child is conditioned to fear a fuzzy Teddy Bear. The child becomes fearful of any black, fuzzy object. This process is called stimulus _______.
a. discrimination
b. transference
c. generalization
d. shaping

Answer: (c)

Q-3. A child who calls all four legged animals “Doggy” is _______.
a. differentiating
b. discriminating
c. generalizing

Answer: (c)

Q-4. The principle of generalization refers to the fact that stimuli which are very similar to the original stimulus will:
a. evoke generally different responses
b. evoke the same response

Answer. (b)

Q-5. The fact that animals trained to salivate to a tone of 500 cycles will also salivate to a tone of 300 cycles is called:
a. stimulus generalization
b. discrimination
c. shaping

Answer. (a)

Q-6. A dog has been trained to salivate to C on a tuning fork. The dog also salivates at slightly higher tones. This is called _______.
a. extinction
b. generalization
c. shaping

Answer: (b)

Q-7. Which of the following is an example of stimulus generalization?
a. A child who has been bitten by a large dog fears all large animals.
b. A child who has been bitten by a large dog does not fear small dogs.
c. A child overcomes his fear of dogs after an explanation by his mother.


Q-8. If you were conditioned to respond to red hair and black eyes with approach movements, and then you found yourself also approaching auburn hair & blue eyes, this event would be an instance of
a. discrimination
b. good judgment
c. spontaneous recovery
d. stimulus generalization

Answer: (d)

Q-9. The student who feels that all Psychology books are alike is
a. differentiating
b. discriminating
c. generalizing
d. chaining

Answer: (c)

Q-10. The child who says that soda pop “tastes like my foot’s asleep” is exhibiting _______.
a. extinction
b. discrimination
c. differentiation
d. generalization

Answer: (d)

Q-11. A child who has been severely punished by a male parent over an extended period of time and then responds to the presence of all males by running away illustrates _______.
a. submission
b. discrimination
c. stimulus generalization
d. retardation

Answer: (c)

Q-12. In conditioning, the process of conditioned discrimination overcomes effects of generalization.
a. True
b. False

Answer: (a)

Q-13. Conditioning response discrimination is accomplished by:
a. reinforcing the subject with food
b. not reinforcing generalized responses
c. reinforcing all generalized responses
d. presenting the reinforcer after every response.

Answer: (b)

Q-14. A process opposite to generalization is
a. extinction
b. conditioning
c. discrimination
d. spontaneous recovery

Answer: (c)

Q-15. When faced with a new problem which has elements in common with an old one, people may first try the old solution. This process is called:
a. generalization
b. discrimination
C. conditioning
d. extinction

Answer: (a)

Q-16. When instructors ask test questions which are not identical in wording to those presented in the text, we attempt to develop _______.
a. summarization
b. generalization
c. extinction

Answer: (b)

A laboratory example of stimulus generalization is that of a pigeon placed in a Skinner Box. The key always has a green light showing on it and the pigeon is reinforced with food every time it pecks this green key. After conditioning has occurred and a steady rate of responding exists the color on this key is changed gradually. Each of 16 colors is displayed on the key in a random fashion. The response emitted by the pigeon in the presence of each wavelength is counted and plotted. The resulting curve is known as the stimulus generalization gradient.

In stimulus generalization stimuli similar to the training stimulus are effective. For example, stimuli 1, 2, and 3 are similar to the training stimuli and produce a response. For example, “Green is on the wall, not in your head.”


1. Pigeon and a Pelican: A fascinating and interesting application of the principle of stimulus control is “Project Pigeon.” During World War II, a research project conducted at the University of Minnesota used a pigeon to guide a war missile to targets. B. F. Skinner wrote about this fascinating project after the military declassified them (Skinner, 1960). The following is a summary of the project.

Skinner and his co-workers, including Keller Breland, used pigeons as a tracking device in a missile called “Pelican.” The pigeon has the response repertoire to make fine visual discriminations. Through laboratory work they learned how the behavior of animals could be controlled through the precise arrangement of stimulus control. Through a series of experiments a pigeon-powered system of missile guidance was developed which could out-perform available electronic systems of the time, while involving much less space and weight. The pigeon was trained to peck at an image of a target projected on a screen. The pigeon was held in place by a harness, in which it could only move its neck and head. It could move enough to eat grain and operate the control system by moving its head in the appropriate direction. In training, an image of the target was projected on a screen. The pigeon was reinforced for pecking at the image. The control of the missile was to result from the point of contact of screen and beak.

