Theories of Classical Conditioning

Report
Theories of Classical
Conditioning
Critical CS-US relationship
• Important (critical) things to note about classical conditioning:
– the CS MUST precede the US
– the CS MUST predict the US
– if the CS does not predict the US, no conditioning
occurs
– the CR does not have to be identical to the UR
• E.g., subtle differences even Pavlov noticed)
• may even be opposite: Morphine studies
• Any response is a classically conditioned response if it
– occurs to a CS
– after that CS has been paired with a US
– but does NOT occur to a randomly presented CS-US pairing
Theories: WHY do
organisms respond to predictability?
• Pavlov: Stimulus substitutability theory
• Kamin: Surprise theory
• Rescorla and Wagner: Computational Model
Pavlov: Stimulus Substitution Theory
– Basic premise of theory
• w/repeated pairings between CS and US, CS becomes
substitute for the US
• thus, the response initially elicited only by US is now also
elicited by CS
– sounds pretty good:
• salivary conditioning: US and CS both elicit salivation
• eyeblink conditioning: both elicit eyeblinks
– Theory was doing well until we found compensatory
CRs
Pavlov: Stimulus Substitution Theory
– Criticisms and Flaws:
• CR is almost never an exact replica of the UR
• an eyeblink to UR of air puff = large, rapid closure
• eyeblink to CS of tone = smaller, more gradual closure
• Defense of theory: Hilgard (1936): Why
differences in CR and UR:
– intensity and stimulus modality of the CS and US are
different
– Thus: differences in Response magnitude and timing
are to be expected
– But still doesn’t explain OPPOSITE CR
Pavlov: Stimulus Substitution Theory
• BIGGER PROBLEM:
– whereas many US's elicit several different R's, as a general rule not all of these
R's are later elicited by the CS
• E.g. Zener (1937)
– dog presented w/food as US:
• found that the dog elicited a number of UR responses to the food
• E.g., salivation, chewing, swallowing, etc.
– CS not elicit all of those responses
• NO CRs of chewing and swallowing
• Just the CR of just salivation
• on other hand: CR may contain some of responses that are not part of CR:
– Zener found that dogs turned head to bell
– But no head turns to presentation of food
Modifications of SST
• MODIFICATIONS OF SST: (Hilgard)
– only some components of UR transferred to CR
– CS such as a bell often elicits unconditioned responses of its own, and
these may become part of CR
• SIGN TRACKING: Hearst and Jenkins 1974
– emphasized this change in form of CR vs. UR
– Also Jenkins, Barrara, Ireland and Woodside (1976)
• Sign Tracking : animals tend to
– orient themselves toward
– approach
– explore any stimuli that are good predictors of important events such
as the delivery of food
1
4
2
Set up:
1. Initial training: Light
turns on above
feederfeeder releases
pieces of hot dog
2. Test:
a. Light turns on above
feeder, then above
each of the other
walls
b. Forms a sequence
of 1234
3. What is optimal response?
3
Jenkins, Barrara, Ireland and Woodside (1976)
4. But: Dog “tracked the
sign”
Modifications of SST
• Strongest data against SST theory: Paradoxical
conditioning
– CR in opposite direction of UR
• Black (1965):
– heart rate decreases to CS paired w/shock
– US of shock elicits UR of heart rate INCREASE
– But CS of light or tone elicits CR of heart rate DECREASE
• Seigel (1979): conditioned compensatory responses
–
–
–
–
Morphine studies
evidence of down regulation in addiction
Actual cellular process in neurons (and other cells, too!)
thus SST theory appears incorrect
Perceptual Gating Theory
• Perceptual gating theory:
– Idea that only if CS is biologically relevant will it get
processed
– If a CS doesn’t get processed it can be predictive/informative
– Animals attend to biologically relevant stimuli
• Problem:
– Data show that under certain circumstances a stimulus is
“attended to” or “processed”, but still does not serve as a CS
with an accompanying CR
– Issue remains: is the stimulus the most predictive?
