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Rescorla-Wagner Model

Rescorla-Wagner Model

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Rescorla-Wagner Model

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  1. Rescorla-Wagner Model • US-processing model • Can account for some Pavlovian Conditioning phenomena: • acquisition • blocking • unblocking with an upshift • conditioned inhibition • US-pre-exposure effect • Cannot account for some Pavlovian Conditioning phenomena: • extinction (i.e., spontaneous recovery) • unblocking with a downshift • latent inhibition • temporal factors (i.e., CS-US interval)

  2. Pearce-Hall Model • attention model of conditioning • a CS-processing model • according to the model, it is highly adaptive to pay pay attention to, or process, CSs that could become valid predictors of important outcomes (i.e., USs) • it is also adaptive not to pay attention to, or process, CSs when the important event is already predicted by something else

  3. Pearce-Hall Model • also based on the concept of surprise • when the subject is surprised, attention to, or processing of the CS occurs • as the US becomes predicted by a CS, and is less surprising, processing of the CS declines • The amount of processing, that is associability of a CS, changes on each trial depending on whether the US was predicted (on the previous trial) • If the US was predicted, then attention to the CS declines • If the US was not predicted, then attention to the CS increases

  4. Pearce-Hall Model ΔVA = k(λ – VT) Recall from the RW Model, k = constant; salience or associability of the CS With the PH Model, k changes across trials (CS processing model, not a US processing model)

  5. Pearce-Hall Model kAN = λN-1 – VAN-1 kAN = associative strength or associability of CSA on trial N λN-1= strength of the US on previous trial VAN-1 = strength of CSA on previous trial (could become VT if more than one CS) Important point: k depends on what happened on the previous trial; on first exposure, novelty causes some attention

  6. Pearce-Hall Model kAN = λN-1 – VAN-1 Early in training, when the strength of the CS is low (i.e., λ – V is high) see high k value and thus, more attention to the CS When the CS is strong in later trials (i.e., λ – V is small) attention to the CS is low The important point is that attention to the CS changes across trials

  7. Pearce-Hall Model Attention to, or processing of, the CS can be measured in terms of an OR (i.e., looking at a L) This is different than the CR Support for the PH Model comes from the finding that subjects orient towards novel stimuli and maintain their orientation, provided the stimulus is a poor predictor of the US

  8. Kaye & Pearce compared the OR in 3 groups of rats Group 1: L alone Group 2: L condensed milk Group 3: L milk/no milk (inconsistent/random) Looked at OR to L Attention (OR) was high on the first trial since the L is novel

  9. kAN = λN-1 – VAN-1 Group 1: L alone k stays low (decrease attention) Group 2: L milk VA gets bigger over time which makes the total term smaller (this means small k and decrease in attention) Group 3: L milk/no milk Attention remains high since VA is low

  10. When the CS is not a good predictor, rats maintained their attention to the cue If the CS is a good predictor (of the US or no US), then attention decreases

  11. Pearce-Hall Model and Blocking • like the RW Model, all CSs combine to predict the US • if one CS already predicts the US, then pay less attention to all CSs on that trial • when a new CS is added, should pay attention to it because it is novel • therefore, should see some conditioning to the new cue on the first trial based on the salience of the CS

  12. Pearce-Hall Model and Blocking • only after first trial is over would the animal know that nothing new had happened • according to the model, should see blocking from trial 2 and onwards • however, in most cases see blocking right from the start

  13. Pearce-Hall Model and Unblocking kAN = λN-1 – VAN-1 • when subjects encounter a US that is not well predicted, or is surprising (either bigger or smaller), then subjects should pay attention to all CSs on that trial and get unblocking • because the formula includes the absolute value of λN-1 – VAN-1 it doesn’t matter if the US is bigger or smaller • if the US changes we’ll see increase in attention and thus, learning

  14. Pearce-Hall Model and latent inhibition When the CS is given by itself, see decrease in attention to the CS over trials (λ = 0) However, a problem with the model is that it cannot explain the context-specificity of LI If CS pre-exposures are given in one context, and conditioning occurs in a second context, there is no retardation of learning According to the model, k should be low regardless of context

  15. The Comparator Hypothesis • developed by Ralph Miller • this is a model of performance, not learning • according to Miller, all CSs have excitatory power; there is no separate inhibitory process • the strength of performance (or CR) depends on the relative strength of the various excitatory associations • a subject compares the excitatory strength of the explicit CS to the strength of other cues present in the situation, such as apparatus cues

  16. The Comparator Hypothesis • when the strength of a CS is relatively greater than the background cues, get a measurable CR • when the strength of a CS is weaker than the background cues, get weakened level of excitation (what others might call inhibition) • according to the theory, the competition between two excitatory reactions controls performance

  17. The Comparator Hypothesis • during normal excitatory, get CS-US pairings – but the US is also paired with background cues and these background cues are the comparator stimuli • because these background cues are also present during the ‘no-US’ condition, they are typically weaker than the explicit CS • so, under normal conditioning procedures, the CS has stronger excitatory strength than the comparator cues

  18. The Comparator Hypothesis • during inhibitory conditioning, the CS is weak relative to the background cues • during inhibitory conditioning, have CS – no US pairings; but the background cues are paired with the US and the absence of the US • thus, the CS is weaker than the background cues and see little CR to the CS

  19. The Comparator Hypothesis • Prediction – After training one can manipulate the excitatory value of the context and this will affect the excitatory value of the CS • E.g. – After conditioning, give repeated exposure to the context alone followed by exposure to CS • One will see greater responding to CS

  20. Temporal Factor Models • designed to explain the effects of time in conditioning • effects of time not considered in US-processing models like the RW model nor in CS-processing models like the PH model • CS-US interval is one important temporal variable • a more critical temporal variable appears to be the ratio of the ISI to ITI

  21. Midterm Exam Thursday, Feb. 17, 2005 • covers everything up to and including today’s lecture • in the case of a storm, the exam will take place during • the very next class