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Blocking. The phenomenon of blocking tells us that what happens to one CS depends not only on its relationship to the US but also on the strength of other CSs that might be present. Kamin (1969) CER paradigm. Group. Phase 1. Phase 2. Phase 3. L. N. Shock. NL. Shock. Exp. Gp.

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Blocking

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Blocking

The phenomenon of blocking tells us that what happens

to one CS depends not only on its relationship to the US

but also on the strength of other CSs that might be present

Kamin (1969) CER paradigm

Group

Phase 1

Phase 2

Phase 3

L

N

Shock

NL

Shock

Exp. Gp.

Con Gp.

Nothing

NL

Shock

L


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Results (CR to L)

.5

.4

Supp.

ratio

.3

.2

.1

0

Con

Exp

Learning to the added cue (L) is blocked by prior conditioning

with the N

See little CR to L in Experimental group because the N is already

a good predictor of the US


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The concept of surprise is important in explaining the

Blocking effect

Conditioning only occurs if the animal is surprised

by the US

T

US

On the first trial, the animal doesn’t expect the US

The US is surprising and learning takes place

Over trials the animal learns that the T predicts

the US

The US is no longer surprising and the rate of learning

slows down


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6

5

4

3

2

1

5

6

7

8

2

1

4

9

10

11

12

3

Typical Learning Curve

asymptote


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On compound trials, give

LT

US

Get little or no conditioning to the new L because the US

is not surprising; it is predicted by the T

The L is redundant; it provides no new information

What would happen if you changed the US on

compound trials?

The US would now be surprising, so should see

conditioning to the added cue

Unblocking

Unblocking is the elimination of the blocking effect – see

learning to the new cue


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Holland demonstrated unblocking by changing the US

(increased and decreased the US in separate groups)

This experiment demonstrates the importance of surprise

4 groups of rats

Phase 1

Phase 2

(1-1) = no surprise

L

1 pellet

LN

1 pellet

L

1 pellet

LN

3 pellets

(1-3) = surprise

L

3 pellets

LN

1 pellet

(3-1) = surprise

L

3 pellets

LN

3 pellets

(3-3) = no surprise

2 groups received the same # of pellets in phases 1 and 2

(blocking)

2 groups received a change in the number of pellets

(unblocking)


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Results

Unblocking

(see learning

To N)

(1-3)

(3-1)

% CR

to N

Blocking

(no learning

To N)

(3-3)

(1-1)

Test sessions


The rescorla wagner model l.jpg

The Rescorla-Wagner Model

  • Based on the concept of surprise

  • better than simple contingency theory

Contingency theory

Associations develop when a subject assesses the correlation

or predictive relationship between the CS and US

Excitatory conditioning occurs when:

p(US/CS) > p(US/no CS)

Problem with contingency theory is that it doesn’t take into

consideration what is happening to other CSs; therefore, it can’t

explain blocking


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The Rescorla-Wagner Model

  • mathematical model

  • US processing model

  • conditioning depends on the degree to which the

  • US is processed

  • according to the model, each US has a certain amount

  • of associative strength that will support conditioning

  • takes into account all the CSs present on a given trial

  • the concept of surprise is important to the RW model

  • surprise is defined as the discrepancy between the US

  • that is expected and the one that actually occurs


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The Rescorla-Wagner Model

Should be able to grasp the general idea of the RW model if you

understand the following 6 basic rules:

  • If the strength of the actual US is greater than the

  • strength of the subject’s expectation, all CSs that are

  • paired with the US will receive excitatory conditioning

  • If the strength of the actual US is less than the strength

  • of the subject’s expectation, all CSs that are paired with

  • the US will receive some inhibitory conditioning

3. If the strength of the actual US is equal to the strength

of the subject’s expectation, there will be no conditioning


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The Rescorla-Wagner Model

  • The larger the discrepancy between the strength of

  • the expectation and the strength of the US, the greater

  • will be the conditioning (either excitatory or inhibitory)

  • that occurs

  • More salient CSs will condition faster than less salient

  • CSs

  • If 2 or more CSs are presented together, the subject’s

  • expectation will be equal to their total strength (with

  • excitatory and inhibitory stimuli tending to cancel each

  • other out)


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Acquisition

L

1 pellet of food

Trial 1:

Rat has no expectation of the US

So, the strength of the US is much greater than the rat’s

expectation (which is ‘0’)

Therefore, this trial produces some excitatory

conditioning (refer to Rule #1)

But conditioning is rarely complete after 1 trial

Trial 2:

The second time the L is presented, it will elicit some

expectation of the US, but still not as strong as the actual

US

So rule #1 applies again and more excitatory conditioning

develops – and so on for trials 3, 4, 5……


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Acquisition

L

1 pellet of food

At each conditioning trial, the rat’s expectation of the food

pellet should get stronger

The difference between the strength of the expectation and

the strength of the US gets smaller

The fastest growth in conditioning occurs on the first few

trials and there is less and less conditioning as the trials

proceed (rule #4)

Eventually, the L elicits an expectation of 1 pellet and 1

pellet is given – the asymptote of learning is reached and

no further conditioning occurs


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6

5

4

3

2

1

5

6

7

8

2

1

4

9

10

11

12

3

Learning Curve

The RW model predicts the typical learning curve

(acquisition)


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Blocking

Now, suppose asymptote is reached and we give:

LT

1 pellet of food

According to rule #6, when 2 CSs are presented the

subject’s expectation is based on the total expectation

from the 2 CSs

T is a new stimulus = ‘0’ expectation

L produces expectation of 1 pellet

The actual US = 1 pellet; expectation matches the US that

is given and no additional conditioning occurs (rule #3)

The L retains its excitatory strength and the T retains

its ‘0’ strength


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The Rescorla-Wagner Model

ΔV = k(λ – V)

Δ = change

so ΔV = change in the strength of the CS

k = constant

Related to the salience of the CS and US

Refers to the associability of the CS

λ = maximum amount of conditioning that the

US can support – its our actual US value

λ – V = the discrepancy between what the animal

expects (V) and the actual US that is given (λ)


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The Rescorla-Wagner Model

Will sometimes see the formula written as:

ΔVA = k(λ – VT)

ΔVA = change in the strength of CSA

VT = strength of all CSs on a given trial

(VT = VA + VB …..)


