PSY 402Theories of LearningChapter 8 – Stimulus ControlHow Stimuli Guide Instrumental Action

Categorization and DiscriminationAnimals 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 SΔ and signal that pecking will not be rewarded.

8.2 Examples of test slides from the tree, water and Margaret categories (Part 1)

8.2 Examples of test slides from the tree, water, and Margaret categories (Part 2)

8.2 Examples of test slides from the tree, water, and Margaret categories (Part 3)

More Complex TasksLater 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.

8.3 Apparatus (Part 1)

8.3 Examples of positive images (Part 2)

8.3 Examples of positive images (Part 3)

8.4 Results of an experiment in which pigeons learned four categories

8.5 Categorization after training with categories of different sizes

Categorization TheoriesFeature 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.

8.6 Faces used in the categorization experiment by Huber and Lenz

Generalization GradientGeneralization – 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.

8.7 A stimulus generalization gradient

redgreen

yellow-orange

orange-red

yellow

yellow-green

DiscriminationThe 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.

8.8 The sharpness of the generalization gradient depends on the type of training

Inhibitory GradientsGradients 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.

8.9 Excitatory and inhibitory generalization theories with line-tilt stimuli

Peak ShiftWhen 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.

8.10 Hypothetical excitatory and inhibitory gradients (Part 1)

Spence subtracts inhibition from excitation

8.10 Hypothetical excitatory and inhibitory gradients (Part 2)

8.11 Peak shift

When the inhibitory stimulus S- is to the right, the peak shifts left

TranspositionA 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).

8.12 Transposition

Animals pick S’ not S+

This is a bigger distance than between S+ and S-

Perceptual LearningPreexposure 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.

8.14 Two similar stimuli are theoretically composed of unique and overlapping elements

Unique features Shared

Acquired EquivalenceWhen 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)

8.15 Example of a sequence of stimuli used in a matching-to-sample experiment

Rewarded

Categorization by AssociationAcquired 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.