Theories of Classical Conditioning: Critical Concepts and Pavlov's Stimulus Substitution Theory
Understand the key aspects of classical conditioning, including the critical CS-US relationship and Pavlov's Stimulus Substitution Theory. Explore why organisms respond predictably, uncover criticisms and flaws in the theory, and delve into the bigger problem of selecting for certain conditioned responses.
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Theories of Classical Conditioning
Critical CS-US relationship Important (critical) things to note about classical conditioning: the CS MUST precede the US the CS MUST predict the US if the CS does not predict the US, no conditioning occurs the CR does not have to be identical to the UR E.g., subtle differences even Pavlov noticed) may even be opposite: Morphine studies Any response is a classically conditioned response if it occurs to a CS after that CS has been paired with a US but does NOT occur to a randomly presented CS-US pairing
Theories of Classical Conditioning: WHY do organisms respond to predictability? Pavlov: Stimulus substitutability theory Kamin: Surprise theory Rescorla and Wagner: Computational Model Current Attentional Models
Pavlov: Stimulus Substitution Theory Basic premise of theory: CS substitutes for US w/repeated pairings between CS and US, CS becomes substitute for the US thus, the response initially elicited only by US is now also elicited by CS sounds pretty good: salivary conditioning: US and CS both elicit salivation eyeblink conditioning: both elicit eyeblinks Theory was doing well until we found compensatory CRs
Pavlov: Stimulus Substitution Theory Criticisms and Flaws: CR is almost never an exact replica of the UR An eyeblink to UR of air puff = large, rapid closure Eyeblink to CS of tone = smaller, more gradual closure Defense of theory: Hilgard (1936): Why differences in CR and UR: Intensity and stimulus modality of the CS and US are different Thus: differences in Response magnitude and timing are to be expected But still doesn t explain OPPOSITE CR
Pavlov: Stimulus Substitution Theory BIGGER PROBLEM: Whereas many US's elicit several different R's, as a general rule not all of these R's are later elicited by the CS CS seems to select for certain CRs E.g. Zener (1937) Dog presented w/food as US: found that the dog elicited a number of UR responses to the food E.g., salivation, chewing, swallowing, etc. CS not elicit all of those responses NO CRs of chewing and swallowing Just the CR of just salivation On other hand: CR may contain some of responses that are not part of CR: Zener found that dogs turned head to bell But no head turns to presentation of food
Modifications of SST MODIFICATIONS OF SST: (Hilgard) Only some components of UR transferred to CR CS such as a bell often elicits unconditioned responses of its own, and these may become part of CR Remember SIGN TRACKING: Brown and Jenkins 1974 Emphasized this change in form of CR vs. UR Also Jenkins, Barrara, Ireland and Woodside (1976) Sign Tracking : animals tend to Orient themselves toward the CS (not the US) Approach Explore any stimuli that are good predictors of important events such as the delivery of food
1 Set up: 1. Initial training: Light turns on above feeder feeder releases pieces of hot dog 4 2 2. Test: a. Light turns on above feeder, then above each of the other walls b. Forms a sequence of 1 2 3 4 3. What is optimal response? 3 4. But: Dog tracked the sign Jenkins, Barrara, Ireland and Woodside (1976)
Modifications of SST Strongest data against SST theory: Paradoxical conditioning CR in opposite direction of UR Black (1965): heart rate decreases to CS paired w/shock US of shock elicits UR of heart rate INCREASE But CS of light or tone elicits CR of heart rate DECREASE Seigel (1979): conditioned compensatory responses Morphine studies evidence of down regulation in addiction Actual cellular process in neurons (and other cells, too!) thus SST theory appears incorrect
Perceptual Gating Theory Perceptual gating theory: Idea that only if CS is biologically relevant will it get processed If a CS doesn t get processed it can be predictive/informative Animals attend to biologically relevant stimuli Problem: Data show that under certain circumstances a stimulus is attended to or processed , but still does not serve as a CS with an accompanying CR Issue remains: is the stimulus the most predictive? Second issue: Defining biologically relevant
Kamins work: 1967-1974 Blocking and overshadowing Overshadowing: use one "weak" and one "strong" CS CS1+CS2 US reaction to weaker stimulus is blotted out by stronger CS Demonstrated by Pavlov Blocking: Train 1 CS, then add a second CS to it: CS1 US CS1+CS2 US test each individually after training Find that only one supports a CR One stimulus blocks learning to second CS Demonstrated by Kamin
