Concept learning of ecological and artificial stimuli in rhesus macaques

The study of concepts in animals is complicated by the possibility that performance reflects reinforcement learning of discriminative cues, which might be used to categorize of stimuli. To minimize that possibility, we trained seven rhesus macaques to respond, in a specific order, to four simultaneously presented exemplars of different perceptual concepts. These exemplars were drawn at random from large banks of images; in some conditions, the stimuli changed on every trial. Subjects nevertheless identified and ordered these stimuli correctly. Three subjects learned to correctly order ecologically relevant concepts; four subjects, to order close-up sections of paintings by four artists with distinctive styles. All subjects classified stimuli significantly better than that predicted by chance, and outperformed a feature-based computer vision algorithm, even when the exemplars were changed on every trial. Furthermore, six subjects (three using ecological stimuli and three using paintings) transferred these concepts to novel stimuli. Our results suggest that monkeys possess a flexible ability to form class-based perceptual concepts that cannot be explained as the mere discrimination of physical features.