Exploring Whether Nonhuman Primates Show a Bias to Overestimate Dense Quantities

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The density bias, documented within the foraging domain for some monkey species and for human infants, emerges when perceived numerosity is affected by interstimulus distance such that densely arranged food items appear more numerous relative to the same amount of food sparsely arranged. In this study, capuchin monkeys and rhesus monkeys were presented with a computerized relative discrimination task that allowed for the control of stimulus size, interelemental distance, and overall array pattern. The main objective was to determine whether the density bias was a more widespread and general perceptual phenomenon that extends beyond the foraging domain, similar to other numerosity illusions and biases. Furthermore, we compared the current results to these same monkeys’ data from a previous study on the Solitaire numerosity illusion to investigate a potential link between a density bias and this related numerical illusion. Capuchin monkeys showed a density bias in their perceptual discrimination of dense versus sparse stimuli; however, rhesus monkeys perceived this bias to a lesser degree. Individual differences were evident, as with the Solitaire illusion. However, there was not a relation between susceptibility to a density bias and susceptibility to the Solitaire illusion within these same monkeys.

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