Natural Images: A Lingua Franca for Primates?

Primates in the laboratory often perform tasks that involve discrimination of pictures shown on computer screens. It is not clear, however, if they perceive the pictures as symbolic representations of real objects. In this study we tested the ability of two monkeys (Macaca mulatta) to categorize pictures on a computer screen. The pictures were photographs of objects from 17 categories, which were grouped as natural and familiar, natural and unfamiliar, or artificial and unfamiliar. The monkeys learnt the categories of the natural and familiar objects faster than those of the artificial and unfamiliar objects. Nevertheless, once they had learnt the categories, they were just as fast and accurate at generalizing to new exemplars of artificial and unfamiliar objects. This finding suggests that the monkeys are able to extract similarities and form categories from a small number of stimuli differing widely across size, color and viewpoint, and for which they have no prior internal representation.