1. 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?

2. 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

3. 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

4. Diagram of Pavlov’s Stimulus Substitution Model
Conditioning enables the CS to elicit the UR
The CS substitutes for the US

5. 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

6. 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

7. 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

8. 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.

9. 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

10. 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

11. 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
T elicits headjerk behavior (rapid
back and forth movements of the head)
Light
Food
L elicits rearing and investigation
of the food cup

12. 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

13. 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

14. Sexual Behavior System
General
search
behavior
Focal
search
behavior
Consummatory
behavior
(copulation)
CS US
CS 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)

15. Some Different Views as to What is Learned in Classical Conditioning
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

16. 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

17. CS US CS US U R UR Early Learning Trial
An association begins to develop between?
The CS center and the US center UR

18. CS-Only Test Trial CS US CS
U R CR

19. 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

20. CS CS US US U R UR Early Learning Trial
An association begins to develop between?
The CS center and the response UR UR

21. CS-Only Test Trial CS CS
U R CR

22. S-S versus 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

23. US-Devaluation Procedure
Phase
Experimental Group
Control Group
Conditioning CS US CS US
US Devaluation
e.g., CTA to chocolate
e.g., no exposure
to US
Decreased CR
Test/Result CS CS CR
i.e., Devaluation of US in Experimental Group results in decreased CR relative to Control Group

24. 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: T
Wintergreen/sucrose L
Nothing
Phase 2:
½ got T
LiCl
½ got L
LiCl
Phase 3:
Measured consumption of wintergreen
Results:
Rats that had T - LiCl pairings consumed
significantly less than rats that had L – LiCl
pairings

25. 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

26. 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)

27. 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.

28. 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.

29. 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

30. ToneCS
US for Group: .1 .2 .4 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10
Time
= 2 minutes of Tone CS
= Shock US in presence of Tone CS
= Shock US in absence of Tone CS

31. 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).

32. 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.

33. 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.

34. Blocking Problem for notion of contiguity and contingency
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

35. Blocking Group Phase 1 Phase 2 Phase 3 Experimental Group (blocking) AUS AB US
Test B
Control
Group
Nothing AB US
Test B
Same # trials
Contiguity
Contingency

36. 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