The effect of heterogeneity on numerical ordering in rhesus monkeys

We investigated how within-stimulus heterogeneity affects the ability of rhesus monkeys to order pairs of the numerosities 1 through 9. Two rhesus monkeys were tested in a touch screen task where the variability of elements within each visual array was systematically varied by allowing elements to vary in color, size, shape, or any combination of these dimensions. We found no evidence of a cost (or benefit) in accuracy or reaction time when monkeys were tested with stimuli that were heterogeneous in color, size, or shape. This was true even though both monkeys experienced extended initial training with arrays that were homogeneous in the color, shape, and size of elements. The implications of this finding for the mechanisms that monkeys use to represent and compare numerosities are discussed.