As it pecked the target, the system corrected to its pecking and a feedback guidance system would turn the missile so that once more it was centered on the target. If the pigeon was to guide the missile to a particular target, it had to peck at an image or projection of that target and at no other image.

Through a program of intermittent reinforcement the pigeons could be trained to peck at a target for a long time after reinforcement was removed. Through discrimination training they could learn to peck at a wide variety of targets.

They were trained to ignore images of clouds or of flak. In a laboratory mock-up the pigeons were able to guide a missile to a particular street intersection on an aerial map of a particular enemy city. The accuracy of pigeon-guiding was used for a group of up to seven pigeons pecking simultaneously. Provisions were made to use the majority vote in case a minority started working on a wrong target.

2. Cumming’s In Defense of Bird Brains: Pigeons were taught to look at small electrical devices called diodes and report whether the paint cover on them was chipped, scratched, non-uniform, or absent. The pigeons were trained to inspect diodes at the rate of about 1000 per hour, and report defective parts with an error rate of two percent, which is slightly better than the company had ever obtained from its human inspectors. The pigeon would work for four hour periods showing no signs of fatigue. This is unlike human inspectors, who get progressively more lax in their standards as a work day progresses.

Work on an assembly line is a boring task which may lead to a poor job. As a problem in industry, it would be easy if all the flaws of the parts being inspected were the same, but each new flaw is different. Because most assembly line inspectors are faced with this problem, Cumming, formerly an experimental psychologist at Columbia University, worked the problem. It’s impossible to build a machine to detect all the various flaws, e.g. a small electronic diode is painted solid black; however, the paint at times is imperfect and the silver metal will show through. You could easily build a machine to tell unpainted diodes from partially painted ones if it were always the same flaw, but sometimes half of it is painted, and sometimes there is only a tiny scratch in the paint. It is just not practical to build a machine to automatically detect so many different types of paint flaws, so we have people doing it. Cumming decided what was needed was a device that has a good visual discrimination system and can form concepts. A concept might be good painted parts and bad painted parts. With these concepts, bad parts would be rejected and good parts would be put in a different category. It would treat all members of a particular concept in the same manner.

There are many animals that have excellent visual systems and are able to form these concepts. A pigeon is one of these animals with excellent vision and with capabilities of forming concepts. They are also very economical, as pigeons will work for chicken feed.

The procedure which Cumming proposed was a discrimination procedure. It was a little more complicated than the usual discrimination procedure which uses a single SD and a single S. Instead of two individual stimuli, we have two classes of stimuli. A stimulus class consists of a set of stimuli, all of which have some common property. The phenomenon of stimulus generalization demonstrates that conditioning takes place to a class of stimuli rather than to a particular stimulus. In this experiment, one set of stimuli, the bad parts, had the common physical property of a paint flaw. They also had a common behavioral property; all of the bad parts served as SD’s for the response of rejecting them and served as S Delta’s for the response of accepting them. Similarly, the stimulus class of good parts consisted of parts which had no paint flaws. They served as SD’s for accepting them and S Delta’s for rejecting them.

Another name for “stimulus class” is “concept.” One says that one has the concept when the observer generalizes and responds the same way to all of the stimuli in a stimulus class, but does not respond in that way to stimuli not in the class.

Q-17. In order for a child to form a concept of “red” in learning terms, he must:
a. extinguish red responses
b. discriminate blue from green
c. generalize to stimuli of the concept

Answer: (c)

The training or the conditioning of one bad part as generalized to the other bad part consists of generalization within a stimulus class (or concept) and discrimination among concepts or stimulus classes.

Conceptual behavior is learned like other discriminations. We reinforce one response in the presence of one stimulus class or concept and extinguish that response in the presence of all other stimulus classes.

Q-18. The concept Orange is:
a. an inherent class in nature
b. It is the same in all cultures
c. defined as a wavelength of light between certain values.
d. generalization of all stimuli in a class and discrimination between classes.

Answer: (d)

Cumming had the pigeons placed in a small enclosure in which they could look through a window at a parts conveyer. The conveyer brought a part before the window. If the part had a paint flaw, the bird was to peck the window. The window also acted as a response key so that food reinforcement would be delivered. Then the conveyor would bring another new part before the window for “inspection.” If the paint was good, the pigeon was to peck a response key to one side of the window. This caused the conveyor to bring up another new part for inspection and so on.

Q-19. Discrimination occurs when:
a. generalization to some stimuli is reduced by extinction
b. reactions to all stimuli are reduced by extinction
c. reactions to the positive conditioned stimulus are reduced by extinction
d. stimulus generalization has failed to occur initially

Answer: (a)

What happened if the bird made a mistake? Two things occurred. First, reinforcement was withheld, and second, punishment was delivered. Whenever the pigeon rejected a good part or accepted a bad part, this incorrect response was punished. Whenever the bird made a mistake, Cumming prevented him from working. This was accomplished by turning off the lights in the pigeon’s chamber for a twenty second period; during that time further responses had no effect.

