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

Abstract

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 associ- ability 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-co nditioning 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 representa- tion of the causal structure of their environ- ment" (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 stimu- lus (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