Stimulus Control
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Paradoxical Effects of Reward • Overtraining extinction effect: more training leads to faster extinction • Reinforcement magnitude effect: Big rewards lead to faster extinction • And, of course, the partial reinforcement extinction effect (PREE) Paradoxical effects of reward: Why? • Discrimination hypothesis: Nonreinforcement is easier to detect after CRF than PRF. Discrimination Hypothesis: Test CRF CRF EXT vs. PRF CRF EXT Paradoxical effects of reward: Why? • Discrimination hypothesis: Nonreinforcement is easier to detect after CRF than PRF. • Frustration hypothesis (Amsel): animals learn to make response as a reaction to nonreward. • Sequential theory (Capaldi): The memory of nonreinforcement becomes a cue that elicits responding. Stimulus Control Stimulus Control of Behavior • Having stimulus control means that the probability of the behavior varies depending upon the stimuli present • Most of our behavior is under stimulus control – A person that contributes to charity generously while in church may watch every penny spent while at work Discrimination 1.2 CS+ CS- Response Strength 1 0.8 0.6 0.4 0.2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Trials Discrimination and Stimulus Control • Discrimination is demonstrated when differential responding occurs to two or more stimuli. Train Test Reynolds (1961) Generalization • Generalization is when responses to one stimulus occur to other, usually similar, stimuli • Generally, as the training and test stimuli become more different responding will decline, producing what is called a generalization gradient Generalization Gradient Guttman & Kalish (1956) – pigeons reinforced for pecking a 580 nm lit key (orange-yellow) (S+) on a VI schedule 350 250 – A test session was then given where many different colored key lights were presented in extinction 200 150 100 50 S+ 0 63 0 61 0 59 0 57 0 55 0 53 0 0 51 Responses 300 Wavelength (nanometers) Interpreting Generalization Gradients Pigeons trained to peck a moderately bright light (S+) to get food. (S- = dim light) After asymptote is reached, present occasional nonreinforced probe trials at various wavelengths or levels of brightness. Excitatory and inhibitory gradients Pigeons trained to peck at a 800 hz tone (S+), with a 500 nm light S-. 1000 Hz Tone S+ / 950 Hz Tone S- 1000 Hz Tone S+ / No Tone S- 1000 Hz Tone always on Peak Shift Effect – Hanson (1959) Control group: Experimental group: 500 550 nm Light S+ 550 nm Light S+ / 590 nm Light S- Responses 400 Control Experimental 300 200 100 0 480 500 520 540 560 580 Wavelength (nanometers) 600 620 Inhibitory or Excitatory Strength Spence’s Theory to Account for Peak Shift 80 S+ Inhibitory Excitatory Difference 60 40 20 0 -20 S- -40 -60 490 510 530 550 570 590 610 630 650 670 Wavelength (nanometers) Interdimensional discrimination 700 Pseudodiscrim Discrimination # Responses 600 500 400 300 200 100 0 501 530 555 576 Wavelength Discrimination: S+ = 555nm Light; S- = Tone 606 How do we learn discriminations with complex stimuli? How do we learn discriminations with complex stimuli? A A + B B Complex Discrimination: Example Pre-exposure Devalue Test -- Saline-LemonLiCL Sucrose-Lemon? Lemon Saline-LemonLiCL Sucrose-Lemon? Another example… Pre-exposure Devalue Test -- Saline-LemonLiCL Sucrose-Lemon? Sal-L/Suc-L Saline-LemonLiCL Sucrose-Lemon? Complex Discriminations: Mechanism # 2 the method of pre-exposure matters… Pre-Exposure: AXBXAXBX | CXCXCXCX Devalue: AXLiCL Test: BX? Question: How much does aversion generalize to BX and CX? CX? A = lemon B = salt Mondragon & Hall (2002) C = sucrose X = quinine What’s going on? • Juxtaposition of stimuli clearly matters • But why? AXAXAX… produces habituation to AX Remember: expected things are less salient or associable AXBXAXBX…. A B Treating Different Stimuli Alike: Categorization “Categorization can be viewed as the ability to treat similar, but not identical, things as somehow equivalent, by sorting them into their proper categories and by reacting to them in the same manner” (Huber, 2001) • Classical view: categories united by a defining feature or features • But Consider: Oak leaves v. Non-oak leaves Chairs v. non chairs What is “Chairness” “family resemblance” Categorization Experiments Train Test Scenes with Trees + New Set tree scenese Scenes w/o trees - New Set of no-tree scenes "A pigeon pecks rapidly at a small photograph of Harvard Yard containing trees, buildings, people, sky. After a few seconds, a hopper of grain appears and the pigeon eats. Now the scene changes to a treeless Manhattan street. The bird emits a few desultory pecks, then turns away and paces about. After a minute or so, a picture of a leafy suburban garden appears and the bird begins pecking again." (Shettleworth 1998) Other categories pigeons can form •Aerial v. non-aerial photos •Chairs •Humans •Cars •Defective pharmaceutical capsules! •Oak leaves versus other leaves Human v. Non-Human How do they do it? • Exemplar theory: remember category members and then generalize. – Vaughn & Greene 1984: pigeons can remember no less than 320 individual slides! Outdoor scenes randomly assigned to + or – Exemplar theory: more evidence • Cook (1990) – Birds versus Mammals used in slides – Real Category Group: Birds v. Mammals – Pseudocategory Group: Random Bird & Mammals versus Random Birds & Mammals Feature Theory • Individual features acquire associative value. • Response rate to stimulus depends on total expectancy (V) evoked. Feature Theory: Evidence Cerella (1980): Train: Charlie Brown +, other characters – Test: Keep all features intact, but alter whole Prototype theory • Abstract the “ideal” (or average) category exemplar. • To test: train with only extreme exemplars, test with average of extremes. Prototype Theory in Humans Posner & Keele 1968 Conclusions: • Not clear whether birds can extract abstract concepts in categorization experiments • Birds may use features and exemplars • Another animals may be capable of more complex feats.
