Increasing trial discriminability in the ephemeral reward task does not improve optimal choice in pigeons (Columba livia)

Abstract

The ephemeral reward task involves presenting subjects with a choice between two stimuli that each provide an identical reward. If one stimulus (stimulus A) is selected, reinforcement associated with that stimulus is provided and the trial ends; however, if the other stimulus (stimulus B) is selected, the reinforcer associated with that stimulus is provided and the subject is also allowed to obtain the reinforcer associated with stimulus A. Thus, it is optimal for the subject to choose stimulus B. Although wrasse (a fish) and parrots are able to quickly learn to maximize reinforcement, other species (including humans, nonhuman primates, rats, and pigeons) struggle to choose optimally. Research with pigeons and rats found that insertion of a delay between choice and initial reinforcement facilitates optimal performance, suggesting that mitigating impulsivity may increase optimal choice; however, research with human participants does not point to a relationship between measured and self-reported impulsivity and suboptimal performance on the task. For this reason, it was posited that subjects sometimes failed to discriminate the relation between choice and the second reinforcer. To test this hypothesis, we proposed that if the time between trials was increased, the relation between the subject’s choice and the second reinforcer would be more discriminable and optimal choice might be achieved. We found, however, that increasing the intertrial interval from 5 s to 30 s did not facilitate optimal choice. We conclude that the earlier effect of increasing the delay between choice and the initial outcome probably reduced impulsive choice, at least for pigeons and rats.