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Chapter 4
• Classical Conditioning: Mechanisms
– What makes effective conditioned and
unconditioned stimuli?
– What determines the nature of the
conditioned response?
– How do conditioned and unconditioned
stimuli become associated?
What determines the nature of the CR?
Classical conditioning is defined by the development
of a CR to a previously neutral CS
Look at why particular responses become conditioned
The first explanation for the nature of the CR is the
Stimulus Substitution model
The Stimulus Substitution Model
• developed by Pavlov
• the association of the CS with the US turns the CS
into a ‘surrogate’ US
• the CS activates neural circuits previously activated
only by the US and elicits responses similar to the US
• with pairings of the CS and US, a new connection
develops between the neural circuits previously
activated by the CS and the neural circuits previously
activated only by the US
Diagram of Pavlov’s Stimulus Substitution Model
CS
center
US
center
Response
center
• Conditioning enables the CS to elicit the UR
• The CS substitutes for the US
The US as a determining factor for the CR
• The quality or nature of the US affects the type of CR
• Different USs elicit different URs
L
Food; elicits salivation
L
Shock; elicits fear response
• More subtle differences in CRs depending on the US
KL
Food
KL
water
• Pigeons peck at the KL in both cases but the pecking
response is different
• Thus, the form of the CR resembles the form of the UR
Learning & Homeostasis
• The concept of Homeostasis was introduced by
Water Cannon
• certain physiological responses are maintained at
acceptable levels (temp., HR, BP)
• change in one direction must be accompanied by an
opposite response to return the system to a baseline or
optimal level
• operates through a negative feedback loop
• i.e., a drop in temp. is detected and serves as a
stimulus to activate compensatory responses
Learning & Homeostasis
• Homeostatic mechanisms operate by a negative
feedback process that counteracts a challenge after
the disturbance has already caused a deviation from
homeostatic level
• Pavlovian conditioning can be beneficial to anticipate
the challenges to homeostasis and start compensatory
mechanisms earlier
Learning & Homeostasis
• For ex., exposure to cold
Feedback processes require a drop in temp. before
compensatory responses (shivering, vasoconstriction)
occur to counteract the cold
• If subjects learn which cues signal when they will get
cold, they can make feed-forward compensatory
adjustments in anticipation of the cold and avoid the
actual drop in temp.
Conditioned Homeostatic Responses
• examined most extensively with psychoactive drugs
• Conditioning model of drug tolerance
• tolerance = repeated presentations of the drug have
progressively less effect
• administration of a psychoactive drug causes
physiological changes that disrupt homeostasis = US
• these physiological changes caused by the drug trigger
unconditioned compensatory changes to counteract the
drug = UR
Conditioned Homeostatic Responses
• stimuli that accompany drug administration become
associated with the drug (syringe, context cues, etc…)
• these cues associated with drug administration trigger
the compensatory response, so the drug has less effect
• the CR then counteracts the drug effect
The CS as a determinant of the form of the CR
The quality or nature of the CS affects CR, even though
the CS is supposed to be innocuous or neutral
Experiment by Holland with rat subjects
Tone
Food
Light
Food
T elicits headjerk behavior (rapid
back and forth movements of the head)
L elicits rearing and investigation
of the food cup
The CS as a determinant of the form of the CR
Timberlake and Grant (1975) – described in textbook
Investigated classical conditioning in rats with food
as the US
The CS was a presentation of another rat
Instead of gnawing and biting (as stimulus substitution
model might predict), the CR was orientation, approach,
sniffing and social contact
Conditioned Behavior and Behavior Systems
Presentation of a US activates the behavior system
relevant to that US
- food activates foraging/feeding system
- mate activates sexual behavior system
As a CS becomes associated with the US, it becomes
integrated into that behavior system
The form of the CR will depend on the CS-US interval
that is used
The CS-US interval determines where the CS becomes
incorporated into the sequence of responses that makes
up the behavior system
Sexual Behavior System
General
search
behavior
Focal
search
behavior
CS
CS
Consummatory
behavior
(copulation)
US
US
Atkins (2000)
Light  Female quail
•60 sec CS-US interval (male searched in area of light)
•20 min CS-US interval (male searched all of chamber)
How does a CS produce responding?
Some Different Views as to What is
Learned in Classical Conditioning
a.
S-S Learning
b.
S-R Learning
S-S Learning Interpretation
States that the CS enters into an association with the US
(or US representation)
The CS triggers a memory of the US which in turn elicits
a response
Early Learning Trial
CS
US
CS
US
U
UR
R
An association begins
to develop between?
