Classical Conditioning:
Mechanisms and Theory
Eyeblink Class Study
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60 conditioning trials (blocks of 20)
7 blocks of 4 probe trials
C1, P1, C2, P2, C3, P3, P4, P5, P6, P7
Acquisition, extinction
Results: Individual
% CR in Block
100
75
50
25
1
2
3
4
Blocks (of 4 probe trials)
5
6
7
Results: Averaged
% CR in Block
100
75
50
25
1
2
3
4
Blocks (of 4 probe trials)
5
6
7
Unconditional/Conditional
• US: elicits response without training
• Cs: elicits response due to training
(association)
• Not quite so clear-cut
Consider
• Aversive conditioning: tone (CS), mild
shock (US)
• Pavlov: mild shock(CS), food (US)
• Sign tracking: light (CS), saccharin (US)
• Taste aversion: flavour of saccharin
(CS), illness (US)
Novelty
• Prior associations
• Familiar vs. unfamiliar stimuli
• Not “unlearning” of familiar stimuli, per
se
• Basically, need to learn something
different
Latent Inhibition/CS
Preexposure
Phase 1
Phase 2
Phase 3
Exp. gr.
“CS” alone CS-US
test
Cont. gr.
nothing
test
CS-US
CR magnitude
• Highly familiar stimuli more difficult to
associate with US than novel stimuli
• Preexposure group
Cont.
Exp.
Latent Inhibition
• Habituation function
• Typically we think of habituating to a
US; ambiguity in CS/US designation
• Attentional processes
• CS- could also explain, but doesn’t
suppress responding to other CS+
US Preexposure
• Subjects exposed to US before CS-US
pairings slower to produce CR
• Associative interference (Hall 2008)
– Association of contextual CS with US
during US preexposure
– In essence, need to extinguish context CS
to associate novel CS with US
• Could this be habituation of US, too?
• Test methodology?
Ayres, Moore & Vigorito (1984)
• Stimulus salience
• Stimulus novelty
• Conditioned suppression
Method
• Stimuli
– CS: tone, light
– US: shock
• Stage 1: pair CS with US; suppression
ratio
• Stage 2: pair second CS (novel or
familiar) with US; suppression ratio
• Stage 3: extinction of second CS
Results
Stage 2: 2nd stim.
& shock
Stage 1: 1st stim. & shock
Suppression Ratio
0.5
Stage 3: 2nd
stim. extinction
Tone
0.4
Light
0.3
0.2
0.1
2
4
6
8
10
1
2 3
1
2
Day
Familiar
T-T
L-L
Novel
L-T
T-L
Familiars (T-T & L-L) show
less suppression than novels
(L-T & T-L): preexposure
Salience and Intensity
• Salience: significance, noticeability,
detectability
• Salience and intensity often used
synonymously
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Low to moderate levels, probably interchangable
Consider high level stimulus
Physiological damage
Not salient, but definitely intense
• Better to treat intensity as a component of
salience
Salience
• Increase via:
• Intensity
• Relevance
– Physiological needs
– Similarity of
environmental
stimuli (e.g.,
naturalistic CS”)
Belongingness: Stimuli
Relevance
• Equipotentiality principle
• Pavlov
• Any stimulus should, relatively, be equally
conditionable with any other stimulus
– E.g., CS1 easily associated with US1, should also
be easily associated with US2
– Easy-to-easy, hard-to-hard
• But doesn’t always work this way
• Garcia & Koelling’s work on taste aversion
Stimuli Relevance
• Biological predispostions; evolved
• Pigeons
– Visual CS associated more easily than
auditory CS with food US
– But auditory CS easier than visual CS
when shock is US
• Fear conditioning in primates (rhesus
monkeys, human children)
– CS of snake vs. flower
Wilcoxon et al. (1971)
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Biological preparedness in conditioning
Rats nocturnal, quails diurnal
Taste aversion
Blue water, sour water
Quails: colour --> stronger CS
Rats: taste --> stronger CS
Higher Order
Second-order
First-order
CS1
US
CR
CS2
CS1
CR
Third-order
CS3
CS2
CR
• Few pairings, higher-order
• Extensive training, CS• Solution: periodic reconditioning of firstorder
Rizley & Rescorla (1972)
• Extinction of CS1 does not affect CS2
• CS1 = tone, US = shock, CS2 = light
• Experimental group: 1. CS1-US, 2. CS2CS1, 3. extinguish CS1, 4. test CS1 & CS2
• Control group: 1. CS1-US, 2. CS2-CS1, 3.
nothing, 4. test CS1 & CS2
Results
Acquisition CS2 (light)
CS2 (light)
test
CS2 still shows suppression
for both exp. & cont.
groups…
CS1 (tone)
test
Suppression Ratio
0.5
0.4
0.3
0.2
0.1
1- 2
Exp. gr.
Cont. gr.
3-4
5-6
7-8
1-2
3-4
1-4
5-8
Even though CS1 shows no
suppression in exp. group.
