The effect of response rate on reward value in a self-control task

To understand how effort, defined by number of responses required to obtain a reward, affects reward value, five pigeons were exposed to a self-control task. They chose between two alternatives, 2s of access to food after a delay of 10s, and 6s of access to food after an adjusting delay. The adjusting delay increased or decreased depending on the pigeons' choices. The delay at which the two alternatives were equally chosen defined the indifference point. To determine whether requiring responses during the delay led to more impulsive (smaller-sooner rewards) or self-controlled (larger-later rewards) choices, we varied the number of required pecks during the 10-s delay to the 2-s reinforcer, and assessed how the requirement affected the indifference points. In the High Rate Phase, they had to peck at least 10 times during the delay; in the Low Rate Phase, they could peck at most 5 times during the delay. For four pigeons the indifference point increased with the response requirement; for one pigeon it decreased. The results suggest that, in general, reward value varies inversely with effort.This work was supported by the Portuguese Foundation for Science and Technology (FCT) Grant (PTDC/MHC-PCN/3540/2012) to Armando Machado and Marco Vasconcelos. Ines Fortes and Marco Vasconcelos were supported by an FCT Doctoral Grant (SFRH/BD/77061/2011) and an FCT Investigator Grant (IF/01624/2013), respectively. We are grateful to the members of the Animal Learning and Behavior Laboratory of University of Minho for their helpful comments on a prior version of this paper.