Linear numerosity illusions in capuchin monkeys (Sapajus apella), rhesus macaques (Macaca mulatta), and humans (Homo sapiens).

Numerosity illusions emerge when the stimuli in one set are overestimated or underestimated relative to the number (or quantity) of stimuli in another set. In the case of multi-item arrays, individual items that form a better Gestalt are more readily grouped, leading to overestimation by human adults and children. As an example, the Solitaire illusion emerges when dots forming a central cluster (cross-pattern) are overestimated relative to the same number of dots on the periphery of the array. Although this illusion is robustly experienced by human adults, previous studies have produced weaker illusory results for young children, chimpanzees, rhesus macaques, capuchin monkeys, and guppies. In the current study, we presented nonhuman primates with other linear arrangements of stimuli from Frith and Frith's (Percept Psychoph 11:409-410, 1972) original paper with human participants that included the Solitaire illusion. Capuchin monkeys, rhesus macaques, and human adults learned to quantify black and white dots that were presented within intermingled arrays, responding on the basis of the more numerous dot colors. Humans perceived the various illusions similar to the original findings of Frith and Frith (1972), validating the current comparative design; however, there was no evidence of illusory susceptibility in either species of monkey. These results are considered in light of illusion susceptibility among primates as well as considering the role of numerical discrimination abilities and perceptual processing mode on illusion emergence.