Concurrent variable-interval schedule performance: Fixed versus mixed reinforcer durations

Six pigeons were trained on concurrent variable-interval schedules. Two different reinforcer durations were arranged on the left key, and which of these was delivered was determined probabilistically. A single reinforcer duration was arranged on the right key. In Parts 1 and 3 of the experiment, the probability of the left-key reinforcer durations (1 and 7 seconds in Part 1; 3 and 10 seconds in Part 3) was varied from 0 to 1, keeping the schedules constant and the right-key reinforcer at 3 seconds. Response allocation to the left key fell as the probability of the shorter left-key reinforcer duration was increased. In Part 2, one left-key reinforcer duration was 3 seconds and the other was varied from 0 to 10 seconds, while again the schedules and right-key reinforcer duration (3 seconds) were kept constant. Left-key response allocation increased as the varied reinforcer duration on the left key increased. An extension of the generalized matching law failed to provide a good description of response allocation in these procedures. In Part 4, the left- and right-key reinforcer durations were 3 seconds and the variable-interval schedules were varied. Response allocation was well described by the generalized matching law. Part 5 arranged equal variable-interval schedules on the left and right keys, 3-second reinforcers on the right key, and the left-key reinforcer durations were varied from 1 to 10 seconds. The relationship between the log response and reinforcer-duration ratios was nonlinear. The effects of varying reinforcer durations in concurrent schedules cannot be described using the generalized matching law with constant parameters. It is suggested more generally that this quantitative relation may not describe performance either when reinforcing situations differ in duration (e.g., with delayed reinforcers) or when response requirements differ in duration.