– Second issue: Defining “biologically relevant”
Kamin’s work: 1967-1974
Blocking and overshadowing
• Overshadowing:
–
–
–
–
use one "weak" and one "strong" CS
CS1+CS2US
reaction to weaker stimulus is blotted out by stronger CS
Demonstrated by Pavlov
• Blocking:
– Train 1 CS, then add a second CS to it:
• CS1 US
• CS1+CS2US
– test each individually after training
– Find that only one supports a CR
– One stimulus “blocks” learning to second CS
– Demonstrated by Kamin
Kamin’s blocking experiment
• used multiple CS's and 4 groups of rats
• the blocking group receives
– series of L+ trials which produce strong CR
– series of L+T+ trials
– then tested to just the T
• control groups receives
– SAME TOTAL NUMBER OF TRIALS AS BLOCKING GROUP
– no first phase
– L+ only; Test T
– T+ only; Test T
– LT+ only: Test T
Kamin’s blocking experiment
• prediction: since both received same # of trials to the tone- should
get equal conditioning to the tone
• results quite different: Blocking group shows no CR to the tone- the
prior conditioning to the light "blocked" any more conditioning to
the tone
• directly contradicts frequency principle (remember associationism!)
Group
Control
Control
Control
Blocking
Phase I
------L+
Phase II
L+
T+
LT+
LT+
Test Phase Result
T
T elicits no CR
T
T elicits CR
T
T elicits a CR
T
T elicits no CR
Things we know about blocking:
•
the animal does "detect" the stimulus:
– can’t be perceptual gating issue
– EXT of CR with either T alone or with LT
– EXT occurred faster with compound LT
• appears to be independent of:
– length of presentation of the CS
– number of trials of conditioning to compound CS
• constancy of US from phase 1 to 2 important!!!!
– US must remain identical between the two phases or no blocking
• influenced by:
– Type of CR measure (used CER, not as stable as non fear CR)
– nature of CS may be important- e.g. modality
– intensity of CS or US stimuli important
• depends on amount of conditioning to blocking stimulus which already
occurred
Change in either US or CS can
prevent/ overcome blocking
• Change the intensity of the CS from phase 1 to phase 2
–
–
–
–
–
Overshadowing could be playing a role
strong vs weak stimulus
e.g. experiments when changed from 1 ma to 4 ma shock
quickly condition to compound stimulus
little or no overshadowing or blocking
• Change in intensity of either CS stimulus– Change in context from Phase 1 to Phase 2
• lT
• Lt
then T
then T
– presents a different learning situation and no blocking
• Any ideas about what is happening?
Explanations of Blocking:
•
Poor Explanation: Perceptual gating theory:
– tone never gets processed
– tone not informative
– data not really support this (evidence that do “hear” tone)
•
Good Explanation: Kamin's Surprise theory:
–
–
–
–
–
•
to condition requires some mental work on part of animal
animal only does mental work when surprised
bio genetic advantage: prevents having to carry around excess mental baggage
thus only learn with "surprise"
situation must be different from original learning situation
Better Explanation: Rescorla Wagner model:
– particular US only supports a certain amount of conditioning
– if one CS “hogs” all that conditioning- none is left over for another CS to be added
– question- how do we show this?
Recorla: Which is more important?
CS-US correlation vs. contiguity
• CS-US contiguity:
– CS and US are next to one another in time/space
– In most cases, CS and US are continguous
• CS-US correlation: CS followed by the US in a
predictive correlation:
• If perfect correlation (most predictive)- most conditioning
• p(US/CS) = 1.0
• p(US/no CS) = 0.0
• But: life not always a perfect correlation
CS-US correlation is more critical
• Rescorla (1966, 1968): Showed how 2 probabilities
interact to determine size of the CS
– CS = 2 min tone; presented at random intervals (M = 8 min)
– for: Group 1: p(shock/CS) = 0.4 during 2 min presentation
– For Group 2: p(shock/no CS) = 0.2
• Which group should show more conditioning?
• WHY?
