Abstract
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.
Highlights
A functional category is a set of stimuli that are regarded to be equivalent independently of their physical properties and elicit the same behavioral responses, whereas a perceptual category, in contrast, is based on physical similarity among stimuli[1,2]
In Experiment 2, to examine whether the monkeys learned to utilize a category in the group reversal task, we studied how they would perform the task with a novel stimulus set that was introduced in a daily session
In Experiments 1, 2 and 3, we examined whether macaque monkeys (Japanese monkeys) were able to form and utilize functional category by using a repetitive group reversal, which is a procedure originally used by Vaughan for pigeons[9]
Summary
A functional category is a set of stimuli that are regarded to be equivalent independently of their physical properties and elicit the same behavioral responses, whereas a perceptual category, in contrast, is based on physical similarity among stimuli[1,2]. It was explained that the development of rapid adaptations to concurrent association reversals indicates that the pigeons had formed an equivalence class for each of the 20 stimuli Using this procedure, subsequent studies have shown the formation and use of functional categories in pigeons[10] and other www.nature.com/scientificreports/. We predicted that if the monkeys had learned a strategy to form a category of stimuli associated with the same outcome in the course of the training with the regular stimulus sets, they would be able to show a quick adaptation even in the first reversal of the sessions with a new stimulus set. In Experiment 4, to study when the category formation is completed after the introduction of a new stimulus set, we examined how the performance after a reversal would be affected as we changed the timing of the reversal with reference to the introduction of a new stimulus set
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