The organization of conspecific face space in nonhuman primates

Abstract

Humans and chimpanzees demonstrate numerous cognitive specializations for processing faces, but comparative studies with monkeys suggest that these may be the result of recent evolutionary adaptations. The present study utilized the novel approach of face space, a powerful theoretical framework used to understand the representation of face identity in humans, to further explore species differences in face processing. According to the theory, faces are represented by vectors in a multidimensional space, the centre of which is defined by an average face. Each dimension codes features important for describing a face's identity, and vector length codes the feature's distinctiveness. Chimpanzees and rhesus monkeys discriminated male and female conspecifics' faces, rated by humans for their distinctiveness, using a computerized task. Multidimensional scaling analyses showed that the organization of face space was similar between humans and chimpanzees. Distinctive faces had the longest vectors and were the easiest for chimpanzees to discriminate. In contrast, distinctiveness did not correlate with the performance of rhesus monkeys. The feature dimensions for each species' face space were visualized and described using morphing techniques. These results confirm species differences in the perceptual representation of conspecific faces, which are discussed within an evolutionary framework.

Figure 1

A hypothetical illustration of a three-dimensional face space (Valentine, 1991). The origin of face space represents the central tendency of each feature dimension and is densely populated with faces of typical appearance. Each dashed line represents an identity vector that codes a unique face identity (A–D). Long vectors (e.g., A) indicate individuals of distinctive appearance in that their features are far from average, while short vectors (e.g., D) reflect typical individuals in that their features are more similar to the average.

Figure 2

An illustration of the matching-to-sample (MTS) paradigm used for chimpanzees. Subjects first contact the sample image (left panel) with the joystick-controlled cursor (shown by a cross). Then they select one of two comparison images (right panel), the correct image being identical to the sample face (lower right). This example shows female chimpanzee composites. The task was identical for the rhesus monkeys, only they selected images on a touchscreen. To view a colour version of this figure, please see the online issue of the Journal.

Figure 3

The mean performance of chimpanzees (Ch) and rhesus monkeys (Rh) over the seventy-six 50-trial sessions (m = slope values for the group means). To view a colour version of this figure, please see the online issue of the Journal.

Figure 4

The derived stimulus configuration for female chimpanzee faces based on a 2D multidimensional scaling (MDS) analysis of subjects’ discrimination performance. The average face is outlined. To view a colour version of this figure, please see the online issue of the Journal.

Figure 5

The derived stimulus configuration for male chimpanzee faces based on a 2D multidimensional scaling (MDS) analysis of subjects’ discrimination performance. The average face is outlined. To view a colour version of this figure, please see the online issue of the Journal.

Figure 6

The derived stimulus configuration for female rhesus monkey faces based on a 2D multidimensional scaling (MDS) analysis of subjects’ discrimination performance. The average face is outlined. To view a colour version of this figure, please see the online issue of the Journal.

Figure 7

The derived stimulus configuration for male rhesus monkey faces based on a 2D multidimensional scaling (MDS) analysis of subjects’ discrimination performance. The average face is outlined. To view a colour version of this figure, please see the online issue of the Journal.

Figure 8

Visualization of the perceptual dimensions of female and male chimpanzee face space by transforming the population average face into the template space of each of the four high/low dimension averages. To view a colour version of this figure, please see the online issue of the Journal.

Figure 9

Visualization of the perceptual dimensions of female and male rhesus monkey face space by transforming the population average face into the template space of each of the four high/low dimension averages. To view a colour version of this figure, please see the online issue of the Journal.