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Psychology 2210A/B

CHAPTER 8 Notes 7/11/2012 5:59:00 PM Measuring Stimulus Control Demonstrating that an organism responds differently to one stimulus than to another is necessary for concluding that behavior is under stimulus control. Imagine a simple experiment. When a key-light is illuminated with red light pecks to the key are reinforced, but when the key is illuminated with green light pecks are not reinforced. After several training sessions, pigeons will display high rates of pecking when the red stimulus is present and near-zero rates when the green stimulus is presented. Pigeons thus show differential responding – responding one way to one stimulus and a different way to a different stimulus. Consider an experiment involving a more complex stimulus. In this experiment, pigeons were reinforced on a VI schedule for pecking a key that was illuminated with a white triangle on a red background. Note that such a stimulus has two prominent elements: red color and a white triangle. (A stimulus with multiple elements is called a compound stimulus – in reality, every stimulus is a compound stimulus). Reynolds (1961) wondered which of these elements controlled responding. To answer this question, he introduced occasional trials in which only the white triangle was projected on the key. On other test trials, only the red color was projected on the key. He found that some pigeons responded at high rates when the red background was presented and at low rates when the white triangle was presented. Other pigeons showed the opposite pattern. Thus, for some birds pecking was strongly controlled by the red background; for others pecking was controlled by the white triangle. Differential responding indicates stimulus control. For an organism’s behavior to be under stimulus control requires that the organism is able to discriminate among different stimuli present. How different must two stimuli be in order to be identified as different stimuli? One way to answer this question is to train a response to a particular stimulus and then confront the subject with additional examples of the stimulus but whose features differ to varying degrees. A classic example of this approach was provided by Guttman and Kalish (1956). Pigeons were trained to respond to a key illuminated with light measuring 580 nanometres (nm). This wavelength appears yellow-orange. After training, subjects were then presented with a variety of stimuli of different wavelengths, ranging from 520 nm (greenish) to 640 nm (reddish), and responses were not reinforced. Maximal responding occurred at 580 nm – the training stimulus. Interestingly, nearly as much responding occurred to wavelengths of 570 nm and 590 nm. Responding thus generalized to these wavelengths. As the wavelength became more different from 580 nm, responding diminished. The graph shown above is called a stimulus generalization gradient. Such gradients provide important information about the nature of stimulus control. A fairly flat gradient, for instance, indicates that the stimulus being varied does not control behavior. A flat gradient is obtained when equal responding occurs to all stimuli presented. A steep gradient, on the other hand, indicates that the stimulus being varied does control behavior.
 What Factors Influence Stimulus Control? 
 Sensory Capacity It is a trivial point that an organism’s sensory capacity will influence which stimuli or stimulus elements it perceives and which stimuli come to control responding. Bats and noctuid moths detect ultrasound, for example, and therefore can come under the control of ultrasonic stimuli. Humans do not hear ultrasound and so obviously our behavior cannot be controlled by such stimuli.
 Motivation When we are hungry we are more sensitive to food-related cues. Do you behave differently walking through the food-court of a mall with a full stomach than you do with an empty stomach? Yes. Yes, you do. 
 Stimulus Salience Behavior is more likely to come under stimulus control by an intense stimulus than by a weak stimulus. This phenomenon is known as blocking. Blocking was demonstrated by Kamin – remember him? Kamin investigated conditioned fear using the conditioned suppression procedures. Rats were trained to press a lever in an automated chamber and were occasionally presented with a stimulus signaling that a shock would be delivered. For one group of rats, the stimulus was a soft tone; for a second group, the stimulus was a compound of the soft tone plus a bright light. After several sessions, both groups of rats were tested in the chamber with only the soft tone presented. Kamin observed conditioned suppression (withholding of lever pressing) by the group trained with the soft tone alone. The group trained with the soft tone plus the bright light failed to show conditioned suppression. For this group, the presence of the light during training interfered with or “blocked” conditioned fear to the soft tone. It should be pointed out, however, that there is another way to interpret this finding. What if subjects did not perceive the tone and the light as separate elements? What if instead, they perceived the tone and the light together as a stimulus configuration? If this was the case, the failure to respond on test trials shown by the overshadowing group may have occurred because of a failure to generalize responding to the test stimulus, which was different than the configural stimulus to which they had been trained. In contrast, test and training trials were identical for the control group (both involved only the soft tone). Thus, the failure to demonstrate conditioned suppression in the overshadowing group might reflect a failure in generalizing responding to the test stimulus. This purported failure would be called “generalization decre
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