Diversity of Adjustments to Reward Downshifts in Vertebrates

Abstract

This review focuses on reward-schedule effects, a family of learning phenomena involving surprising devaluations in reward quality or quantity (as in incentive contrast), and reward omissions (as in appetitive extinction), as studied in three taxonomic groups of vertebrates: mammals, birds, and amphibians. The largest database of dependable data comes from research with mammals in general, and with rats in particular. These experiments show a variety of behavioral adjustments to situations involving reward downshifts. For example, rats show disruption of instrumental and consummatory behavior directed at a small reward after receiving a substantially larger reward (called successive negative contrast, SNC)—a reward-schedule effect. However, instrumental SNC does not seem to occur when animals work for sucrose solutions—a reversed reward-schedule effect. Similar modes of adjustment have been reported in analogous experiments with avian and amphibian species. A review of the evidence suggests that carry-over signals across successive trials can acquire control over behavior under massed practice, but emotional memory is required to account for reward-schedule effects observed under widely spaced practice. There is evidence for an emotional component to reward-schedule effects in mammals, but similar evidence for other vertebrates is scanty and inconsistent. Progress in the comparative analysis of reward-schedule effects will require the intense study of a set of selected species, in selected reward-downshift situations, and aiming at identifying underlying neural mechanisms.