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Jessica F. Cantlon, Elizabeth M. Brannon; Relative salience of number, shape, color, and surface area in rhesus monkeys. Journal of Vision 2005;5(8):339. doi: 10.1167/5.8.339.
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Many species have been shown to discriminate number as a property of a set of objects however, little is known about how salient number is relative to other object properties like size, shape, and color. Some researchers have suggested that animals only attend to number as a ‘last resort strategy’ (Davis & Memmott, 1982). In this study we investigated the relative salience of number versus alternative stimulus dimensions for macaque monkeys. Monkeys with varying degrees of experience on numerical tasks were trained to a 70% criterion on a delayed match-to-sample task where the correct choice matched the sample in both number and a second nonnumerical property (shape, color, or surface area). Following training, monkeys were tested with nondifferentially reinforced probe trials in which one choice was a number match and the other was a shape, color, or area match (e.g., sample = 2 red tulips and choices = 4 red tulips or 2 red cars). Results indicate that monkeys' propensity to use number as a basis for matching increased with the numerical difference between the nonnumerical and numerical match. Thus when number was highly discriminable, monkeys were more likely to use it as a basis for matching over other object features. In addition, monkeys were more likely to use number than area even at the closest distance.
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