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Chapter 8

PSYC 2210 Chapter Notes - Chapter 8: Stimulus Control, Red Color, 10 Nanometer

Course Code
PSYC 2210
Anthony Nield

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Chapter 8: Stimulus Control of Behaviour
As I pointed out earlier, both Thorndike and Skinner recognized that instrumental responses
and reinforcers occur in the presence of particular stimuli. The stimulus control of instrumental
behaviour is evident in many aspects of life. Studying, for example, is under strong control of
school-related stimuli. The stimulus control of behaviour is an important aspect of how
organisms adjust to their environment. The survival of animals (including human animals)
depends on their ability to perform responses that are appropriate to their circumstances.
Identification And Measurement of Stimulus Control
- How can a researcher tell that an instrumental response has come under the control of certain
Differential Responding and Stimulus Discrimination
→ Let us look at an experiment by Reynolds, where two pigeons were reinforced on a VI
schedule for pecking a circular response key. Reinforcement for pecking was available whenever
the response key was illuminated by a visual pattern consisting of a white triangle on a red
background. Thus the stimulus on the key had two components: the white triangle and the red
color of the background. Reynolds was interested in which of these stimulus components gained
control over the pecking behavior. The results showed that one of the pigeons pecked a great deal
more when the response key was illuminated with the red light than when it was illuminated with
the white triangle. This outcome shows that its pecking behaviour was much more strongly
controlled by the red color than by the white triangle. By contrast, the other pigeon pecked a
great deal more when the white triangle was projected on the response key than when the key
was illuminated by the red light. Thus, for the second bird, the pecking behaviour was more
strongly controlled by the triangle. This experiment illustrates several important ideas. First, it
shows how to experimentally determine whether instrumental behaviour has come under the
control of a particular stimulus The stimulus control of instrumental behaviour is demonstrated
by variations in responding (differential responding) related to variation in stimuli. If an
organism responds one way in the presence of one stimulus and in a different way in the
presence of another stimulus, its behaviour has come under control of those stimuli. Such
differential responding was evident in the behaviour of both pigeons Reynolds tested.
Differential responding to two stimuli also indicates that the pigeons were treating each stimulus
as different from the other. This is called stimulus discrimination. An organism is said to
exhibit stimulus discrimination if it responds differently to two or more stimuli. Stimulus
discrimination and stimulus control are two ways of considering the same phenomenon. One
cannot have one without the other. If an organism does not discriminate between two stimuli, its
behaviour is not under control of those cues. Another interesting Aspect of reynolds experiment
was that pecking behaviour of each bird came under the control of a different stimulus. Reynolds
did not direct attention to one of the stimuli at the expense of the other. Therefore, it is not
surprising that each bird came to respond to a different aspect of the situation. The experiment is
comparable to showing a group of children a picture of a cowboy grooming a horse. Some of the
children may focus on the cowboy; others may find the hose more interesting.
Stimulus Generalization.
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→ Numerous factors are involved in the identification and differentiation of stimuli.
Experimental analysis of the problem have depended mainly on the phenomenon of stimulus
generalization. In a sense, stimulus generalization is the opposite of differential responding, or
stimulus discrimination. An organism is said to show stimulus generalization if it responds in a
similar fashion to two or more stimuli. The phenomenon of stimulus generalization was first
observed by Pavlov. He found that after one stimulus was used as a CS, his dogs would also
make the conditioned response to other, similar stimuli. A landmark study of stimulus
generalization in instrumental conditioning was by Guttman and Kalish whom first reinforced
pigeons on a variable-interval schedule for pecking a response key illuminated by a yellowish-
orange light with a wavelength of 580 nanometers (nm). The results showed that the highest rate
of pecking occurred in response to the original 580-nm color. But, the birds also made substantial
numbers of pecks when lights of 570-nm and 590-nm wavelengths were tested. This indicates
that responding generalized to 570 and 590 stimuli. However, as the color of the test stimuli
became increasingly different from the color of the original training stimuli, progressively
different from the color of the original training stimulus, progressively fewer responses occurred.
The results showed a gradient of responding as a as a function of how similar each test stimulus
was to the original training stimulus. This is an example of a stimulus generalization.
Stimulus Generalization Gradients as Measures of Stimulus Control
→ With the use of stimulus generalization gradients, investigators can determine exactly how
much a stimulus has to be changed to produce a change in behaviour. The wavelength of the
580nm training stimulus had to be changed by more than 10nm before a decrement in
performance was observed. This aspect of the stimulus generalization gradient provides precise
information about how large a variation in the stimulus is required for the pigeons to respond to
the variation. How do you suppose the pigeons would have responded if they had been color
blind? In that case they could not have distinguished lights on the basis of color or wavelength.
Therefore, they would have responded in much the same way regardless of what color was
projected on the response key. Thus, the stimulus generalization gradient would have been flat. A
comparison of the results obtained by Guttman and Kalish and our hypothetical experiment with
color-blind pigeons indicates that the steepness of a stimulus generalization gradient provides a
precise measure of the degree of stimulus control. A steep generalization gradient indicates good
control of behaviour by the stimulus dimension that is tested. In contrast, a flat generalization
gradient indicates poor stimulus control.
Page 263: Generalization of Treatment Outcomes (Stimulus generalization & behavioural
Stimulus And Response Factors In Stimulus Control
→ The central issue in the analysis of the stimulus control is what determines which of the
numerous features of a stimulus situation gains control over the instrumental behaviour. In the
present section we will consider stimulus and response factors that determine which cues come
to control behaviour.
Sensory Capacity And Orientation
→ The most obvious variable that determines whether a particular stimulus feature comes to
control responding is the organism’s sensory capacity and orientation. Sensory capacity and
orientation determine which stimuli are included in an organism’s sensory world. Limitations on
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