Robert Rescorla (1966)
Examined predictability 6 types of Groups
• CS-alone
– present CS alone with no US pairing
– problem: not have same number of US trials as
experimental animals do, may actually be extinction
effect
• Novel CS group:
– looks at whether stimulus is truly "neutral"
– may produce habituation- animal doesn't respond
because it "gets used to it"
• US-alone
– present US alone with no CS pairing
– problem: not have same number of CS trials
Rescorla: 6 types of control groups
• Explicitly unpaired control
– CS NEVER predicts US
– that is- presence of CS is really CS-, predicts NO US
– animal learns new rule: if CS, then no US
• Backward conditioning:
– US precedes CS
– assumes temporal order is important (but not able to
explain why)
– again, animal learns that CS predicts no US
• Discrimination conditioning (CS+ vs CS-)
– use one CS as a plus; one CS as a minus
– same problem as explicitly unpaired and backwardworks, but can work in certain circumstances (taste
avoidance)
Rescorla: Results with 6 Groups
• CS-alone: No conditioning, but habituation to CS
• Novel CS group: novel worked better than CS with previous experience.
• US-alone: habituation to CS
• Explicitly unpaired control:
– Got GREAT conditioning
– Learned that the CS NEVER predicts the US!
• Backward conditioning:
– US preceded CS
– assumed temporal order is important
– It was: Animal learned that CS predicts NO US, but US predicted CS
• Discrimination conditioning (CS+ vs CS-)
– use one CS as a plus; one CS as a minus
– Got discrimination
– Animals paid attention to whatever stimulus was MOST PREDICTIVE!
CS-US correlation: Summary of Results
• whenever p(US/CS) > p(US/NO cs):
– CS = EXCITATORY CS
– that is, CS predicts US
– amount of learning depended on size difference between p(US/CS) and
p(US/no CS)
• whenever p(US/CS) <p(US/NO CS):
– CS = INHIBITORY CS
– CS predicts ABSENCE of US
– amount of learning depended on size difference between p(US/CS) and
p(US/no CS)
• whenever p(US/CS) = p(US/NO cs):
– CS = NEUTRAL CS
– CS doesn’t predict or not predict CS
– no learning will occur because there is no predictability.
CS-US correlation vs. contiguity
• Thus: appears to be the CORRELATION
between the CS and US, not the contiguity
(closeness in time) that is important
• Can write this more succinctly:
– correlation carries more information
– if r = + then excitatory CS
– if r = - then inhibitory CS
– if r = 0 then neutral CS (not really even a CS)
Classical condition is “cognitive”
(oh the horror of that statement, I am in pain)
• PREDICTABILITY is critical
• Learning occurs slowly, trial by trial
– Each time the CS predicts the US, the strength of the correlation is
increased
– The resulting learning curve is monotonically increasing:
• Initial steep curve
• Levels off as reaches asymptote
– There is an asymptote to conditioning to the CS:
• Maximum amount of learning that can occur
• Maximum amount of responding that can occur to CS in anticipation of the
upcoming US
• We can explain this through an equation!
Answers to
Blocking and Overshadowing
• Overshadowing:
– use one "weak" and one "strong" CS
– reaction to weaker stimulus: less CR
– Reaction to stronger stronger stimulus: more CR
• Blocking:
– What is being predicted
– Does LT give any more information/predictability than
L alone?
– If not, then L “blocks” learning to LT
Assumptions of
Rescorla-Wagner (1974) model
• Model developed to accurately predict and map learning as it occurs trial
by trial
• Assumes a bunch of givens:
– Assume animal can perceive CS and US, and can exhibit UR and CR
– Helpful for the animal to know 2 things about conditioning:
• what TYPE of event is coming
• the SIZE of the upcoming event
• Thus, classical conditioning is really learning about:
– signals (CS's) which are PREDICTORS for
– important events (US's)
Assumptions of R-W model
• assumes that with each CS-US pairing 1 of 3 things can happen:
– the CS might become more INHIBITORY
– the CS might become more EXCITATORY
– there is no change in the CS
• how do these 3 rules work?
