Optimality and concurrent variable-interval variable-ratio schedules

Despite claims to the contrary all leading theories about operant choice ma y be seen as models of optimality. Although melioration is often contrasted with global maximization, both make the same core assumptions as other Ver sions of optimality theory, including momentary maximizing, hill climbing, and the various versions of optimal foraging theory. The present experiment aimed to test melioration against more global optimality and to apply the visit-by-visit analysis suggested by foraging theory. Rats were exposed to concurrent schedules in which one alternative was always variable-ratio 10 and the other alternative was a variable-interval schedule. Although choice relations varied from rat to rat, the overall results roughly confirmed th e matching law a result often taken to support melioration. Pooling the dat a across sessions and across rats, however, resulted in no increment in uns ystematic variance, lending support to the contention by Ziriax and Silberb erg (1984) that the choice relation is partly constrained. When the data we re analyzed at the level of visits, the results either disconfirmed predict ions of melioration or showed regularities about which melioration is silen t. Instead, performance tended toward a rough optimization, in which respon ding favored the variable ratio, but with relatively brief visits to the va riable interval. There were no asymmetries in travel or variability that wo uld indicate that different processes were involved in generating visits at the two different schedules. The findings point toward a more global optim ality model than melioration and demonstrate the value of per-visit analysi s in the study of concurrent performances.