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 S D and signal opportunity for food. The items that are not members of the category are S  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.