Abstract
Almost all nonhuman animals can recognize when one item is the same as another item. It is less clear whether nonhuman animals possess abstract concepts of “same” and “different” that can be divorced from perceptual similarity. Pigeons and monkeys show inconsistent performance, and often surprising difficulty, in laboratory tests of same/different learning that involve only two items. Previous results from tests using multi-item arrays suggest that nonhumans compute sameness along a continuous scale of perceptual variability, which would explain the difficulty of making two-item same/different judgments. Here, we provide evidence that rhesus monkeys can learn a two-item same/different discrimination similar to those on which monkeys and pigeons have previously failed. Monkeys’ performance transferred to novel stimuli and was not affected by perceptual variations in stimulus size, rotation, view, or luminance. Success without the use of multi-item arrays, and the lack of effect of perceptual variability, suggests a computation of sameness that is more categorical, and perhaps more abstract, than previously thought.
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Acknowledgments
David P. Charles passed away on October 17, 2006. We thank Luke Humphrey for help testing monkeys. We thank an anonymous reviewer for suggesting an additional analysis. This work was supported by the Intramural Research Program of the National Institute of Mental Health. The authors declare that they have no conflict of interest. All procedures were reviewed and approved by the NIMH Animal Care and Use Committee and complied with US law.
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Basile, B.M., Moylan, E.J., Charles, D.P. et al. Two-item same/different discrimination in rhesus monkeys (Macaca mulatta). Anim Cogn 18, 1221–1230 (2015). https://doi.org/10.1007/s10071-015-0891-z
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DOI: https://doi.org/10.1007/s10071-015-0891-z