Examining the Underlying Mechanisms of the Differential Outcomes Effect in Pigeons

Abstract

When the rewards associated with the various stimuli of a matching-to- sample task differ from one another, animals are able to perform the task with much higher accuracy than when the rewards are the same across the various stimuli. This effect is known as the differential outcomes (DO) effect. There are currently two theories that are thought to account for the DO effect. The first is Enhanced Discriminability Theory, where DO enhances the discriminability between the sample stimuli, effectively making them easier to remember. The second is Expectancy Theory, where the DO makes it easier for the subjects to remember the expected reward throughout the delay period of the matching-to-sample task. In the present study we examined which theory can best account for the DO effect by examining the response properties of single neurons in both the nidopallium caudolaterale (NCL), an area responsible for processing re- ward information, and the entopallium (ENTO), an area responsible for processing visual memory. Of the 45 delay cells recorded from NCL we found sustained inhibitory delay activity during rewarded trials only. For excitatory delay activity we found sustained activity during both rewarded and non-rewarded trials in NCL. Of the 56 delay cells recorded from ENTO, we found sustained delay activity during both rewarded and non-rewarded trials. We suggest that a combination of both Expectancy Theory and Enhanced Discriminability Theory provides the best account for the DO effect.