WITHIN-SESSION CHANGES IN THE VI-RESPONSE FUNCTION - SEPARATING FOOD DENSITY FROM ELAPSED SESSION TIME

Previous studies examining the relationship between response rate and reinforcement rate on variable-interval schedules (the variable-interv al response function) have confounded elapsed session time with within -session changes in food density. The present experiments attempted to manipulate these factors independently and thus isolate their effects on responding. In Experiment 1, 7 rats pressed a bar for food on a se ries of four variable-interval schedules (7.5 s, 15 s, 30 s, and 480 s ). Elapsed session time was held constant while food density was manip ulated via a presession feeding. Changes in food density altered the f orm of the variable-interval response function, independently of elaps ed session time. In Experiment 2, 8 rats responded on the same series of variable-interval schedules as in Experiment 1, but food density wa s held constant and elapsed session time was manipulated via the use o f timeout periods. The results revealed no evidence for an effect of e lapsed session time independent of food density. The present results e xtend a recent analysis of the variable-interval response function by Dougan, Kuh, and Vink (1993) by identifying food density as an importa nt factor determining the form of the function. The present results al so help clarify the controversy over the correct empirical form of the variable-interval response function by further defining the variables responsible for differences in the form of that function.