International Journal of Comparative Psychology

A downshift from a more preferred to a less preferred incentive leads to a transient rejection of the lower incentive. This phenomenon, known as successive negative contrast (SNC), has been reported in studies with mammals, but not with fish, amphibians, or reptiles, all showing gradual adjustments to the new incentive conditions. It is assumed that an understanding of the brain systems involved in the onset of SNC in mammals will suggest likely brain areas for a comparative analysis in non mammalian vertebrates. Studies reviewed in this article show that opioid receptors are normally engaged during SNC, participate in the detection of the incentive downshift, play a role in SNC onset (delta receptors), and modulate recovery from SNC (kappa receptors). However, opioid receptors do not seem to be involved in the consolidation of the downshift memory. These results suggest a relationship between the evolution of the opioid system and the evolution of learning mechanisms involved in the adjustment to incentive downshifts in vertebrates.