The CS center and
the US center
CS-Only Test Trial
CS
US
U
CS
R
CR
S-R Learning Interpretation
States that because the CS occurs close in time to the
UR, it comes to trigger the response directly
An association forms between the CS and the response
Early Learning Trial
CS
CS
US
US
U
UR
R
An association begins
to develop between?
The CS center and
the response UR
CS-Only Test Trial
CS
CS
U
R
CR
S-S versus S-R Learning
S-R Learning
Support: studies where CR is identical to UR
Problem: sensory preconditioning
S-S Learning
Support: US-devaluation experiments
Problem: does not specify the nature of the CR
US-Devaluation Procedure
Phase
Experimental
Group
Conditioning
CS
US Devaluation
e.g., CTA to
chocolate
Test/Result
CS
US
Decreased
CR
Control Group
CS
US
e.g., no exposure
to US
CS
CR
i.e., Devaluation of US in Experimental Group
results in decreased CR relative to Control Group
Support for S-S theory comes from study by Holland
He showed that the memory of an event could substitute
for the event itself in the formation of a new association
Phase 1:
Phase 2:
Phase 3:
Results:
T
Wintergreen/sucrose
L
Nothing
½ got T
LiCl
½ got L
LiCl
Measured consumption of wintergreen
Rats that had T - LiCl pairings consumed
significantly less than rats that had L – LiCl
pairings
Explanation
According to Holland, the T in phase 2 calls up a
representation or memory of the flavor and this
memory gets associated with the LiCl.
Even though the flavor was never paired with LiCl,
we see an aversion because the T was paired with LiCl
The 2 stimuli essentially substitute for each other
Evidence for S-S theory
How do Conditioned and Unconditioned
Stimuli Become Associated?
Contiguity
Two stimuli become associated if they occur close
together in time
While contiguity is important, it is not sufficient for
conditioning to occur
Contingency
Refers to the predictive relationship between stimuli
The CS has to convey information about US occurrence
p(US/CS) > p(US/no CS)
Rescorla’s (1968) Classic
Experiment on Contingency
Method
Used the CER procedure with rats:
First phase, operant conditioning to
establish steady bar pressing.
Second phase, CER training: classical
conditioning to establish CER.
Method (continued)
CER training (daily for 5 days):
• All rats exposed to 12 tones (the CS).
• The tones were 2-min long and the mean
inter-tone interval was 8 min.
• The probability of shock (the US) during
the tone was .40 for all rats.
• Groups differed in the probability of
shock during the inter-tone interval.
Method (continued)
During the Inter-tone interval:
• Group 0 : no shocks
• Group .1: shocked with a probability of .1
• Group .2: shocked with a probability of .2
• Group .4: shocked with a probability of .4
Tone
CS
US for Group:
0
.1
.2
.4
1 1 2 2
Time
3
34 4 5
56 6 7
7
8
89 9 10 10
= 2 minutes of Tone CS
= Shock US in presence of Tone CS
= Shock US in absence of Tone CS
Method (continued)
Third phase, after CER training, the rats were
returned to bar pressing for food.
While the rats were bar pressing, the tones were
presented as before, but no shocks were given
(what procedure is this?)
Conditioning was assessed by a suppression ratio
(the lower the ratio, the greater the suppression,
the stronger the conditioning).
Rats that experienced as many
shocks in the absence of the
tone as they did in its presence
showed no suppression.
Rats that only
experienced shock
with the tone, showed
total suppression.
• These results suggest that contiguity is
not the only associative principle
necessary to produce learning.
• All rats experienced the same degree of
contiguity between tone and shock, they
differed in the extent to which the shock
was contingent on the tone.
Problem for notion of contiguity and contingency
Blocking
• one of the more important phenomena in classical
conditioning
• shows that simple contiguity between the CS and US
is not sufficient for conditioning
• blocking experiments are typically done in 3 phases
Blocking
Group
Phase 1
Experimental
A
Group (blocking)
Control
Group
US
Nothing
Phase 2
Phase 3
AB
US
Test B
AB
US
Test B
Same # trials
Contiguity
Contingency
Blocking
In Control group, should get equal conditioning to both
A and B
- both are equally contiguous with the US
- same predictive relationship
What happens in the Blocking group?
- if during phase 1, A perfectly predicts the US then
should see little conditioning to B in phase 2
- B adds no new information since the US is already
predicted by A
B should evoke a stronger CR in the control group on test
This is called Blocking