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Laurie R. Santos, Jennifer Barnes, Neha Mahajan; Numerical representation in four lemur species. Journal of Vision 2005;5(8):735. doi: 10.1167/5.8.735.
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© ARVO (1962-2015); The Authors (2016-present)
Although much is known about how some primates— in particular, monkeys and humans— visually represent, enumerate, and track different numbers of objects, very little is known about the numerical and cognitive processing of visual stimuli in prosimian primates. Here, we explore how four lemur species (Eulemur fulvus, Eulemur mongoz, Lemur catta, and Varecia rubra) visually represent small numbers of objects. Specifically, we presented lemurs with three expectancy violation looking time experiments aimed at exploring their expectations about a simple 1 + 1 addition event. In three experiments, we presented subjects with displays in which two lemons were sequentially added behind an occluder and then measured subjects' duration of looking to expected and unexpected visual outcomes. In Experiment 1, subjects looked reliably longer at an unexpected outcome of only one object than at an expected outcome of two objects. Similarly, subjects in Experiment 2 looked reliably longer at an unexpected outcome of three objects than at an expected outcome of two objects. In Experiment 3, subjects looked reliably longer at an unexpected outcome of one object twice the size of the original than at an expected outcome of two objects of the original size. These results suggest that some prosimian primates have the ability to track objects hidden behind occluders as well as the capacity to quantify these occluded objects across time and motion. In addition, since our looking tasks involve no training, our results necessarily tap into capacities that are naturally available to these animals without extensive preparation and suggest that the looking time paradigm may be used in future experiments assessing the visual processing abilities in a wide variety of primate species.
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