Delay of reinforcers in a concurrent-chain schedule: An extension of the hyperbolic-decay model.

Six pigeons were trained in concurrent-chain schedules with equal aperiodic initial links and delays to reinforcers in the terminal links. The terminal links always lasted 30 s. In Experiment 1, two reinforcers were delivered in each terminal link, with the first reinforcer delivered either 1 s (Experiment 1A) or 5 s (Experiment 1B) after choice. In these experiments, the delay between the first and second reinforcers in one terminal link was 10 s, and the delay between the first and second reinforcers on the other key was varied. This variation produced little change in preference. In Experiment 1C, the first and second delays on one key were 10 s, and on the other key they were varied within the restriction that the sum of delays was 20 s. Preference for the varied terminal link increased as the first delay was decreased. A hyperbolic model of the value of reinforcer delay provided a good description of the data from Experiment 1. In Experiment 2, a single reinforcer was delivered in each terminal link after a delay of either 0.2 or 19.8 s, and these delays were reversed between conditions. The initial-link schedule providing terminal-link access was varied from means of 5 s to 480 s. As the initial-link duration was increased, preference for the shorter delay became less extreme. An extension of the hyperbolic-decay model, in which the decay constant was a hyperbolic function of the initial-link duration, described the results well. Differences between the procedure used here (constant-duration terminal links) and that used in conventional concurrent-chain research precludes use of the model as a general account of concurrent-chain performance.