Relative value perception in an insect: positive and negative incentive contrasts in ants

Abstract

Humans tend to value things not on their absolute values, but relative to reference points such as former experience or expectations. People rate the quality of a new salary relative to their previous salary and the salaries of their peers, instead of appreciating its absolute value. Here, we demonstrate a similar effect in an insect: ants, which had previously experienced a low quality food source, showed higher acceptance of medium quality food (e.g. 0.1M then 0.5M; positive contrast) than if they had received the medium food all along (e.g. 0.5M then 0.5M; control), and vice versa for high expectations. Further experiments demonstrate that these contrast effects arise from cognitive rather than mere sensory or pre-cognitive perceptual causes. Pheromone deposition also correlates with perceived reward value, and ants showed successive contrasts in their pheromone deposition. Relative value perception can therefore be expected to have strong effects not only on individual behaviour, but also on collective decision-making. Contrast effects were also social: the quality of food received from other ants affected the perceived value of food found later. Value judgement is a key element in decision making, and thus relative value perception will strongly influence how animals interact with their environment.