9-12
Holland & Rescorla (1975)
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CS1 = light, CS2 = tone, US = food
CS1-US then CS2-CS1
Then satiate (devalue) US
Test:
– CS1’s CR weakened
– CS2’s CR unaffected
• Manipulating CS1-US relationship doesn’t
seem to affect CS2’s representation
Sensory Preconditioning
• Pair two stimuli (e.g., light and tone)
• Pair one with US… becomes a CS
• Now second stimulus also makes CR
Blocking
• Pair CS1 and US repeatedly
• Make compound CS1-CS2 and keep
pairing with US
• CS1 gives strong CR
• CS2 gives weak CR
Value of Classical
Conditioning
• Preparedness
• Evolution, survival mechanisms
• Foresight, anticipation
Zamble et al. (1985)
• Male rats
• Give male repeated access to receptive
females; pair with explicit CS
• With CS, initiates copulation sooner,
ejaculates quicker
• Competitive advantage over other
males
Hollis (1984)
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Blue gourami
Males hold territory
Attack intruders
Condition light with intruder
– Resident attacks intruder sooner
– Resident won conflict more often
Learning and Homeostasis
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Preparation
Homeostatic systems
Feedback lag in control system
Classical conditioning associations can
influence homeostatic systems
• Prepare for events that will perterb the
system
• Minimize lag
Effects
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Generally, very adaptive
However, sometimes difficulties
Conditioned compensatory responses
Drug tolerance
S. Siegel’s work on drug tolerance
– Contextual CS prepare opposing CR to
maintain homeostasis
– Difficulties if contextual CS absent
Stimulus Substitution Theory
• Pavlov’s theory
• Through repeated pairings of the CS
and the US the CS becomes a
substitute for the US so that all
responses initially elicited only by the
US are now also produced by the CS
Jenkins & Moore (1973)
• Pigeons
• Food or water as US
• CRfood = pecked
response key as if
eating; rapid pecks with
open beak
• CRwater = pecked
response key as if
drinking; slower pecking
with beak closed, often
with swallowing
water
food
Problems with Stimulus
Substitution Theory
• CS not a complete substitute for US
– E.g., eyeblink differences
– Magnitudes
• CSs produce different responses
– Omissions and additions
– E.g., conditioned suppression in rats
• US = shock, UR = flinch, CS = tone, CR = freeze
• Conditioned compensatory responses
What is Learned in Classical
Conditioning?
• US centre, Response centre, CS centre
Nervous System
US
Response
Centre
US Centre
S-S
S-R
CS
CS Centre
Response
S-S or S-R Connections?
• Stimulus-Stimulus (S-S)
Theory
US Centre
Response Centre
– Two associations
• Learned CS centre to US
centre
• Innate US centre to response
centre
CS Centre
• Stimulus-Response (S-R)
Theory
US Centre
– One association
• CS centre to response centre
CS Centre
Response Centre
Rescorla’s (1973) Experiment
• Habituation to weaken US-response link
• Conditioned suppression procedure
– Loud noise
• Experimental protocol
Group
Phase 1
Phase 2
Test
Habituation Light
Noise
Noise
(habituation)
Light: low CR
Light
Noise
No stimuli
Light: high CR
Control
Results
Control: high CR
Habituation: low CR
Supports S-S theory
CS/US Influence on CR
• Timberlake & Grant (1975)
– Second rat as CS in sign tracking
– CR --> social, not consumatory behaviour
• Akins (2000)
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Male quails’ behaviour sequence
General or focal search for female
CS = visual stimulus, US = female quail
CS-US interval short (1 min.) or long (20 min.)
Akins (2000)
General Search
% Time near CS
Crossings (Pacing)
Focal Search
1
20
CS-US Interval (min.)
1
20
CS-US Interval (min.)
