PSY402 Theories of Learning. Chapter 10 Stimulus Control of Behavior

Transcription

1 PSY402 Theories of Learning Chapter 10 Stimulus Control of Behavior

2 The Role of Environmental Stimuli In operant conditioning, the stimulus becomes associated with the reinforcer or punishment. Reward or punishment is the UCS. The stimulus signaling reward or punishment is the CS. The CR motivates operant behavior. Responding can be used as a measure of the strength of a CR.

3 Definitions of Terms Stimulus control -- Environmental stimuli signal the opportunity for reward or punishment. Generalization responding in the same way to similar stimuli. Discrimination responding to some stimuli but not to others.

4 Generalization Gradient Degrees of generalization occur. In some situations, the same response occurs to similar stimuli. In other situations, the amount of response varies with the similarity. Generalization gradient a graph showing how the strength of response changes with similarity. Steep gradients mean narrow response (stimuli must be very similar).

5 Kinds of Gradients Excitatory conditioning (S+) a CS- UCS response to a stimulus is learned. Excitatory gradient the S+ is varied and the CR is measured. Inhibitory conditioning (S-) a CS signals absence of the UCS and thus inhibits the CR. Inhibitory gradient the S- is varied and the CR is measured.

6 yellow-orange yellow yellow-green orange-red green red

7 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.

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

9 Flat Gradients A flat gradient means all stimuli are being responded to as if they were the same. Responding with a gradient to a tone occurred only when the tone signaled reward during training.

10 Generalization of Inhibition Inhibition example: fear of dating. A good experience leads to less fear of dating a different person. Inhibition gradients are similar to excitatory gradients the more the stimulus varies, the less inhibition.

11 Excitatory and Inhibitory Generalization with Line Tilt Stimuli

12 Explanation Lashley-Wade theory people and animals generalize because they are unable to discriminate. Can t tell the difference between stimuli A contrast is needed during training to enable discrimination. Discrimination training leads to steeper generalization gradients. Perceptual experience matters.

13 Discrimination Learning Important to recognize when reinforcement is not available so that responding can be withheld. Discriminative stimulus: S D reinforcement is available (S+) S Δ reinforcement is unavailable (S-) Conditioned stimuli always produce a response. Discriminative stimuli signal the opportunity to respond.

14 Two-Choice Discrimination Tasks The discriminative stimuli are on the same dimension: Red vs green light. Need not be presented simultaneously. Two-choice discrimination includes one S D and one S Δ. Other tasks can use multiple multiple S D or multiple S Δ.

15 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 Δ and signal that pecking will not be rewarded.

16 Test Slides Tree Category

17 Test Slides Water Category

18 Test Slides -- Margaret Category

19 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.

20 Apparatus (Part 1)

21 Examples of positive images

22 Examples of positive images

23 Three Phases Subjects begin by responding equally to both stimuli prediscrimination phase. Discrimination phase -- with training, response to S D increases and response to S Δ declines. Shift back to non-differential reinforcement to show that behavior was caused by reinforcement.

24 Conditional Discrimination Availability of reinforcement depends on the condition of a stimulus. The stimulus does not always signal the same thing. More difficult to learn. Nissen s chimpanzees: Large, small squares, white or black. S D = large white, small black.

25 Behavioral Contrast Behavioral contrast the increased responding to the differential stimulus, decreased response to S Δ Contrast also occurs with changes in the duration of reinforcement. VI-10 to VI-3 Local contrast emotional Sustained contrast related to the differential reinforcement.

26 Anticipatory Contrast Williams sustained contrast occurs due to anticipation of a future reinforcement contingency. Not due to recall of past contingency. VI-3, VI-6, VI-3 Compare the first and third VI-3 behavior. VI-6 affected the first VI-3 more than the last VI-3 -- a prospective effect.

27 Occasion Setting A conditioned stimulus (CS) can create the conditions for operant responding to a second conditioned stimulus (CS). Occasion setting ability of one stimulus to enhance the response to another stimulus. The facilitating stimulus does not produce a CR by itself.

28 S D as an Occasion Setter A Pavlovian occasion-setter can increase operant responding. Example: A meal elicits CR craving for cigarette. Requesting a cigarette after a meal an operant behavior caused by CR. Conditional occasion-setting: Second stimulus modifies meaning of first discriminative stimulus.

29 Conclusions An occasion-setter can increase operant responding. A discriminative stimulus (S D ) can increase response to a CR (Pavlovian conditioning). This implies interchangeability of Pavlovian occasion-setters and discriminative stimuli.

30 Peak Shift When both inhibitory and excitatory stimuli are conditioned, inhibition changes the shape of the gradient. Peak shift maximum responding occurs to a stimulus not previously trained as the S +. The peak shifts away from the S - stimulus. The amount of response is the difference between inhibitory and excitatory conditioning.

31 Hypothetical Excitatory and Inhibitory gradients Spence subtracts the inhibition on the next slide from this excitation

32 Hypothetical Excitatory and Inhibitory Gradients Overall predicted response is less because this amount of inhibition is subtracted from it.

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

34 Errorless Discrimination Learning When an S Δ is gradually introduced the pigeon learns to inhibit response without making mistakes. Three fading steps are involved: Brief introduction of S Δ for 5 sec-30 sec Slowly change color of S Δ from dark to green Slowly increase duration of S Δ from 30 sec to 3 minutes

35 Implications of Errorless Training Errorless learning seems to condition response to S D without inhibition to S Δ. This means that errorless learning is not aversive. As a result, no peak shift occurs. Errorless learning is harder to condition to some stimuli than others (e.g., colors but not lines).

36 Application of Errorless Training Examples with humans: Preschool children recognizing scapes using a fading technique. Oral reading. Dorry & Zeaman taught mentally handicapped children to identify vocabulary words (pictures faded out). Not all training works problems with transfer and with reversed consequences.

37 Is Learning Relational? Are animals learning the relationships between stimuli rather than an absolute response? Transposition occurs when stimuli are changed: The brighter of two lights, louder of two tones is responded to. Different results support both views of learning: Hull-Spence & Kohler.

38 Mackintosh s Attentional View Stimuli with multiple dimensions arouse the relevant dimension analyzer. This depends on the salience and intensity of the dimension. The predictive value of the dimension determines arousal. Discrimination learning depends on predictiveness.

39 8.17 Examples of computer stimuli presented to pigeons by Cook (Part 1)

40 8.17 Examples of computer stimuli presented to pigeons by Cook (Part 2)

41 8.17 Examples of computer stimuli presented to pigeons by Cook (Part 3) Less popout with conjoined features.

42 8.18 Same and different displays used in the experiment by Wasserman et al

43 Continuity Theory Hull-Spence suggest that excitation and inhibition gradually increase with trials. Excitation to S D, inhibition to S Δ. Non-continuity theory suggests that a hypothesis is formed & tested. Learning occurs rapidly with attention to the right dimension. There is support for both theories.