Differences in delay, not ratios, control choice in concurrent chains

In two experiments, pigeons were trained with concurrent-chains schedules, wherein responding to equal initial links measured preference between variable-interval terminal links. Absolute terminal-link duration was varied by keeping constant the difference between the terminal-link delays and forcing their ratio to change. Delay-reduction theory scales value relative to a common temporal context and requires that delay differences control choice. Thus, preference should remain invariant. Most competing accounts, including the matching law and a strong form of Weber's law, require that preference vary with the delay ratio. Experiment 1 employed standard concurrent chains, in which terminal-link position and color were confounded. Although average preference remained constant, individual preferences were highly variable and inconsistent, possibly due to carryover of position biases across conditions. In an attempt to reduce variability, Experiment 2 used a modified concurrent-chains procedure. Preference at different terminal-link durations was assessed simultaneously to prevent order effects, and terminal-link position was alternated randomly across trials to minimize the impact of position biases. In Experiment 2, both individual and mean preferences showed the constant-difference invariance. Overall, choice was controlled by terminal-link differences, not ratios.