Visual nesting of stimuli affects rhesus monkeys' (Macaca mulatta) quantity judgments in a bisection task

Abstract

Nonhuman animals are highly proficient at judging relative quantities presented in a variety of formats, including visual, auditory, and even cross-modal formats. Performance typically is constrained by the ratio between sets, as would be expected under Weber's law and as is described in the approximate number system (ANS) hypothesis. In most cases, tests are designed to avoid any perceptual confusion for animals regarding the stimulus sets, but despite this, animals show some of the perceptual biases that humans show based on organization of stimuli. Here, we demonstrate an additional perceptual bias that emerges from the illusion of nested sets. When arrays of circles were presented on a computer screen and were to be classified as larger than or smaller than an established central value, rhesus monkeys (Macaca mulatta) underestimated quantities when circles were nested within each other. This matched a previous report with adult humans (Chesney & Gelman, Attention, Perception, & Psychophysics 24:1104-1113, 2012), indicating that macaques, like humans, show the pattern of biased perception predicted by ANS estimation. Although some macaques overcame this perceptual bias, demonstrating that they could come to view nested stimuli as individual elements to be included in the estimates of quantity used for classifying arrays, the majority of the monkeys showed the bias of underestimating nested arrays throughout the experiment.

Figure 1

Example trial type. The top box shows a control trial with non-nested stimuli, with nine circles, that should be classified as a “More” (M). The bottom box also shows nine circles, but now nested in four locations. This should also be classified as a “More” (M).

Figure 2

Mean percentage of “larger” responses for each block of 1,500 trials at each central value. The first column shows, from top to bottom, the first, second, and third runs of 1,500 trials with central value 5. The middle column shows, from top to bottom, the first, second, and third runs of 1,500 trials with central value 6. The right column shows, from top to bottom, the first, second, and third runs of 1,500 trials with central value 7. Performance in all panels is shown for control trials (in grey) and nested trials (in black). Errors bars represent standard errors of the mean.

Figure 3

Mean percentage of correctly made “larger” responses for the subset of trials examined (see text) to determine the effects of more or less nesting on performance. Smaller numbers of discrete subsets would typically mean greater nesting of the circles in the array, and this led to lower overall performance. Error bars represent standard errors of the mean.