The Rescorla–Wagner model is a model of classical conditioning, in which learning is conceptualized in terms of associations between conditioned and unconditioned stimuli. A strong CS-US association means, essentially, that the CS signals or predicts the US. One might say that before conditioning, the subject is surprised by the US, but after conditioning, the subject is no longer surprised, because the CS predicts the coming of the US. The model casts the conditioning processes into discrete trials, during which stimuli may be either present or absent. The strength of prediction of the US on a trial can be represented as the summed associative strengths of all CSs present during the trial. This feature of the model represented a major advance over previous models, and it allowed a straightforward explanation of important experimental phenomena, most notably the blocking effect. Failures of the model have led to modifications, alternative models, and many additional findings. The model has had some impact on neural science in recent years, as studies have suggested that the phasic activity of dopamine neurons in mesostriatal DA projections in the midbrain encodes for the type of prediction error detailed in the model. The Rescorla–Wagner model was created by Yale psychologists Robert A. Rescorla and Allan R. Wagner in 1972.
Basic assumptions of the model
The change in the association between a CS and a US that occurs when the two are paired depends on how strongly the US is predicted on that trial - that is, informally, how "surprised" the organism is by the US. The amount of this "surprise" depends on the summed associative strength of all cues present during that trial. In contrast, previous models derived the change in associative strength from the current value of the CS alone.
The associative strength of a CS is represented by a single number. The association is excitatory if the number is positive, inhibitory if it is negative.
The associative strength of a stimulus is expressed directly by the behavior it elicits/inhibits.
The salience of a CS and the strength of the US are constants and do not change during training.
Only the current associative strength of a cue determines its effect on behavior and the amount of learning it supports. It does not matter how that strength value was arrived at, whether by simple conditioning, reconditioning, or otherwise.
The first two assumptions were new in the Rescorla–Wagner model. The last three assumptions were present in previous models and are less crucial to the R-W model's novel predictions.
Equation
and where
is the change in the strength, on a single trial, of the association between the CS labelled "X" and the US
is the total associative strength of all stimuli present, that is, X plus any others
The revised RW model by Van Hamme and Wasserman (1994)
Van Hamme and Wasserman have extended the original Rescorla–Wagner model and introduced a new factor in their revised RW model in 1994: They suggested that not only conditioned stimuli physically present on a given trial can undergo changes in their associative strength, the associative value of a CS can also be altered by a within-compound-association with a CS present on that trial. A within-compound-association is established if two CSs are presented together during training. If one of the two component CSs is subsequently presented alone, then it is assumed to activate a representation of the other CS as well. Van Hamme and Wasserman propose that stimuli indirectly activated through within-compound-associations have a negative learning parameter—thus phenomena of retrospective reevaluation can be explained. Consider the following example, an experimental paradigm called "backward blocking," indicative of retrospective revaluation, where AB is the compound stimulus A+B:
Phase 1: AB–US
Phase 2: A–US
Test trials: Group 1, which received both Phase 1- and 2-trials, elicits a weaker conditioned response to B compared to the Control group, which only received Phase 1-trials. The original RW model cannot account for this effect. But the revised model can: In Phase 2, stimulus B is indirectly activated through within-compound-association with A. But instead of a positive learning parameter when physically present, during Phase 2, B has a negative learning parameter. Thus during the second phase, B's associative strength declines whereas A's value increases because of its positive learning parameter. Thus, the revised RW model can explain why the CR elicited by B after backward blocking training is weaker compared with AB-only conditioning.
Some failures of the RW model
;Spontaneous recovery from extinction and recovery from extinction caused by reminder treatments ;Extinction of a previously conditioned inhibitor ;Facilitated reacquisition after extinction ;The exclusiveness of excitation and inhibition ;Pairing a novel stimulus with a conditioned inhibitor ;CS-preexposure effect ;Higher-order conditioning ;Sensory preconditioning
Success and popularity
The Rescorla–Wagner model owes its success to several factors, including
