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Rescorla – Wagner Model

1. There is a maximum associate strength that candevelop between a CS & a US

- limit determined by US

2. Associative strength increases with each training trial– the amount of increase on each trial is dependent onlevel of prior training

3. Rate of conditioning varies depending on CS & USused.

4. The level of conditioning on a particular trial isinfluenced not only by the amount of conditioning tothat stimulus, but also by the level of conditioning toother stimuli associated the US (i.e. blocking)

- same l (asymptote)- a & b are different (rates of change)

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- different l (asymptote)- a & B same (rates of change)

direction of learning is determined by the relationshipbetween V and V max.

1. When V is less than V max then associate strengthincreases

2. when V is greater than V max then associativestrength decrease

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3. When V = V max then associative strength does notchange.

- eg –rat receives 1OV shock – if not expectingshock then ↑ in fear – if it expected 2OV shockthen Ø in fear – if expected OV shock no changein fear level

Rescola/Wagner

Explaining the old:Equation can predict the shape of the acquisition curve.

Equation can explain extinction, the decrease in associativestrength with presentations of the US without the CS.

Explains Blocked (as shown above)

Model can also explain the effects of contingency. Whydoes presenting the US without a CS interfere withconditioning?

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- The US is not presented in a Vacuum! – thereare many stimuli from the environment that canbe associated with the US. – these environmentalcues are also present when CS predicts USso...CSe Æ US, e Æ US, CSe Æ US, e Æ US,

- initially conditioning occurs to both CS & e,eventually although VCS and Ve are each lessthan Vmax, together they exceed Vmax so the USwill actually Ø the fear associated with CS & e,and when e Æ US is next presented it is below Vmax so will ↑ associative strength – sobackground will be stronger association than CS!

- seen in ↑ fear of apparatus by animal- no unsignalled US’s not so much fear shown,

1. the strength of conditioning depends not just on thecharacteristics of the US, but also on whether it wasexpected.

2. Conditioning occurs not just to the nominal CS butalso to background cues that may be present. (Contextconditioning) -

Predicting the NewTheory also predicts that under some circumstances CS’s

will lose associative strength despite being paired withUS!- keep in mind model views learning as an

adjustment of expectancies to outcomes.

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Phase 1 Phase 2 TestA Æ US [A+B] Æ US A

?B Æ US B

- common sense says no change in A or B, but R/Wmodel predicts that if an animal receives less of anUS than expected – then a decrease in USexpectation or Associative Strength is predicted!

END OF Phase 1 VA -= l, VB = l

Start Phase 2 VAB = VA + VB = 2l

End of Phase 2 VAB = l; VA = _ l, VB = _ l

Model Limitations- some predictions incorrect

Latent InhibitionLubow & Moore (1959)

Gp 1 Gp 21) Flashing light –2) light Æ shock light Æ shock

What effect will pre-exposure to light have?

R/W model says when light is presented alone noconditioning will occur because no US is presented

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- V(max) = 0 when no US presentedDV = ab (Vmax – V) = ab (0 - 0) = 0!

So exposure to light should have no effect – but Lubow &Moore found conditioning significantly slower in the grouppre-exposed to the light

- they called this Latent Inhibition because theybelieved that the CS becomes inhibitory

- however no evidence for inhibition – just difficultto condition - perhaps habituation decreasesselective attention – maybe attention should bebuild into a model!

Configural Learning- model says – associative strength of a compound

stimulus is equal to the sum of its partsA US = .5 Vab = Va + Vb = .5 + .5 = 1.0B US = .5

- so predicts when A & B presented together shouldget greater responding.

- Bellingham, Gillete – Bellingham & Kehoe(1985)

tone Æ foodlight Æ foodtone/light Æ

- rats responded to each alone on 90% of trials- when presented together responding dropped to

30%- rats behaved as if compound was a unique, novel

stimulus – now termed a “configuration”

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Occasion SettingRescorla 1985 trained pigeons

Noise Æ light Æ foodLight Æ ______

-noise “set the occasion” for when the light would befollowed by food.- Rescorla then presented light 144 without food-

under normal CC this would lead to extinction. Inthis case when noise and light were presentedtogether the birds responded three times moreoften then if light alone was presented.

- So presenting light alone had no effect on its roleas occasion setter.

- Holland hypothesized that occasion setters are notdirectly associated with the US, but serve tomodulate or facilitate the association between theCS and the US.

So the Wagner/Rescorla model accounts for many aspectsof learning, but not all.

There are several alternate theories:

MacIntosh’s Theory of Attention!

Masses of info coming in all the time - can’t attend to it all– must be selective

- cocktail party phenomenon – can attend to 1conversation despite large amt of competing input

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- theories of attention propose that on aconditioning trial when several stimuli arepresented together the subject will attend to only 1(or only a few) of those stimuli, and learning willoccur only for those stimuli attended to.

Mackintosh ’75 proposed a theory of attention andclassical conditioning that is the major competitor of R/Wmodel.

- animals seek out info that predicts the occurrenceof biologically significant events (US’s)

- once it has found a stim that reliability predicts anevent – it ignores all other stimuli

- thus the animal plays an active role in theconditioning process. Animal must recognize thecorrelation (or lack of correlation) between events(CS + US)

- CS Preexposure – animal learns CS is irrelevant –will stop attending to it + will have difficultylearning that CS is correlated with US.

- “learned irrelevance”- evidence for this – Baker & MacKintosh (1977)

- significantly greater interference is producedby uncorrelated presentations between CS &US prior to conditioning than to either CS orUS presentations alone.

