Evidence for the hierarchical structure of instrumental learning

In three experiments using rats, we examined the role of a discriminative stimulus (S) in governing the relation between a response (R) and an outcome (O) in an appetitive instrumental learning paradigm. In each experiment, we attempted to distinguish between a simple S-O association and a hierarchical relation in which S is associated with the R-O association. We used three variations on discriminative training procedures and three different assessment techniques-for revealing the hierarchical structure. In Experiment 1, we employed a training procedure in which S signaled a change in the R-O relation but no change in the likelihood of O. Although such an arrangement should not produce an excitatory S-O association, it nevertheless generated an S that controlled responding and transferred that control to other responses. In Experiment 2, we used a discrimination procedure in which two Ss each had the same two Rs and Os occur in their presence but each S signaled that a different R-O combination would be in effect. This design provided the opportunity for equivalent pairwise associations among S, R, and O but unique hierarchical relations. The subjects learned the hierarchical structure, as revealed by the specific depressive effect of a subsequent lithium-chloride-induced devaluation of O on responding only in the presence of the S in which that response had led to that outcome. In Experiment 3, one S signaled two different R-O outcomes. Then, two new stimuli were presented with the original S; the R-O relations were retained in the presence of one of the added stimuli but were rearranged in the presence of the other. The added S came to control less responding when it was redundant with respect to the R-O relations than when it was informative. Although all of the results were of modest size and each has an alternative interpretation, together they provide converging evidence for the hierarchical role of S in controlling an R-O association.