– if US is larger than expected: CS = excitatory
– if US is smaller than expected: CS= inhibitory
– if US = expectations: No change in CS
• The effect of reinforcers or nonreinforcers on the change of associative
strength depends upon:
– the existing associative strength of THAT CS
– AND on the associative strength of other stimuli concurrently present
More assumptions
• Explanation of how an animal anticipates what type of CS is coming:
– direct link is assumed between "CS center" and "US center":
• e.g. between a tone center and food center
• In 1970’s: other researchers thought R and W were crazy with this idea
• Now: neuroscience shows formation of neural circuits!
– assumes that STRENGTH of an event is given
• the conditioning situation is predicted by the strength of the learned
connection
– THUS: when learning is complete:
• the strength of the association relates directly to the size or intensity of the CS
• Asymptote of learning = max learning that can occur to that size or intensity of
a CS
• Maximum amount of learning that a given CS can support
More assumptions
• The change in associative strength of a CS as the result of
any given trial can be predicted from the composite
strength resulting from all stimuli presented on that trial:
– Composite strength = summation of conditioning that
occurs to all stimuli present during a conditioning trial
– if composite strength is LOW:
• the ability of reinforcer to produce increments in the strength of
component stimuli is HIGH
• More can be learned for this trial
– if the composite strength is HIGH:
• reinforcement is relatively less effective (LOW)
• Less can be learned for this trial- approaching max of learning
More assumptions:
• Can expand to extinction, or nonreinforced trials:
– if composite associative strength of a stimulus
compound is high, then the degree to which a
nonreinforced presentation will produce a
decrease in associative strength of the
components is LARGE
– if composite associative strength is lownonreinforcement effects reduced
The Equation!:
•
Yields an equation: THE Rescorla Wagner (1974) model!!!!!
Vi =αißj(Λj-Vsum)
•
Vi = amount learned (conditioned) on a given trial
•
Αi = the salience of the CS
•
ßj = the salience of the US
•
(Λj-Vsum) = total amount of conditioning that can occur to a particular CS-US pairing
•
•
What does this equation say?
The amount of conditioning that will occur on a given trial is a function of:
• The size of the salience of the CS multiplied by
• The size of the salience of the US multiplied by
• (The maximum amount of learning minus the amount of learning that has already occurred).
Let’s use this in an example:
First example:
•
A rat is subjected to conditioned suppression procedure:
–
CS (light) ---> US (1 mA shock)
–
Question: what is associative strength?
–
1 = associative strength that a 1mA shock can support at asymptote ( λ j )
•
•
–
(I am arbitrarily setting this value for easy math)
So, we will say that the associative strength of a 1 mA shock = 100 units of association/learning
VL = associative strength of the light (strength of the CS-US association)
•
thus: λ 1 = animal’s maximum reaction to the size of the observed event (actual shock)
•
VL = measure of the Subjects current "expectation" about the light predicting the light.
•
VL will approach λ 1 over course of conditioning: VL = λ 1
First trial: CSL USshock
• CS (light+tone) --> 1 mA shock on trial 1 (no previous pairing)
– Λj = max amount of conditioning that can occur to the
CSL : Let’s set it at 100
– Vsum = assoc. strength of all paired trials so far (0)
– Can set αi = 0.5
– Can set ßj = 1.0
–
VL = αißj(Λj-Vsum) just plug in numbers
– VL = 0.5*1.0(100-0) = 50 units of conditioning/learning
Second example: 2CS's:
• CS (light+tone) --> 1 mA shock
– Vsum = VL + VT = assoc. strength of the 2 CS's
– (still 0 on trial 1)
– Vsum = αißj(λn)
– if VL and VT equally salient:
• VL = 0.5αißj;
• VT = 0.5αißj
– VT = 0.5*0.5*(100-0) = 25 units of learning
WHY is this equation important?