Kamins blocking experiment Used multiple CS's and 4 groups of rats The blocking group receives series of L+ trials which produce strong CR series of L+T+ trials then tested to just the T The Control groups receives SAME TOTAL NUMBER OF TRIALS AS BLOCKING GROUP no first phase L+ only; Test T T+ only; Test T LT+ only: Test T
Kamins blocking experiment Prediction: since both received same # of trials to the tone- should get equal conditioning to the tone Results quite different: Blocking group shows no CR to the tone- the prior conditioning to the light "blocked" any more conditioning to the tone Directly contradicts frequency principle (remember associationism!) Group Control Control Control --- LT+ T T elicits a CR Blocking L+ LT+ T T elicits no CR Phase I Phase II Test Phase Result --- L+ T T elicits no CR --- T+ T T elicits CR
Things we know about blocking: The animal does "detect" the stimulus: can t be perceptual gating issue EXT of CR with either T alone or with LT EXT occurred faster with compound LT Appears to be independent of: length of presentation of the CS number of trials of conditioning to compound CS Constancy of US from phase 1 to 2 important!!!! US must remain identical between the two phases or no blocking Influenced by: Type of CR measure (used CER, not as stable as non fear CR) nature of CS may be important- e.g. modality intensity of CS or US stimuli important Depends on amount of conditioning to blocking stimulus which already occurred
Change in either US or CS can prevent/ overcome blocking Change the intensity of the CS from phase 1 to phase 2 Overshadowing could be playing a role strong vs weak stimulus e.g. experiments when changed from 1 ma to 4 ma shock quickly condition to compound stimulus little or no overshadowing or blocking Change in intensity of either CS stimulus- Change in context from Phase 1 to Phase 2 lT then T Lt then T presents a different learning situation and no blocking Any ideas about what is happening?
Explanations of Blocking: Poor Explanation: Perceptual gating theory: tone never gets processed tone not informative data not really support this (evidence that do hear tone) Good Explanation: Kamin's Surprise theory: to condition requires some mental work on part of animal animal only does mental work when surprised bio genetic advantage: prevents having to carry around excess mental baggage thus only learn with "surprise" situation must be different from original learning situation Better Explanation: Rescorla Wagner model: particular US only supports a certain amount of conditioning if one CS hogs all that conditioning- none is left over for another CS to be added question- how do we show this?
A Brief Aside Must determine how CS-US relationship works Rescorla (1966) spent a lot of time on control groups What exactly IS a control group in classical conditioning? Why is this important? Question of contiguity vs. predictability at play here.
Recorla: Which is more important? CS-US correlation vs. contiguity CS-US contiguity: CS and US are next to one another in time/space In most cases, CS and US are continguous CS-US correlation: CS followed by the US in a predictive correlation: If perfect correlation (most predictive)- most conditioning p(US/CS) = 1.0 p(US/no CS) = 0.0 But: life not always a perfect correlation
CS-US correlation is more critical Rescorla (1966, 1968): Showed how 2 probabilities interact to determine size of the CS CS = 2 min tone; presented at random intervals (M = 8 min) Group 1: p(shock/CS) = 0.4 during 2 min presentation Group 2: p(shock/no CS) = 0.2 Which group should show more conditioning? WHY?
Robert Rescorla (1966) Examined predictability 6 types of Groups CS-alone present CS alone with no US pairing problem: not have same number of US trials as experimental animals do, may actually be extinction effect Novel CS group: looks at whether stimulus is truly "neutral" may produce habituation- animal doesn't respond because it "gets used to it" US-alone present US alone with no CS pairing problem: not have same number of CS trials
Rescorla: 6 types of control groups Explicitly unpaired control CS NEVER predicts US that is- presence of CS is really CS-, predicts NO US animal learns new rule: if CS, then no US Backward conditioning: US precedes CS assumes temporal order is important (but not able to explain why) again, animal learns that CS predicts no US Discrimination conditioning (CS+ vs CS-) use one CS as a plus; one CS as a minus same problem as explicitly unpaired and backward- works, but can work in certain circumstances (taste avoidance)
Rescorla: Results with 6 Groups CS-alone: No conditioning, but habituation to CS Novel CS group: novel worked better than CS with previous experience. US-alone: habituation to CS Explicitly unpaired control: Got GREAT conditioning Learned that the CS NEVER predicts the US! Backward conditioning: US preceded CS assumed temporal order is important It was: Animal learned that CS predicts NO US, but US predicted CS Discrimination conditioning (CS+ vs CS-) use one CS as a plus; one CS as a minus Got discrimination Animals paid attention to whatever stimulus was MOST PREDICTIVE!