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Pearce & Hall (1980)

Proposed a theory of CS effectiveness

- CS’s become ineffective whenever the US isalready well predicted

- if the situation is changed so that the US is againsurprising (i.e. by making the US more intense)the CS’s will quickly regain effectiveness

Comparitor theories

- other theories we’ve discussed have 2 things incommon 1) predictions are based on trial by trialcalculations and 2) assume the presence of one CScan interfere with a subjects learning about otherCS’s (blocking)

- Comparator theories (Gibbon & Balsam, 1981;Miller & Schachtman 1985) – assume that theanimal compares the likelihood that the US willoccur in the presence of the CS with the

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likelihood that the US will occur in the absenceof the CS.

- What’s important is the overall long-termcorrelation between CS & US

- comparison of CS and context does not affect thelearning of the CR, but rather its performance.

e.g. p (US/CS = .5p (US/noCS) = .5

comparator theory says that this CS will not elicit a CR –NOT because CS has no excitatory strength – but becauseboth the CS and the contextual stimuli have acquired equalstrengths because both paired with US 50% of the time.

- theory assumes CS will not elicit CR unless it hasgreater excitatory strength than the contextualstimuli

- so assume that animal has learned somethingabout CS – that the US sometimes occurs in itspresence – but will not respond to CS unless it’s abetter predictor of US than context.

Test – after condit keep animal in context w/o US does thisextinguish context component and make CS betterpredictor? Yes – extinction of context does increaseresponding to the CS.

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What is learned in C.C.?

- when a tone is paired with food what is it that isassociated?

- Pavlov – simultaneous activation of tone & foodcenters results in the formation of a new pathwaybetween them.

- so when tone presented the excitation it produceswill be transmitted to the food center

- Hull (1943) – when tone is followed by salivationan association forms between tone & salivarycenters – so tone will directly elicit salivation

Pavlov’s approach called S-S Theory – because it assumesassociation is between two stimuli – the CS & the US

Hull’s approach is the S-R theory because it assumes adirect link between the CS and the response.

To test – what if – after conditioning we make the foodaversive?

Pavlov - TONE Æ FOOD Æ SALIVATION- if FOOD ≠ Saliv then tone won’t either

Hull - TONE Æ SALIVATION- If food ≠ saliv – no change!

Holland & Straub 1979 – 1 Osec Noise Æ FOOD pellets

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- conditioning measured by activity level- after condit activity to tone doubled after condit

made food unattractive by pairing with rotation athigh speed

later present tone- what will happen with activity?- significant decrease in activity to tone

so supports S-S theory

-However – another test is 2nd order conditioning

CS1 Æ US CS2 Æ CS

SS Learning SR Learning

CS1 CS2 Æ CR↑ Ø

CS2 CR

Test by extinguishing CS1 – what happens to CS2?- Results are mixed – in some experiments 2nd order

condit. involves S-R learning – in others S-Slearning

- so research is focused on which conditionssupport which forms of learning?!

Is the CS a signal or a substitute for the US?Tolman (1932) believed that the CS signaled food or set upthe expectation of food “Oh boy, I’m going to get food?”

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There is evidence to support the idea that the CS acts as asignal that the US is coming.Colwill and Motzkin (1994)

CS1 Æ sucroseCS2 Æ food pellets

When either CS presented rats approach the food hoperThe researchers then “devalued” one of the US’s – thismeans they paired on the CS’s with lithium (illness= tasteaversion). If the rats know which CS predicted which USthen since CS1 predicted sucrose they should now avoid it,but still respond on CS2. This is what happened- providingevidence that the CS led to an expectation of a specificreward.Pavlov – CS-US centers linked, so excitation in CSproduced excitation in US – so stimulus substitution “Ohboy what great food?”

- Pavlov found that after pairing light with foodwhen dog released from harness it tried to lickbulb

Auto shaping (sign tracking) –pigeons responses to food and water are different

- food – peck mouth open & eyes closed- water – peck beak closed, tongue down, eyes open- Jenkins & Moore (’73) – autoshaped to food or

water – what form does peck to the key-light take.- support for stimulus substitution – pigeons pecked

key as if it was food or H20!

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However if stimulus substitution only way, then CR shouldalways be identical to UR but this is not the case

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Fear condit UR to shock – jump in airCR to shock – freeze

UR to shock - ↑ in respirationCR to shock - Ø in respiration

In salivation – chemical composition of saliva to USdiffers to saliva to CS

- Modified Stimulus – substitution model- CS elicits innately programmed responses that

prepare the organism for the forthcoming US- animals have UR’s to important events, also have

preparatory responses to prepare for events- in some cases preparatory response = UR, in

others it differs..

The Two-System Hypothesis-evidence for both stimulus substitution and expectation-hypothesis is that there are two systems of learning- aprimitive stimulus substitution system and a moresophisticated cognitive system based onanticipation/expectation.

There has been a gradual shift in thinking about CC- froma simple contiguity association idea to a more complexsystem of determining the most effective predictors ofUS’s. This means it is a sophisticated system of detecting

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relationships so that we can learn to anticipate importantevents and be prepared for them.

`Medical DiagnosisGluck and Bower 1988- gave university students 250 setsof medical records (fictitious)-patients had some combination of: bloody nose, stomachcramps, puffy eyes and discolored gums.- students asked to decide which of two diseases the patienthad. After each diagnosis they were told whether they wereright or wrong.- after reading all 250 records students were shown eachsymptom and asked to estimate the probability whether thepatient had disease 1 or disease 2. (the symptoms weredesigned so that they had predictive values ranging from 0-1.-this can be seen as CC with each symptom acting as a CSand the disease as a US.-if this is true then the Rescorla/Wagner model could beused to predict subjects probability estimates.-model’s predictions were more accurate than morecomplex cognitive information processing models.-