Cumming initially conditioned the pigeons to discriminate between a completely painted and completely unpainted part. Since these stimuli were maximally different, discrimination was taught quickly. Very soon, a little generalization occurred between these highly dissimilar parts. Only after a discrimination had been established between very dissimilar parts did Cumming introduce parts with only minor flaws in their paint. In such stimuli, so similar to the good parts, there would have been a large amount of stimulus generalization between the types of stimuli and good discrimination would not be easily formed. When two stimuli are physically very similar, considerable generalization will occur between such stimuli and good discrimination behavior will be difficult. If there is only a small amount of stimulus generalization between them a good discrimination will be easily established.

Q-20. Of the following the most difficult conditioned discrimination to establish would be that between metronomes beating:
a. 92 and 144 times per minute
b. 122 and 144 times per minute
c. 132 and 144 times per minute

Answer: (c)

If an observer responds in nearly the same way to two different stimuli, then he/she is showing a large amount of generalization and very little discrimination.

Q-21. A process opposite to generalization is extinction spontaneous recovery.
a. true
b. false

Answer: (b)

Q-22. Stimulus generalization may be changed or counteracted by_______.
a. response generalization
b. secondary reinforcement
c. discrimination training
d. classical conditioning

Answer: (c)

Q-23. A professional wine taster can make very fine _______ when he/she shows little _______ among various wines.


A schedule of continuous reinforcement and punishment had been used so that each correct response was reinforced and each incorrect one was punished. After the conceptual behavior was well established, a variable-ratio schedule of reinforcement was instituted.

By introducing known coded parts which would be identified so that a machine could tell whether they were good or bad parts, the factory used an intermittent reinforcement schedule as a double-check. (This ensured that the pigeons would be exposed to at least a minimum of bad parts and would recieve adequate reinforcement to keep them working at an effective level. If the pigeons were not exposed to a minimal amount of bad parts they would have recieved too little reinforcement for even watching the parts go by.) In the factory, the bad machine identifiable parts could be mixed in with the new batches of uncoded parts and then removed and randomly placed back in the next batch of parts to be inspected. In that manner, an accurate discrimination could be maintained with only a small amount of effort. Cumming found that only 2% of the parts needed to be coded so that the bird could maintain a high level of accuracy on the uncoded ones.

The pigeons’ accuracy rate was between 95% and 98%. Although no formal data is available on the accuracy of human inspectors on the assembly line, it is known that they don’t do nearly as accurate a job.

The basic stimulus generalization experiment involves conditioning a response in the presence of one stimulus and measuring the amount of responding which occurs when some property of that stimulus is changed. The greater the amount of responding, the greater the amount of stimulus generalization.

Q-24. The results of generalization experiments are usually expressed in terms of _______.


Such gradients show that as some property of the stimulus becomes more and more different from the discriminative stimulus used during conditioning, the response rate decreases. In other words, the more dissimilar two stimuli are, the less a stimulus generalization occurs, the better the discrimination.

Q-25. A concept is a set of stimuli having some common property. This may be a physical property such as the color red or it may be a functional property such as serving as a discriminative stimulus for a particular response. Another name for “concept” is _______.


The accuracy of a pigeon inspection system could be greatly increased by having several birds inspect each part with a procedure somewhat like the one used by Skinner in “Pigeon in a Pelican.” A majority, three birds, was required before a part could be accepted or rejected.

Some of the birds were able to inspect at a rate of over a 1,000 parts per hour. They could maintain this high strength in rate and accuracy for a consecutive four hours without a “coffee break.” The experimenter was unable to go beyond the four hour session but the birds were still going strong at the end of the session.

3. Inspector General: T. Verhave, a psychologist working for a pharmaceutical company, used pigeons to inspect drug capsules, a task previously done by teams of several dozen human inspectors. A capsule could be defective by being broken, torn, incomplete, poorly tinted, or double-capped. Verhave chose only the last fault, the double capsule, since the company had recently wasted a fair amount of money on an unsuccessful automatic system to detect double capsules. With just one week of practice, there were two pigeons whose reports of double capsules were 99.9% correct. This was a stimulus discrimination training procedure. Company management refused to use pigeons because of various fears, in particular, the hazard that would accompany the incorporation of trained pigeons to the company image.