• We can use the three rules to make predictions about amount
and direction of classical conditioning
• λ j > Vsum = excitatory conditioning
– The degree to which the CS predicted the size of the US was GREATER
than expected, so you react MORE to the CS next trial
• λ j < Vsum = inhibitory conditioning
– The degree to which the CS predicted the size of the US was LESS than
expected, so you react LESS to the CS next trial
• λ j = Vsum = no change:
– The CS predicted the size of the US exactly as you expected
Now have the Rescorla-Wagner Model:
• Model makes predictions on a trial by trial
basis
• for each trial: predicts increase or decrement
in associative strength for every CS present
• Can specify amount and direction of the
change in conditioning!
Now have the Rescorla-Wagner Model:
• Restate the equation: Vi =αißj(λ j -Vsum)
• Vi = change in associative strength that occurs for any CS, i,
on a single trial
• λ j= associative strength that some US, j, can support at
asymptote
• Vsum = associative strength of the sum of the CS's (strength of
CS-US pairing)
• αi = measure of salience of the CS (must have value between
0 and 1)
• ßj = learning rate parameters associated with the US
(assumes that different beta values may depend upon the
particular US employed)
Can say this easier!
• How much you will learn on a given trial (Vi) is a
function of:
– αi or how good a stimulus the CS is (how well it grabs
your attention)
– ßj or how good a stimulus the US is (how well it grabs
your attention
– Λj or how much can learning can be learned about the
CS-US relationship
– AND Vsum or how much you have learned ALREADY!
Okay, you got all that?
Let’s put this baby to work……..
…….we will try a few examples
The equation: Vi =αißj(λ j-Vsum)
•
Vi = change in associative strength that occurs for any CS, i, on a single trial
•
αi = stimulus salience (assumes that different stimuli may acquire associative
strength at different rates, despite equal reinforcement)
•
ßj = learning rate parameters associated with the US (assumes that different beta
values may depend upon the particular US employed)
•
Vsum = associative strength of the sum of the CS's (strength of CS-US pairing)
•
λ j= associative strength that some CS, i, can support at asymptote
•
In English: How much you learn on a given trial is a function of the value of the
stimulus x value of the reinforcer x (the absolute amount you can learn minus the
amount you have already learned).
Acquisition
•
first conditioning trial: Assume (our givens)
– CS = light; US= 1 ma Shock
– Vsum = Vl; no trials so Vl = 0
– thus: λ j-Vsum = 100-0 = 100
– -first trial must be EXCITATORY
• BUT: must consider the salience of the light:
– αi = 1.0
– ßj = 0.5
Acquisition
•
first conditioning trial: CS = light; US= 1 ma Shock
– Vsum = Vl; no trials so Vl = 0
– thus: λ j-Vsum = 100-0 = 100
– -first trial must be EXCITATORY
• BUT: must consider the salience of the light: αi = 1.0 and
learning rate: ßj = 0.5
• Plug into the equation: for TRIAL 1
– VL = (1.0)(0.)(100-0) = 0.5(100) = 50
– thus: V only equals 50% of the discrepancy between Aj an
Vsum for the first trial
Acquisition
• Plug into the equation:
–for TRIAL 1
–VL = (1.0)(0.)(100-0) = 0.5(100) = 50
–thus: VL only approaches 50% of
the discrepancy between Aj and
Vsum is learned for the first trial
Acquisition
• TRIAL 2:
– Same assumptions!
– VL = (1.0)(0.5)(100-50) = 0.5(50) = 25
– Vsum = (50+25) = 75
Acquisition
• TRIAL 3:
– VL = (1.0)(0.5)(100-75) = 0.5(25) = 12.5
– Vsum = (50+25+12.5) = 87.5
Acquisition
• TRIAL 4:
– VL = (1.0)(0.5)(100-87.5) = 0.5(12.5) = 6.25
– Vsum = (50+25+12.5+6.25) = 93.75
• TRIAL 10: Vsum = 99.81, etc., until reach ~100 on approx. trial
14
• When will you reach asymptote?
R-W explains 1 CS learning
100
Amt of learning
80
60
40
20
0
0
2
4
6
8
Trials
learning to Vlight
Total amount learned (Vsum)
10
12
How to explain overshadowing?