CS-US correlation: Summary of Results Whenever p(US|CS) > p(US|NO cs): CS = EXCITATORY CS that is, CS predicts US Amount of learning depended on size difference between p(US/CS) and p(US/no CS) Whenever p(US|CS) <p(US|NO CS): CS = INHIBITORY CS CS predicts ABSENCE of US Amount of learning depended on size difference between p(US/CS) and p(US/no CS) Whenever p(US|CS) = p(US|NO CS): CS = NEUTRAL CS CS doesn t predict or not predict CS No learning will occur because there is no predictability.
CS-US correlation vs. contiguity Thus: appears to be the CORRELATION between the CS and US, not the contiguity (closeness in time) that is important Can write this more succinctly: correlation carries more information than contiguity if r = + then excitatory CS if r = - then inhibitory CS if r = 0 then neutral CS (not really even a CS)
Classical condition is cognitive (oh the horror of that statement, I am in pain) PREDICTABILITY is critical Learning occurs slowly, trial by trial Each time the CS predicts the US, the strength of the correlation is increased The resulting learning curve is monotonically increasing: Initial steep curve Levels off as reaches asymptote There is an asymptote to conditioning to the CS: Maximum amount of learning that can occur Maximum amount of responding that can occur to CS in anticipation of the upcoming US We can explain this through an equation!
The Rescorla Wagner Equation!: Yields an equation: THE Rescorla Wagner (1974) model!!!!! Vi= i j( j-Vsum) Vi = amount learned (conditioned) on a given trial i = the salience of the CS j = the salience of the US ( j-Vsum) = total amount of conditioning that can occur to a particular CS-US pairing
The Rescorla Wagner Equation!: Yields an equation: THE Rescorla Wagner (1974) model!!!!! Vi= i j( j-Vsum) What does this equation say? The amount of conditioning that will occur on a given trial is a function of: The size of the salience of the CS multiplied by The size of the salience of the US multiplied by (The maximum amount of learning) - (the amount of learning that has already occurred).
Can say this easier! How much you will learn on a given trial (Vi) is a function of: i or how good a stimulus the CS is (how well it grabs your attention) j or how good a stimulus the US is (how well it grabs your attention j or how much can learning can be learned about the CS-US relationship AND Vsum or how much you have learned ALREADY!
Assumptions of Rescorla-Wagner (1974) model Model developed to accurately predict and map learning as it occurs trial by trial Assumes a bunch of givens: Assume animal can perceive CS and US, and can exhibit UR and CR Helpful for the animal to know 2 things about conditioning: what TYPE of event is coming the SIZE of the upcoming event Thus, classical conditioning is really learning about: signals (CS's) which are PREDICTORS for important events (US's)
Assumptions of R-W model Assumes that with each CS-US pairing 1 of 3 things can happen: The CS might become more INHIBITORY The CS might become more EXCITATORY There is no change in the CS How do these 3 rules work? If US is larger than expected: CS = excitatory If US is smaller than expected: CS= inhibitory If US = expectations: No change in CS The effect of reinforcers or nonreinforcers on the change of associative strength depends upon: The existing associative strength of THAT CS AND on the associative strength of other stimuli concurrently present
More assumptions Explanation of how an animal anticipates what type of CS is coming: Direct link is assumed between "CS center" and "US center": E.g. between a tone center and food center In 1970 s: other researchers thought R and W were crazy with this idea Now: neuroscience shows formation of neural circuits! Assumes that STRENGTH of an event is given The conditioning situation is predicted by the strength of the learned connection THUS: when learning is complete: The strength of the association relates directly to the size or intensity of the CS Asymptote of learning = max learning that can occur to that size or intensity of a CS Maximum amount of learning that a given CS can support
More assumptions The change in associative strength of a CS as the result of any given trial can be predicted from the composite strength resulting from all stimuli presented on that trial: Composite strength = summation of conditioning that occurs to all stimuli present during a conditioning trial If composite strength is LOW: the ability of reinforcer to produce increments in the strength of component stimuli is HIGH More can be learned for this trial If the composite strength is HIGH: reinforcement is relatively less effective (LOW) Less can be learned for this trial- approaching max of learning
More assumptions: Can expand to extinction, or nonreinforced trials: if composite associative strength of a stimulus compound is high, then the degree to which a nonreinforced presentation will produce a decrease in associative strength of the components is LARGE if composite associative strength is low- nonreinforcement effects reduced
WHY is this equation important? We can use the three rules to make predictions about amount and direction of classical conditioning j > Vsum = excitatory conditioning The degree to which the CS predicted the size of the US was GREATER than expected, so you react MORE to the CS next trial j < Vsum = inhibitory conditioning The degree to which the CS predicted the size of the US was LESS than expected, so you react LESS to the CS next trial j = Vsum = no change: The CS predicted the size of the US exactly as you expected
The Equation: Lets USE it to Explain Learning, Overshadowing and Blocking!: Vi= i j( j-Vsum) Vi = amount learned (conditioned) on a given trial i = the salience of the CS j = the salience of the US ( j-Vsum) = total amount of conditioning that can occur to a particular CS-US pairing
Okay, you got all that? Let s put this baby to work .. .we will try a few examples
The equation: Vi =ij( j-Vsum) Vi = change in associative strength that occurs for any CS, i, on a single trial i = stimulus salience (assumes that different stimuli may acquire associative strength at different rates, despite equal reinforcement) j = learning rate parameters associated with the US (assumes that different beta values may depend upon the particular US employed) Vsum = associative strength of the sum of the CS's (strength of CS-US pairing) j= associative strength that some CS, i, can support at asymptote In English: How much you learn on a given trial is a function of the value of the stimulus x value of the reinforcer x (the absolute amount you can learn minus the amount you have already learned).
