Rhesus monkeys (Macaca mulatta) lack expertise in face processing

Abstract

Faces are salient stimuli for primates that rely predominantly on visual cues for recognizing conspecifics and maintaining social relationships. While previous studies have shown similar face discrimination processes in chimpanzees and humans, data from monkeys are unclear. Therefore, three studies examined face processing in rhesus monkeys using the face inversion effect, a fractured face task, and an individual recognition task. Unlike chimpanzees and humans, the monkeys showed a general face inversion effect reflected by significantly better performance on upright compared to inverted faces (conspecifics, human and chimpanzees faces) regardless of the subjects' expertise with those categories. Fracturing faces alters first- and second-order configural manipulations whereas previous studies in chimpanzees showed selective deficits for second-order configural manipulations. Finally, when required to individuate conspecific's faces, i.e., matching two different photographs of the same conspecific, monkeys showed poor discrimination and repeated training. These results support evolutionary differences between rhesus monkeys and Hominoids in the importance of configural cues and their ability to individuate conspecifics' faces, suggesting a lack of face expertise in rhesus monkeys.

Figure 1

(a) Dedicated testing box containing cage, touchscreen, speakers and infrared camera. (b). Home cage testing apparatus showing front and back of two touchscreens, automatic feeder, transport cart and attachment handles.

Figure 2

Mean trials-to-criterion (TTC, ± 95% confidence intervals) for subjects when learning to discriminate each category of stimulus in its upright orientation.

Figure 3

Mean performance (±95% confidence intervals) by subjects when discriminating images in their upright and inverted orientations.

Figure 4

An illustration of the three types of configural manipulations used in Experiment 2: An unaltered face, a face showing inner features (Inner), a fractured face (Fractured), and a fractured and rearranged face (Rearranged).

Figure 5

Subjects’ mean performance (±95% confidence intervals) on each of the configural Probe trials and their control.

Figure 6

An example of two individual recognition trials, one that subjects performed very well on and another which causes difficulty. The correct choice for each panel is the face in the lower left.