Yep, it is good old Rescorla-Wagner
to the rescue!
Remember Overshadowing
• Pavlov: compound CS with 1 intense CS, 1 weak
– after a number of trials found: strong CS elicits
strong CR
– Weak CS elicits weak or no CR
• Note: BOTH CSs are presented at same time
– Why would one over shadow or overpower the
other?
– Why did animal not attend equally to both?
Overshadowing
• Rescorla-Wagner model helps to explain why:
• Assume
– αL = light = 0.2; αT = tone = 0.5
– ßL = light = 1.0 ; ßt = tone = 1.0
• Plug into equation:
– Vsum = Vl + Vt = 0 on trial 1
– VL = 0.2(1)(100-0) = 20
– Vt = 0.5(1)(100-0) = 50
– after trial 1: Vsum = 70
Overshadowing
•
TRIAL 2:
– VL = 0.2(1)(100-(50+20)) = 6
– Vt = 0.5(1)(100-(50+20)) = 15
– Vsum = (70+(6+15)) = 91
• TRIAL 3:
–
–
–
–
VL = 0.2(1)(100-(91)) = 1.8
Vt = 0.5(1)(100-(91)) = 4.5
Vsum = (91+(1.8+4.5)) = 97.3 and so on
thus: reaches asymptote (by trial 6) MUCH faster w/2 CS's
• NOTE: CSt takes up over 70 units of assoc. strength CSl takes up only
30 units of assoc. strength
Overshadowing
R-W explains 2 CS learning
120
100
Amt of learning
80
60
40
20
0
0.0
0.5
1.0
1.5
Trials
Vsum for light
V sum for tone
V sum total
2.0
2.5
3.0
3.5
Blocking
• Similar explanation to overshadowing:
– Does not matter whether VL has more or
less saliency than Vt,
– CS has basically absorbed all the associative
strength that the CS can support
• Why?
Blocking
• give trials of A-alone to asymptote:
– reach asymptote: VL = λ j =100 =Vsum
•
NOW add trials to compound stimuli:
– CS of the light has salience: αL =.5465
– CS of tone has salience of: ßt =0.464
– Note that CStone has higher salience!
– Eh, oh, the math is going to be TOO HARD to
do!!!!!
Blocking
• Or IS the math to hard to do?
• First compound V1 Trial:
• Vt= αß(Λj-Vsum)
• What is Vsum after the training to the CS light?
• That’s right Vsum = ___________
• Vt=0.*1.0*(100-100)= _____________
• No learning!
How could one eliminate blocking effect?
• increase the intensity of the US to 2 mA
with λ j now equals = 160
– Learning so far: Vsum still equals 100
(learned to 1 mA shock)
– But now: TOTAL learning is increased to
160!
How could one eliminate blocking effect?
• plug into the equation:
• (assume Vl and Vt equally salient)
– Vt = 0.2(1)(160-100) = 0.2(60) = 12
– Vl = 0.2(1)(160-100) = 0.2(60) = 12
– Vsum = 100+12+12 =124
How could one eliminate blocking effect?
• on trial 2:
– Vsum = 124
– Vt = 0.2(1)(160-124) = 0.2(36) = 7.2
– Vl = 0.2(1)(160-124) = 0.2(36) = 7.2
– Vsum now = (124+14.4) = 138.
– Again, monotonically increasing curve.
• Thus, altering the salience of the US alters the learning
• Does altering the CS make the same change?
Can also explain why probability of reward given
CS vs no CS makes a difference:
•
π = probability of US given the CS or No US given No CS
•
can make up three rules:
– if πax > πa then Vx should be POSITIVE
– if πax < πa then Vx should be NEGATIVE
– if πax = πa then Vx should be ZERO
•
modified formula: (assume λ1 =1.0; λ 2 =0; ß1 =.10; ß2=.05; α1=.10; α2=.5)
•
Πa = probability of reward.
