Spatial representation of magnitude in rhesus macaques: Investigating SNARC effects in quantity and size dimensions

The Spatial Numerical Association of Response Codes (SNARC) effect reflects an association of magnitude with space. For example, humans typically map small numbers to one side of space and large numbers to the other. Research with a variety of animal species has revealed similar spatial-magnitude associations in species without relevant cultural practices, suggesting ancient origins. Human spatial-numeric associations are known to be modified by cultural practices, such as reading direction. Some studies of nonhumans have suggested that spatial-numeric associations are fixed, with small quantities represented to the left, while others suggest this relationship is not preprogrammed. Here, we report variable and flexible spatial-numeric associations in rhesus monkeys. Monkeys were required to pick both the smaller and the larger array of dots across counterbalanced conditions. We found clear spatial-numeric associations that varied among individuals and reversed between the Pick Small and Pick Large quantity conditions-indicating flexible rather than fixed associations. We found similar but nonsignificant patterns in subsequent tests using size rather than quantity. These results, like those from adult birds and apes, suggest that while the cognitive architecture for spatial-magnitude mapping is evolutionarily conserved, the specific spatial associations shown by individuals likely result from experience.