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Lecture 7

Thursday, Sept 27/2012 - Lecture 7 Notes

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University of Guelph
PSYC 2330
Francesco Leri

Thursday, September 25 2012 PSYC 2330 Lecture 8 Activity of dopaminergic neurons in the ventral tegmental area should increase when a: a) neutral stimulus such as a tone is presented before conditioning b) a US is presented after a CS c) US is not presented after a CS d) a CS is presented e) all of the above I guess (d) (Correct!) Blocking (Leon Kamin) • The blocking group receive extra conditioning compared to the control group • In phase 2, a tone is added to the light at the same time • The question is how much conditioning is generated to the tone that was added • Everyone is looking for meaningful predictors. Once you have an effective predictors, additional predictors are ignored because their predictive value is redundant • This experiment is important because it shows that: ◦ Conditioning is not an automatic result of CS-US pairings ◦ For conditioning to occur, the CS must be informative about the US in a novel was Blocking • When two CSs are used (a & b), the association or expectation at the beginning of a trial would be the sum of the strengths ◦ sum of Vab = Va + Vb • Therefore, the amount of conditioning on a compound trial in which a and b occur together would be ◦ deltaVa = deltaVb = alpha * beta (Vmax – Sum of Vab) • In the blocking group, if the Vmax for the light (L) = 1.0, then: ◦ VL = 1.0 at the end of phase 1 (because of extensive L conditioning) ◦ The animal is perfectly conditioned at the end of the conditioning • When the light and the tone (T) are presented in combination on trial 1 of phase 2 ◦ sumVLT = VL + VT = 1.0 + 0 = 1.0 • Therefor, the amount of conditioning to the T in the blocking group after 1 trial of conditioning with the LT compound is: ◦ deltaVT = alpha * beta (Vmax – sum of VLT) = 0.3(1.0 – 1.0) = 0 ▪ alpha * beta = 0.3 ◦ How much do the animals learn after adding the tone? ◦ They learn nothing new about the light (They already know all there is to know about the light) ◦ They learn nothing new about the tone (They are using the information about the light only) Overexpectation Effect • CS/US info ◦ Tone followed by shock ◦ Light followed by shock ◦ Tone + Light followed by shock ◦ What will be the strength of the reaction when the tone is played? • Assume that only few trials were given before the compound trial, and that Vmax = 1 and Beta = 0.3 ◦ VT = 0.2 and VL = 0.2, thus the sum of VTL = 0.4 ◦ DeltaVT = DeltaVL = 0.3(1.0 – 0.4) = 0.18 ◦ Animals learn 0.18 more about the tone and light when we pair them together ◦ The model predicts an increase in associative strength for both T and L when presented during the compound trial • But, if there was extensive conditioning before the compound trial such that: ◦ VT = 0.9 and VL = 0.9, thus the sum of VTL = 1.8 ◦ DeltaVT = DeltaVL = 0.3(1.0 – 1.8) = -0.24 ◦ Therefor, the model predicts a decrease associative strength for both T and L when presented during the compound trial Rescorla (1970) • Extensively trained rats • Tone followed by shock • Light followed by shock • Experimental group (E) -> tone + light followed by shock • Control group (C) -> nothing • Suppresion ratio: low numbers are higher fear and higher numbers are lower fear • Experimental group were less afraid of the light and were less afraid of the tone • The formula held • Why is there a difference in the fear of the light and fear? ◦ Animals are biologically prepared to (or predisposed) form CS-US pairings of certain types ◦ Arat is more predisposed to form an association between visual stimuli and painful stimuli than an association between auditory stimuli and painful stimuli ◦ If a behaviour is totally unnatural for an animal, it may never be able to learn what you are attempting to teach Implications for Random Contingency • No conditioning to the CS does not mean no conditioning at all • Even if you don't get conditioning to th CS, you still get contextual learning • Contextual stimuli and context should always be considered as an additional CS • We know how powerful context is from both experimental data and by personal experiences • The brain has special centers for encoding the context as a set of cues, which is different from the centers that are specialized for learning classical conditioning to a single stimuli Example of the Overexpectation Effect • Trial 1: CS + Context --------------------> US = + associative strength to compound • Trial 2: Context alone -------------------> US = + associative strength to context • Trial 20: CS + Context ------------------> US = -associative streng
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