Evidence for the hierarchical structure of instrumental learning

In threeexperimentsusing rats,we examinedthe role ofa discriminative stimulus(S) in govern-ing the relation between a response (R) and an outcome (0) in:anappetitive instrumental learn-ing paradigm. In each experiment, we attempted to distinguish between a simple S-O associa-tion and a hierarchical relation inwhich S is associated with~.theR—O association. We used three variations on discriminative training procedures and three dtfferent 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 0. Although such an arrangement should notproduce 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 presencebut each S signaled that a different R-O combination would be ineffect. This de-sign provided the opportunity for equivalent pairwise associations- among S, R, and 0 but unique hierarchical relations. The subjects learned the hierarchical structure, as revealedby the specific depressive effect of a subsequent lithium-chloride-induced devaluation of 0 on responding only in the presence of the S in which that response had led to that outcome. In Experiment 3, on