You have seen how generalization and discrimination can be combined to yield a procedure for concept formation. A concept consists of discrimination between two elements of stimuli and generalization within each class.

4. The Concept of Man (Herrnstein): Plato defined man as a two-legged animal without feathers. When Sly Diogenes heard this, he picked the feathers from a chicken and brought it to the academy. The academicians took note and realized that they would have to change their definition. They thought awhile and ultimately pronounced that, “Man is a two-legged animal without feathers but with broad, flat nails.” But then a chimpanzee fits the rule and yet is not a man. A person without arms or legs does not fit the rule but he is still a man. It may well be an impossible task to give a definition and set of rules which describe the concept of “man.” Interestingly, we can use the concept of “man” even though we cannot find a good, explicit definition of the concept. It seems that almost “intuitively” we know what a man is. We behave intuitively when we respond in accord with some concept which we cannot define. For example, “I may not be an artist, but I know what good art is.” The speaker would probably be unable to give you a set of rules which would allow you to select good art from bad art; yet he may be able to make quite a reliable discrimination. He has the “intuitive” concepts of “good” and “bad” art.

How do we learn such intuitive concepts if no one knows the rules for defining the concept? The answer probably lies in the fact that when the child correctly points to a man or a picture of a man and says “man,” he is reinforced by his parent’s approval. When he points to a picture of a chimpanzee and says “man,” he is told “No.” After many trials of this sort, he may come to form the concept of (Stimulus Class of) “man.”

This same process of “intuitive” discrimination has been demonstrated in a very interesting experiment by Herrnstein at Harvard University. He taught pigeons to look at projections of 35 millimeter color slides and to report whether or not a slide contains any human beings. The scene in which the pigeon picked out a human subject included a complex jungle scene. We have found this job is easily within the capacity of the bird. The conditioning procedure which Herrnstein used was straightforward. A wide variety of pictures were projected on a viewing screen in the pigeon’s test chamber. Pecking at a response key was reinforced when the pictures projected on the screen contained people. The response was not reinforced when the pictures did not contain people. Pictures containing people served as the SD’s for the key peck response, whereas pictures without people served as the S-Delta. The photographs were taken in a wide variety of natural settings, such as countryside’s, cities, expanses of water, lawns, and meadows. Some slides contained human beings partly obscured by intervening objects: trees, automobiles, window frames, and so on. The people were distributed throughout the pictures, i.e. 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, nude, adults or children, men or women, sitting, standing, black, white, or yellow. Lighting and coloration varied: Some were dark, others were light, others had either reddish or bluish tints and so on. As you can see, a tremendous variety of stimuli were used. The S slides contained many different specific examples of humans, and the S-Delta contained many non-human pictures. The birds were able to master the discrimination rapidly. In fact, occasionally the birds appeared to make mistakes and when the experimenters looked more closely, in some obscure corner of the picture there would be a human. The pigeons were almost equal to the experimenters at the detection of humans in the picture.

We have seen that the pigeons were able to discriminate between the concept of human and the concept of non-human. After considerable conditioning, when a new picture of a person or a non-person was shown, the birds responded correctly to it. This is one of the most important aspects of concept behavior. It provided a means for responding correctly in novel situations.

Frequently, critics of the behavioral approach to psychology state that this approach is too limited. They do not see how behavioral psychology can explain the occurrence of appropriate responses in new situations which the organism has never previously experienced. We see, however, that the notions of stimulus generalization and concept formation give us a mechanism whereby an organism can be trained in a variety of specific situations, can form a concept, and then can respond appropriately to an entirely new situation. The training generalizes to these new situations.

If our behavior was so limited that we could respond correctly only in situations in which we had been specifically trained, we would be severely limited. Everyday we find ourselves in new slightly different situations. Our conditioning, in the presence of earlier examples of the concept, generalizes to the new instance and we respond accordingly. (See Malott’s Elementary Psychology for an elaboration.)

Hippies were easily betrayed because membership requirement is easy to obtain. “Hippie” Cops and F.B.I. Agents can get in because their “individualism” makes the generalization gradient very broad.


Discriminative stimulus (SD):

-A stimulus in the presence of which

-a particular response will be reinforced or punished

S-delta (SΔ):

-A stimulus in the presence of which

-a particular response will not be reinforced or punished

Discrimination training procedure:

-Reinforcing or punishing a response

-in the presence of one stimulus

-and extinguishing it

-or allowing it to recover

-in the presence of another stimulus

Stimulus generalization:

-The behavioral contingencies

-In the presence of one stimulus

-Affect the frequency of the stimulus

-In the presents of another stimulus