Discriminative stimulus control and the effects of concurrent operants

Discriminative-stimulus-control functions were investigated in a concurrent operant context. Variable-interval reinforcement schedules were arranged for pigeons on two response keys. One key, illuminated with a white vertical line on green background (position irrelevant), was programmed with a given schedule value across groups. For different groups of pigeons, the alternative key, illuminated with green alone, was programmed with twice, the same, or half the reinforcement frequency of the other key. Stimulus-control gradients were collected from both keys as line orientation was varied. On the green-plus-line alternative, flattest gradients were observed when twice the reinforcement frequency was concurrently programmed and the steepest were observed when equal values were concurrently programmed. Also examined were the effects of a programmed changeover delay, important in maintaining the independence of concurrent operants. The changeover delay was found to have relatively minor effects upon stimulus control, despite its typical and marked effects upon steady-state responding.