Influence of Rule and Reward-based Strategies on Inferences of Serial Order by Monkeys

Knowledge of transitive relationships between items can contribute to learning the order of a set of stimuli from pairwise comparisons. However, cognitive mechanisms of transitive inferences based on rank order remain unclear, as are contributions of reward magnitude and rule-based inference. To explore these issues, we created a conflict between rule- and reward-based learning during a serial ordering task. Rhesus macaques learned two lists, each containing five stimuli, that were trained exclusively with adjacent pairs. Selection of the higher-ranked item resulted in rewards. "Small reward" lists yielded 2 drops of fluid reward, while "large reward" lists yielded 5 drops. Following training of adjacent pairs, monkeys were tested on novels pairs. One item was selected from each list, such that a ranking rule could conflict with preferences for large rewards. Differences in associated reward magnitude had a strong influence on accuracy, but we also observed a symbolic distance effect. That provided evidence of a rule-based influence on decisions. Reaction time comparisons suggested a conflict between rule and reward-based processes. We conclude that performance reflects the contributions of two strategies, and that a model-based strategy is employed in the face of a strong countervailing reward incentive.