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Jessica Taubert, Lisa Parr; The other “other-species” effect: Understanding important differences in primate face discrimination. Journal of Vision 2011;11(11):568. doi: https://doi.org/10.1167/11.11.568.
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As human adults, we find it difficult to discriminate between individual monkey faces. A sizable body of research attributes this “other-species effect” to insufficient experience. That is, our expertise with faces is limited to the species that we have been exposed to and have interacted with. In recent years, however, evidence has emerged to suggest that monkeys do not share this same expertise. Here, we advance our understanding of face discrimination in humans by comparing two nonhuman primate species using multiple markers of human face expertise. Chimpanzees (Pan troglodytes, N = 6) are our closest-living relative, and thus represent the most direct comparison for understanding human cognitive specializations. Rhesus monkeys (Macaca mulatta, N = 6) are more distantly related but more frequently used as neurological models of human face perception. All 12 subjects were born at the Yerkes National Primate Research Center (Atlanta, GA). Using a 2AFC MTS procedure we measured how face discrimination performance (% correct) in these two species was influenced by changes in orientation, viewpoint, geometric stretching, feature scrambling and contrast-reversal. We also tested whether the composite face effect was contingent on contrast-polarity and sensitivity to first-order configurations in Mooney objects. Although these experiments reveal gross continuity across species they also point to a number of informative differences. For example, chimpanzees were more finely tuned to chimpanzee faces than monkeys to monkey faces. We conclude that while there is a common behavioural signature for face processing in both species, the data suggest that chimpanzees have an online face processing system, specialized for the subordinate-level classification of familiar faces. Monkeys, on the other hand, appear to process all faces uniformly. We propose that these differences across species (the other “other-species effect”) can be used to track the developmental origins of the face processing system in humans.
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