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ΔVA = k(λ – VT)

Pair CSA Food

for 5 trials

VT = VA; since only 1 CS

k = 0.5; constant

λ = 100; US

Trial 1: ΔVA = k(λ – VT)

ΔVA = 0.5 (100 – 0)

= 50

So, change in strength of CS is 50 units

Trial 2: ΔVA = 0.5 (100 – 50)

= 0.5 (50)

= 25

CS gains additional 25 units


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ΔVA = k(λ – VT)

Trial 3: ΔVA = 0.5 (100 – 75)

= 0.5 (25)

= 12.5

Trial 4: ΔVA = 0.5 (100 – 87.5)

= 0.5 (12.5)

= 6.3

Trial 5: ΔVA = 0.5 (100 – 93.8)

= 0.5 (6.2)

= 3.1

ΔVA across 5 trials: 50 + 25 + 12.5 + 6.3 + 3.1 = 96.9

With more trials, V = λ = 100


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Most conditioning occurs on trial 1 (50 units)

On subsequent trials the CS acquires additional strength

which is a fixed proportion of the strength that is still

available

As conditioning progresses, the discrepancy between the

expected and actual US declines; (λ – VT) gets smaller

So, the RW model predicts the typical learning curve


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100

80

60

40

20

1

2

3

4

5

6

Learning Curve

VCS

or CR

Trials


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Rescorla-Wagner Model and Blocking

At the end of trial 5, in the previous example, the total

strength of all CSs is 96.9 (VT = VA = 96.9)

This means 3.1 units from the original 100 are available

on trial 6

Trial 6:

ΔV = k(λ – VT)

Food

CSA/CSB

ΔVB = .5 (100-96.9)

Assume k = .5

VT = VA + VB

= .5 (3.1)

= 1.5

Recall, CSA on first trial gained 50 units of strength

But, CSB gained only 1.5 units of strength

Conditioning to CSB is blocked


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The RW model can explain acquisition and blocking

The RW model can also explain extinction and

conditioned inhibition

ΔVA = k(λ – VT)

Suppose after asymptote is reached with L-Food pairings

we give:

Assume k = .5

VA = 100 (asymptote)

LT

No food

Here, rule #2 applies:

The strength of the expectation will exceed the strength

of the actual US

λ = 0 (no US is given); VT = 100, because L is given


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According to rule #2, both CSs will acquire some

inhibitory strength

How does this inhibitory conditioning affect the L and T?

Because the L starts with strong excitatory strength,

the trials without food and the inhibitory conditioning

they produce will begin to counteract this excitatory

strength

This is an example of extinction – presenting the CS

without the US

ΔVA = .5(0-100)

= -50

ΔVA = k(λ – VT)

L starts with 100, now reduced to 50 – but still excitatory

Repeated trials without the US would reduce L to ‘0’


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Remember, Lfood (until asymptote)

Then, LT no food

The T begins this phase with ‘0’ strength because it

hasn’t been presented before

Therefore, trials without food will cause the strength

of the T to decrease below 0 – it will become a CI

ΔVA = .5(0-100)

= -50

ΔVA = k(λ – VT)

T starts with 0, now at –50; so its inhibitory


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The RW model can also explain the US-pre-exposure effect

Phase 1: US alone

Phase 2: CSUS pairings

On US alone trials, the background cues become

conditioned

Then when CS-US trials are given, it becomes a blocking

experiment

US

Context + CS


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Unusual prediction of the RW Model

Loss of associative value despite pairings with the US

Phase 1:

Phase 2:

A

US

A + B

US

B

US

A and B are trained to asymptote in phase 1

Then in phase 2, both CSs are presented together with

the same US

The expectation in phase 2, would be 2X the US

However, only 1 US is given, so the expectation exceeds

the actual US

So, should see a decrease in CR to both A and B


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Evaluation of the RW Model

The RW model cannot account for latent inhibition (LI)

Phase 1: CS alone

Phase 2: CSUS pairings

According to the model the CS should not gain or lose

strength when no US is present

The RW Model is a US-processing model

So, change in strength of CS on

first trial is ‘0’

ΔVA = k(λ – VT)

The next trial would be the same

ΔVA = .5(0 – 0)

= 0

No increase or decrease in strength

of the CS


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Evaluation of the RW Model

Another problem for the model is unblocking

There are 2 types of unblocking:

Unblocking with an upshift (i.e., the US is increased)

Unblocking with an downshift (i.e., the US is decreased)

The RW model can explain unblocking with an upshift

If the US is increased, λ is increased, there is room to see

conditioning to the added CS

The model cannot explain unblocking with a downshift

If the US is decreased, λ is decreased. Should never see

excitatory conditioning to the added CS


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Evaluation of the RW Model

Extinction

According to the model, extinction should reduce the

strength of the CS to ‘0’

i.e., extinction is the reverse of acquisition

However, we know that extinction is the not the

reverse of acquisition

Spontaneous Recovery: if we give the animals a rest

period, responding the CS recovers

Temporal factors in conditioning

Temporal factors like the CS-US interval are important

but the RW Model cannot account for these factors


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