Abstract
Pigeons were trained on concurrent schedules in which key pecking was required by both schedules (concurrent variable-interval variable-interval schedules) and on concurrent schedules in which key pecking was required by only one of the schedules (concurrent variable-interval variable-time schedules). The distribution of reinforcements was systematically varied with both types of concurrent schedules. The distribution of time between the schedules depended on the reinforcement distribution and was independent of the symmetry of the response requirement. The relation between time and reinforcement distributions appears to be invariant over a wide range of manipulations of responding maintained by concurrent schedules.