A model for Pavlovian learning: Variations in the effectiveness of conditioned but not of unconditioned stimuli.

Several formal models of excitatory classical conditioning are reviewed. It is suggested that a central problem for all of them is the explanation of cases in which learning does not occur in spite of the fact that the conditioned stimulus is a signal for the reinforcer. We propose a new model that deals with this problem by specifying that certain procedures cause a conditioned stimulus (CS) to lose effectiveness; in particular, we argue that a CS will lose associability when its consequences are accurately predicted. In contrast to other current models, the effectiveness of the reinforcer remains constant throughout conditioning. The second part of the article presents a reformulation of the nature of the learning produced by inhibitory-conditioning procedures and a discussion of the way in which such learning can be accommodated within the model outlined for excitatory learning. A recent review of classical conditioning (Dickinson Mackintosh, 1978) suggests that the underlying associative process is responsible for organisms learning about the relationships between events, enabling them to build up an associative representation of the causal structure of their environment (p. 588). According to this view the occurrence of a conditioned response (CR) as a result of the pairing of a conditioned stimulus (CS) and an unconditioned stimulus (US) is regarded as being merely an index of the formation of some internal representation of the relationship between CS and US. The first task of a theory of classical conditioning becomes that of This work was supported by grants from the United Kingdom Science and Medical Research Councils. We have made free use of many suggestions, especially concerning inhibitory learning, made by A. Dickinson. We thank him and also C. Adams, P. Bailey, S. Channel!, N. Mackintosh, and E. Macphail for their comments on earlier versions of this article. Requests for reprints should be sent to John Pearce,