Explaining loss of associate value
despite pairings with the US:
• R-W model makes a unique prediction:
Conditioned properties of stimuli can
DECREASE despite continued pairings with the
US
• Lose associative value if presented together
on conditioning trial after they have been
trained separately
Explaining loss of associate value
Phase 1
A
I pellet
Phase 2
Phase 3 TEST
A
A
I pellet
B
I pellet
B
A
• At the end of Phase 1: VAand VB= Ʌ; both equally and perfectly predict 1
pellet
• Phase 2: Compound stimuli with same US
• No change in US
• Should VAand VB remain unchanged?
• But animal interprets differently: VAand VB=2 Ʌ
• Animal is surprised (disappointed): get suppression to A and B in Phase 3
Conditioned Inhibition
• Two kinds of trials:
– CS+: CS predicts US
– CS+ and CS-: predicts NO US
• Must consider CS+ and CS+ & CS- trials separately:
– CS+: pairs CS+  US, V+ approaches Ʌ
• Excitatory conditioning ceases as V+ approaches Ʌ
– On Non reinforced trials: CS+ and CS•
•
•
•
No excitatory conditioning to CS+, but disappointment
BUT: inhibitory conditioning to CSValue of CS+ + CS- must sum to 0 to get inhibition
CS- value is then NEGATIVE: CS+ - CS- = 0
Extinction of excitation and inhibition
• V for CS+ has reached Ʌ
– Now begin presenting CS+ without US
• CS+ begins to lose its excitatory value
– V for CS+ will approach 0
Critique of the Rescorla-Wagner Model:
• R-W model really a theory about the US
effectiveness:
– says nothing about CS effectiveness
• How WELL a CS predicts as a combo of salience and
probability
– states that an unpredicted US is effective in
promoting learning, whereas a well-predicted US is
ineffective
• Reason has to do with brain processing of all of
this
Critique of the Rescorla-Wagner Model:
• Fails to predict the CS-pre-exposure effect:
– two groups of subjects (probably rats)
– Grp I
CS-US pairings
Control
– Grp II CS alone
CS-US pairings PRE-Expos
• Bob and Tom effect
– Bob always hangs with Tom
– You are dating Tom
– You have a BAAAAAD breakup with Tom
– Now you hate Bob….why?
Critique of the Rescorla-Wagner Model:
• In pre-exposure effect, simply being around a neutral stimulus
alters its ability to become conditioned
• Original R-W model doesn't predict any difference,
– Assumes no conditioning trials occur when CSs presented
in absence of US so Vsum = 0
– This appears to be wrong
• Conditioning likely occurring any time 2 stimuli are together
– Form an incidental association
– Need to modify the equation to account for this
– They have, but we won’t!
Critique of the Rescorla-Wagner Model:
• Original R-W model implies that salience is fixed for any given CS
– R-W assume CS salience doesn't change w/experience
– these data strongly suggest CS salience DOES change w/experience
• Newer data supports changes salience
– data suggest that Salience to a CS DECREASES when CS is repeatedly
presented without consequence
– CS that is accidentally paired with another CS INCREASES in salience
– NOW: appears that CS and US effectiveness are both highly important
• Model has stood test of time, now widely used in neuroscience
• Given birth to attentional models of CC
Attentional Models of CC
• Alternative focus: how well the CS commands attention
– Assumes that increased attention facilitates learning about
a stimulus
– Procedures that disrupt attention to CS disrupt learning
• Different attentional models differ in assumptions
about what determines how much attention a CS
commands on any given trial
– Single attentional mechanisms: Kamin’s surprise
– Multiple attentional mechanisms:
Multiple attentional mechanisms:
• Three attentions:
– looking for action: attention a CS commands after it has become
a good predictor of the CS
– Looking for learning: how well the organism processes cues that
are not yet good predictors of the US, and thus have to be
“learned about”
– Looking for liking: the emotional/affective properties of the CS
• Assume that the outcome of a given trial alters the degree
of attention commanded by the CS on future trials
– Surprise? Then an increase in looking for learning on next trial
– Pleasant outcome? Increases emotional value of CS on next trial
Timing and Information Theory
Models
• Recognized that time is important factor in CC
– Focal search responses become conditioned when
CS-US interval is short
– General search responses become conditioned
when CS-US interval is long
– Suggests that organisms learn both
• What is predicted
• WHEN what is predicted will occur
Temporal coding hypothesis
• Organisms learn when the US occurs in relation
to the CS
• Use this information in blocking, second-order
conditioning, etc.
