Prototype abstraction by monkeys (Macaca mulatta)

The authors analyze the shape categorization of rhesus monkeys (Macaca mulatta) and the role of prototype- and exemplar-based comparison processes in monkeys' category learning. Prototype and exemplar theories make contrasting predictions regarding performance on the Posner-Homa dot-distortion categorization task. Prototype theory-which presumes that participants refer to-be-categorized items to a representation near the category's center (the prototype)-predicts steep typicality gradients and large prototype-enhancement effects. Exemplar theory-which presumes that participants refer to-be-categorized items to memorized training exemplars-predicts flat typicality gradients and small prototype-enhancement effects. Across many categorization tasks that, for the first time, assayed monkeys' dot-distortion categorization, monkeys showed steep typicality gradients and large prototype-enhancement effects. These results suggest that monkeys-like humans-refer to-be-categorized items to a prototype-like representation near the category's center rather than to a set of memorized training exemplars.