A functional category is a set of stimuli that are regarded as equivalent independently of their physical properties and elicit the same behavioral responses. Major psychological theories suggest the ability to form and utilize functional categories as a basis of higher cognition that markedly increases behavioral flexibility. Vaughan claimed the category use in pigeons on the basis of partition, a mathematical criterion for equivalence, however, there have been some criticisms that the evidence he showed was insufficient. In this study, by using a group reversal task, a procedure originally used by Vaughan, we aimed to gather further evidence to prove the category use in animals. Macaque monkeys, which served as subjects in our study, could efficiently perform the task not only with familiar stimulus sets as Vaughan demonstrated but also with novel sets, and furthermore the task performance was stable even when the number of stimuli in a set was increased, which we consider as further evidence for the category use in animals. In addition, by varying the timing of the reversal, we found that a category formation takes place soon after encountering new stimuli, i.e. in a few blocks of trial after a novel stimulus set was introduced.
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