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Chapter 8

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University of Guelph
PSYC 2330
Francesco Leri

Chapter 8: Stimulus Control of Behaviour Identification and measurement of Stimulus Control Differential Responding and Stimulus Discrimination - The stimulus control of instrumental behaviour is demonstrated by variations in responding (differential responding) related to variations in stimuli  Example: Reynolds (1961) on pigeons. Two pigeons reinforced on pecking visual response key when it was lit up, response key was a white triangle on a red background.  The two key components of the stimuli were the white triangle, and the red background.  Once the pigeons were pecking steadily at the stimuli, they were presented only one part of it on its own (red background, or white triangle)  One pigeon pecked persistently at just the red background, but not the white triangle, the other pigeon pecked persistently at the white triangle, but not the red background - The differential responding to the two stimulus indicates that the pigeons were treating each stimulus 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 Generalization - Psychologists and physiologists have long been concerned with how organisms identify and distinguish different stimuli - Stimulus generalization is, in a sense, 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  First observed by Pavlov. He found that after one stimulus was used as a conditioned stimulus, his dogs would also make the conditioned response to other stimuli, similar to the original conditioned stimulus - Stimulus generalization gradient is a gradient of responding as a function of how similar each test stimulus was to the original training stimulus  The more dissimilar the stimulus is to the original stimulus, they fewer responses it will get Stimulus and Response Factors in Stimulus Control - The example of Reynolds experiment given earlier, the stimulus used clearly had two features, the red background, and the white triangle - All stimulus situations can be analyzed in terms of multiple features, even if Reynolds had used just the red background, it could be characterized by its brightness, shape, size, etc. - The central issue in the analysis of stimulus control is what determines which of the numerous features of a stimulus situation gains control over the instrument behaviour Sensory Capacity and Orientation - Sensory capacity and orientation determine which stimuli are included in an organisms sensory world, making them the most obvious variable in determining whether a particular stimulus feature comes to control responding  For example, people cannot hear sounds with a pitch above 20,000 cycles per second, any stimulus above that pitch would have no effect on a human, where it might effect a dog as they can hear sounds of much higher pitch Relative Ease of Conditioning Various Stimuli - Whether a stimulus comes to control behaviour is also dependent on presence of other cues in the situation - In particular, how strongly organisms learn about one stimulus depends on how easily other cues in the situations can be conditioned  This is called overshadowing  Overshadowing illustrates competition among stimuli for access to the process of learning  Pavlov was the first to observe that if two stimuli are presented at the same time, the presence of the more easily trained stimulus may hinder learning about the other one Type of Reinforcement - The development of stimulus control also depends on the type of reinforcement that is used - Certain types of stimuli are more likely to gain control over the instrumental behaviour in appetitive than in aversive situations Type of Instrumental Response - The nature of the response required for reinforcement is another factor that can determine which of several features of a compound stimulus gain control over behaviour - Responses that are differentiated by location (for example a dog lifting its right vs. left leg) are more likely to come under the control of the spatial feature of auditory cues (sound coming from in front, behind etc.) - In contrast, responses that are differentiated by quality (for example, a dog lifting its leg vs. not lifting its leg) are more likely to come under the control of the quality of auditory cues (the type of sound used) - This phenomenon is called the quality-location effect Stimulus Elements versus Configural Cues in Compound Stimuli - The assumption that organisms treat stimulus features as distinct and separate elements is known as the stimulus ele
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