The importance of surface-based cues for face discrimination in non-human primates

Abstract

Understanding how individual identity is processed from faces remains a complex problem. Contrast reversal, showing faces in photographic negative, impairs face recognition in humans and demonstrates the importance of surface-based information (shading and pigmentation) in face recognition. We tested the importance of contrast information for face encoding in chimpanzees and rhesus monkeys using a computerized face-matching task. Results showed that contrast reversal (positive to negative) selectively impaired face processing in these two species, although the impairment was greater for chimpanzees. Unlike chimpanzees, however, monkeys performed just as well matching negative to positive faces, suggesting that they retained some ability to extract identity information from negative faces. A control task showed that chimpanzees, but not rhesus monkeys, performed significantly better matching face parts compared with whole faces after a contrast reversal, suggesting that contrast reversal acts selectively on face processing, rather than general visual-processing mechanisms. These results confirm the importance of surface-based cues for face processing in chimpanzees and humans, while the results were less salient for rhesus monkeys. These findings make a significant contribution to understanding the evolution of cognitive specializations for face processing among primates, and suggest potential differences between monkeys and apes.

Figure 1.

An illustration of each image type in (a) chimpanzees and (b) rhesus monkeys. From left to right: positive, negative, colour and the top face parts used in the control condition (shown positive contrast only).

Figure 2.

An illustration of three types of test trials (a) following positive contrast training (illustrating rhesus monkeys): (left to right) positive–positive (PP), positive–negative (PN) and positive–colour (PC); and (b) following negative training (illustrating chimpanzees): negative–negative (NN), negative–positive (NP) and negative–colour (NC).

Figure 3.

Mean performance by (a) chimpanzees and (b) rhesus monkeys matching faces that differed in contrast or colour. The brackets note significant differences, p < 0.017 (corrected).