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PSY 402. Theories of Learning Chapter 8 – Stimulus Control How Stimuli Guide Instrumental Action. Categorization and Discrimination. Animals respond to stimuli in ways that suggest they form categories. Pigeons can classify a variety of items, including new images not seen before.
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PSY 402 Theories of Learning Chapter 8 – Stimulus Control How Stimuli Guide Instrumental Action
Categorization and Discrimination • Animals respond to stimuli in ways that suggest they form categories. • Pigeons can classify a variety of items, including new images not seen before. • The items to be learned as members of a category are SD and signal opportunity for food. • The items that are not members of the category are SD and signal that pecking will not be rewarded.
More Complex Tasks • Later pigeons were asked to place images into four categories by pressing one of four buttons (rewarded by food if correct). • They are “naming” the object shown. • Pigeons do equally well with natural and manufactured objects (cars, chairs). • Transfer to new stimuli is worse but above chance. • The more examples used, the better they do.
Categorization Theories • Feature theory – birds learn which features signal reinforcement. • Prototype theory – birds learn a prototype that is typical of category members. • Not much different than learning a set of features. • Exemplar theory – birds remember each example (exemplar) used in training then generalize to new stimuli.
Generalization Gradient • Generalization – responding to a new stimulus as if it were a previously presented one. • Responding depends on the perceived similarity to the original stimulus. • Generalization gradient – the change in responding as the features of the stimulus change. • Pigeons trained to peck a color (580 nm) show less responding as the color changes.
Discrimination • The shape of the gradient can be changed by training. • When birds are exposed to two different tones (S+ or S-), they must discriminate between them. • Responding is less generalized because the competing tone produces no reward. • The shape of the gradient becomes steeper and more narrow at the top.
Inhibitory Gradients • Gradients also appear when animals must withhold responding to a stimulus (inhibition) • Two groups of birds were trained: • Group 1 was trained to peck when a black bar was present (S+), but not peck when a white key appeared (S-). • Group 2 was trained to peck when a white key was present, but not when the black bar appeared. • Both groups were tested with bars at angles.
Peak Shift • When an inhibitory stimulus and an excitatory one are both conditioned, inhibition changes the shape of the generalization gradient. • Peak shift – maximum responding occurs to a stimulus not previously trained as the S+. • The peak shifts away from the S- stimulus. • Spence suggested that the amount of response is the difference between inhibitory and excitatory conditioning.
Transposition • A bright excitatory and a darker inhibitory stimulus are both presented during learning, then later an even brighter S’ is presented. • Greatest responding occurs to S’ – this is called transposition. • This suggests that the animal has learned about the relationship among the stimuli, not the absolute values (excitation minus inhibition).
Perceptual Learning • Preexposure to stimuli (without reward) makes them easier to discriminate (tell apart). • This seems contradictory because conditioned inhibition should occur. • Latent inhibition is for the shared features, not the rest of the stimulus, making the differences easier to identify. • With repeated exposure to multiple features, pattern completion occurs – a unitized figure.
Acquired Equivalence • When two stimuli are both associated with a third stimulus, they will start to be treated as alike. • Also called mediated generalization. • Matching to sample task • When red and vertical lines both signal the same thing, they will be treated as the same (equivalent)
Categorization by Association • Acquired equivalence enables birds to form larger categories, consisting of the various things that are rewarded. • These items have psychological similarities due to their association with reward, not because they are alike in other ways. • Categorizing on the basis of something besides physical appearance is a step toward concept formation in animals.
