Perception of the average size of multiple objects in chimpanzees (Pan troglodytes)

Abstract

Humans can extract statistical information, such as the average size of a group of objects or the general emotion of faces in a crowd without paying attention to any individual object or face. To determine whether summary perception is unique to humans, we investigated the evolutional origins of this ability by assessing whether chimpanzees, which are closely related to humans, can also determine the average size of multiple visual objects. Five chimpanzees and 18 humans were able to choose the array in which the average size was larger, when presented with a pair of arrays, each containing 12 circles of different or the same sizes. Furthermore, both species were more accurate in judging the average size of arrays consisting of 12 circles of different or the same sizes than they were in judging the average size of arrays consisting of a single circle. Our findings could not be explained by the use of a strategy in which the chimpanzee detected the largest or smallest circle among those in the array. Our study provides the first evidence that chimpanzees can perceive the average size of multiple visual objects. This indicates that the ability to compute the statistical properties of a complex visual scene is not unique to humans, but is shared between both species.

Keywords: average; chimpanzees; ensemble coding; perception; size; summary statistics.

Figure 1.

Examples of the stimuli used in Experiment 1. In the Single, Homogeneous and Heterogeneous conditions, the average diameter(s) of the circle(s) of one array (right side of the figure) was larger than that (those) of the other array. The figure shows examples of the stimuli in each condition, in which the size differences of circle(s) between the arrays were 20%.

Figure 2.

The proportion of choice of the array having the circles of larger average size in chimpanzees (n = 5) (a) and humans (n = 18) (b). The percentage of trials correctly completed under each condition as a function of differences in average sizes of circles between the arrays, averaged across participants. Standard errors are indicated by the error bars at each point.

Figure 3.

Examples of the stimuli used in Experiment 2. The test session consisted of Both-cue, Smallest-cue and No-cue conditions. The average diameters of 12 circles of one array (right side of the figure, comparison) were 20% larger than those of the other array.

Figure 4.

The proportion of choice of the array having circles of larger average size in chimpanzees (n = 4). The percentage of trials correctly completed under the three conditions, averaged across participants. Standard errors are indicated by the error bars on each column.