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J Comp Psychol. 2001 Sep;115(3):248-57.

Spontaneous representation of number in cotton-top tamarins (Saguinus oedipus).

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Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, USA.


The "violation of expectancy" looking-time methodology has proven a powerful tool for exploring prelinguistic mental representations in human infants as well as in nonhuman primates. Four studies applying this methodology to the question of spontaneous number representations in cotton-top tamarins (Saguinus oedipus) are reported here. Monkeys were shown 1 + 1 events in which objects were placed behind a screen, 1 by 1. The screen was removed, revealing consistent (2 objects) and inconsistent (1, 3, or 1 large object twice the mass of original object) outcomes. In all studies, monkeys looked longer at the inconsistent than at the consistent outcome. When the monkeys view a 1 + 1 operation, they expect exactly 2 objects. It is likely that these numerical representations are spontaneously available to a variety of primate species and could provide a foundation on which humans' number sense was constructed over evolution and development.

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