Paired CS &US
Unpaired CS & US
Re: S-S Theory
• Requires flexibility in CR-UR
relationship
• CSs not associated with all aspects of
US
• CS and US can interact
• CR depends on sensory properties of
CS and presentation context
Rescorla-Wagner Model
• Learning is a discrepancy between
– Expectation
– Occurrence
• Level of surprise --> degree of
conditioning
– More surprising, more learning
– Early vs. later trials
Mathematical Model
 Vn = k( - Vn)
• V = CS-US associative strength
 V = change in associative strength per
trial
• k = salience of stimuli
  = asymptotic maximum of V (due to
US)
  - V = “suprisingness”
R-W and Blocking
• CS1 paired with US repeatedly
• Vcs1 approaches l
• By the time CS2 added, very little
associative strength left to be acquired
• CS1 very predictive of US; little
“surprise” left, so not much need for CS2
Overexpectation Effect
• Predicted by Rescorla-Wagner model
before being empirically demonstrated
Group
Phase 1
Phase 2
Test
Overexpectation L ... 1 food L+T ... 1 food L
T ... 1 food
T
L ... 1 food no stimuli
L
Control
T ... 1 food
T
Results
moderate CR
moderate CR
strong CR
strong CR
Conditioned Inhibition
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Excitatory CS paired with inhibitory CS
Opponent process system
Example: tone = CS+, light = CSStart values:
Vtone = 100.0, Vlight = 0.0, k = 0.2,  = 0.0
• We define
Vsum = Vtone + Vlight
• And
Vn = k(Vmax - Vsum)
CS+
CS-
100
Vn
Vtone
Vlight
Vsum
1
2
3
4
5
6
7
8
9
10
-20.0
-12
-7.2
-4.3
-2.6
-1.6
-0.9
-0.6
-0.5
100
80
68
60.8
56.5
53.9
52.3
51.4
50.8
50.4
0.0
-20
-32
-39.2
-43.5
-46.1
-47.7
-48.6
-49.2
-49.6
100
60
36
22
13
8
5
3
2
0
V1=0.2(0-100)=-20
V2=0.2(0-60)=-12
V3=0.2(0-36)=-7.2
60.8 -39.2 = 22
Associative Strength
Trial
80 -20 = 60
Vtone
50
Vsum
0
Vlight
-50
Trials
CS Preexposure Effect
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Preexposure group: give CS alone
Control group: pair CS with US
Test: how long to get CR in both groups
“Habituation” in preexposure group
– Learn to pay less attention to CS
– CS irrelevant, nonpredictive
– Must “unlearn” during test phase
CS Preexposure Effect
• Not explained by Rescorla-Wagner model
• First preexposure trial
– No prior conditioning
Vn = 0,  = 0
• But, something is learned
• Salience variable, not constant
– Salience of CS decreases during preexposure
– Pay less attention to CS
Other Issues
• Extinction
– Not return to zero
• CS– Extinguished not by being presented by
itself but by extinguishing CS+
• Stimuli must be either CS+ or CS-, not
both depending on context
Attentional Models
• Numerous stimuli in environment
• Can’t attend to them all
• Selectively process (remove signal from
noise)
• E.g., cocktail party effect
– Attend to one conversation at a time
– Your name
Mackintosh’s Theory
• Treats salience as a variable, not a
constant
• Consider two stimuli, L and T
• If L is a better predictor of the US, then
the salienceL will increase and salienceT
will decrease
• Attend to the more informative stimulus
Mackintosh: Blocking
• Trained on CSL; salienceL high because
CSL is predictive of US
• In compound CS phase this means
salienceT will drop towards zero
• CST will receive little attention, hence
the weak CR
Pearce & Hall’s Theory
• CSs become ineffective whenever the US is
already well predicted
• If situation changes so that US is surprising
then more learning about the CS
• Attention to CS depends on surprisingness of
US on previous trial
• Assume surprisingness of US will alter
attention paid to CSs on subsequent trials
– Contrast to R-W (surprisingness of US on trial
determines what is learned on that same trial)
Attentional Theories and
Blocking
• On first compound CS trial, CS2 should
be quite surprising (previously only CS1
paired with US)
• Should result in heightened attention to
(and learning about) CS2 on
subsequent trials
• But, CS1 blocks learning about CS2 on
first compound CS trial
Temporal Coding Hypothesis
• Contiguity
• ISI: short delay vs. long delay and trace
• ITI: generally, stronger CR with it is
spaced further apart
• CS duration: can also influence learning
• Learn not only that CS is paired with US
but also when the US will occur
• US = food CS = noise
CR = time at food cup
• Two CS durations (trial
duration = T) of 10 or 20
seconds
• Six it is (15 to 960
seconds)
• Results explainable by
I/T ratio
Time at Food Cup (CR)
Holland (2000)
1.5
3
6
12
I/T Ratio
T = 10 sec.
T = 20 sec.
24
48
Rate Estimation Theory
• Extension of relative-waiting-time hypothesis
– CS only informative about US if you spend less
time waiting for US when CS is present
• Nonassociative theory
• CRs reflect subjects estimates of rate of US
presentations during CS and absence of CS
• Doesn’t fit will with neurophysiological data
on associative learning
• Heavy computational burden
– May work in controlled (restricted) laboratory
environment, but in real world environment?
Comparator Hypothesis
• Traditional interpretation is that blocking from
failure to learn about CS2
• Comparator assumes that subject learns
about CS2, but ability to respond is blocked
• Revaluation effects
• Extinguishing CS1 can result in CS2 now
producing a CR
Comparator Hypothesis
• Theory of performance, not learning
• Condioned responding depends on:
– Associations between CS and US
– Associations between US and other stimuli
(comparator cues; may include
experimental context)
Comparator Hypothesis
• Only allows formation of excitatory
associations with US
• Excitation or inhibition determined by relative
strengths of excitatory conditioning to target
CS as opposed to other comparator stimuli
• If excitatory value of CS greater than that of
comparator stimuli, then CS+; if lower, then
CS• In essence, another opponent process model