Acquisition first conditioning trial: Assume (our givens) CS = light; US= 1 ma Shock Vsum = Vl; no trials so Vl = 0 thus: j-Vsum = 100-0 = 100 -first trial must be EXCITATORY BUT: must consider the salience of the light: i = 1.0 j = 0.5
Acquisition first conditioning trial: CS = light; US= 1 ma Shock Vsum = Vl; no trials so Vl = 0 thus: j-Vsum = 100-0 = 100 -first trial must be EXCITATORY BUT: must consider the salience of the light: i = 1.0 and learning rate: j = 0.5 Plug into the equation: for TRIAL 1 VL = (1.0)(0.)(100-0) = 0.5(100) = 50 thus: V only equals 50% of the discrepancy between Aj an Vsum for the first trial
Acquisition Plug into the equation: for TRIAL 1 VL = (1.0)(0.)(100-0) = 0.5(100) = 50 thus: VL only approaches 50% of the discrepancy between Aj and Vsum is learned for the first trial
Acquisition TRIAL 2: Same assumptions! VL = (1.0)(0.5)(100-50) = 0.5(50) = 25 Vsum = (50+25) = 75
Acquisition TRIAL 3: VL = (1.0)(0.5)(100-75) = 0.5(25) = 12.5 Vsum = (50+25+12.5) = 87.5
Acquisition TRIAL 4: VL = (1.0)(0.5)(100-87.5) = 0.5(12.5) = 6.25 Vsum = (50+25+12.5+6.25) = 93.75 TRIAL 10: Vsum = 99.81, etc., until reach ~100 on approx. trial 14 When will you reach asymptote?
R-W explains 1 CS learning 100 80 60 Amt of learning 40 20 0 0 2 4 6 8 10 12 Trials learning to Vlight Total amount learned (Vsum)
Now: Back to Explaining Blocking and Overshadowing Overshadowing: use one "weak" and one "strong" CS reaction to weaker stimulus: less CR Reaction to stronger stimulus: more CR Blocking: 1st CS blocks learning to 2nd CS At issue: What is predicting what? Does LT give any more information/predictability than L alone? If not, then L blocks learning to LT
How to explain overshadowing? Yep, it is good old Rescorla-Wagner to the rescue!
Remember Overshadowing Pavlov: compound CS with 1 intense CS, 1 weak after a number of trials found: strong CS elicits strong CR Weak CS elicits weak or no CR Note: BOTH CSs are presented at same time Why would one over shadow or overpower the other? Why did animal not attend equally to both?
Overshadowing Rescorla-Wagner model helps to explain why: Assume L= light = 0.2; T = tone = 0.5 L = light = 1.0 ; t = tone = 1.0 Plug into equation: Vsum = Vl + Vt = 0 on trial 1 VL = 0.2(1)(100-0) = 20 Vt = 0.5(1)(100-0) = 50 after trial 1: Vsum = 70
Overshadowing TRIAL 2: VL = 0.2(1)(100-(50+20)) = 6 Vt = 0.5(1)(100-(50+20)) = 15 Vsum = (70+(6+15)) = 91 TRIAL 3: VL = 0.2(1)(100-(91)) = 1.8 Vt = 0.5(1)(100-(91)) = 4.5 Vsum = (91+(1.8+4.5)) = 97.3 and so on thus: reaches asymptote (by trial 6) MUCH faster w/2 CS's NOTE: CSt takes up over 70 units of assoc. strength CSl takes up only 30 units of assoc. strength
Overshadowing R-W explains 2 CS learning 120 100 80 Amt of learning 60 40 20 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Trials Vsum for light V sum for tone V sum total