• What is learned in one phase of training
influences what is learned in subsequent phase
• Large literature supports this
Importance of Inter-trial interval
•
More conditioned responding observed with longer inter-trial interval
– Intertrial interval and CS duration (CS-US interval) act in combination to determine responding
– Critical factor: relative druation of these two temporal intervals rather than absolute value of
either one by itself
•
Holland (2000)
– Conditioned rats to an autidory cue that was presented just before delivery to food
– CR to CS: nosing of food cup (goal tracking)
– Each group conditioned with
•
•
1 of 2 CS durations: 10 or 20 sec
1 of 6 intertrial intervals: 15 to 960 sec
– Characterized responses in terms of ratio of the intertrial interval (I) and the CS duration (T).
•
Time spent nosing the food cup during CS plotted as function of relative value of I/T
– Results: as IT ratio increases, the percentage of time the rats spend with the nose in the food
cup increases
Importance of Inter-trial interval
• Relative Waiting Time Hypothesis
– Organism making comparison between events during the I and T
– How long one has to wait for the US during the CS vs. how long one
has to wait for the US during the intertrial interval
• When US waiting time during CS is shorter than intertrial interval:
– I/T ratio is high and CS is highly informative about the next occurrence
of the US
– Lots of responding
• When US waiting time during CS is same or longer than intertrial
interval wait:
– I/T ratio is low, CS is not highly informative
– Less responding
Comparator Hypothesis
• Comparator hypothesis assumes that animal compares what
happens in one situation to what happens in another: animal
COMPARES expectations across settings
• Revaluation effects: e.g. in blocking
– Not that can’t learn to second CS, but that responding is blocked to
CS2
– Can get responding to CS2 by presenting alone, with out the US!
• Anytime there is a change in the predictive value of a CS the
organism will re-evaluate its value
• Result is a disruption in responding to the changed CS
Comparator Hypothesis
• Note that this model is a PERFORMANCE model
– It is not what is learned, but what is performed that is
critical
– Organism compares cues that may occur in various
settings and alters responding depending on value of
the cues in a given setting
• Not changing excitatory value of the US, but
comparing the value of the predictive CSs for that
US.
Comparator Models
Model assumes organism learns three associations during course of conditioning:
1. Association between target CS and US
2. Association between the target C S and the comparator cues
3. Association between comparator stimuli and the US
Comparison between the direct and indirect activations determines the degree of excitatory
or inhibitory responding
Comparator model predictions
• The comparison between the CS-US and the comparator-US associations
at testing are important:
• Allows prediction that extinction of comparator-US associations following
training of a target CS will enhance responding to that CS
• Thus, in blocking, extinction of CSA will unmask conditioned responding to
CSB
• Not that responding to CSB was blocked, but that it was masked because,
– when comparing CSA to CSA+CSB, the compound CSs provided no increased
predictability
– Only when lessen predictiveness of CSA does CSB become “important”
• Organism responds to the BEST predictor under the circumstances!
Dopamine and
Rescorla Wagner Model
• Turns out that changes in dopamine (DA) levels in dorsal striatal limbic
cortical pathway vary as we learn
• And guess what: these levels can be predicted by the RW model!
• But, once a CS-US pairing (or an operant R-SR pairing) become well
learned, the circuit begins to involve lower parts of the brain
– Circuit begins to involve basal striatal areas
– Becomes an “automated” or mastered behavior
– No longer involves being “surprised”; is the most robust predictor amongst
comparitors
• A response to another CS will occur along the DA pathway if the CS-US
relation change!!!!!
– Change in the conditional value of a CS

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