Visual search for orientation of faces by a chimpanzee (Pan troglodytes): face-specific upright superiority and the role of facial configural properties

Masaki Tomonaga

A previous experiment showed that a chimpanzee performed better in searching for a target human face that differed in orientation from distractors when the target had an upright orientation than when targets had inverted or horizontal orientation [Tomonaga (1999a) Primate Res 15:215-229]. This upright superiority effect was also seen when using chimpanzee faces as targets but not when using photographs of a house. The present study sought to extend these results and explore factors affecting the face-specific upright superiority effect. Upright superiority was shown in a visual search for orientation when caricaturized human faces and dog faces were used as stimuli for the chimpanzee but not when shapes of a hand and chairs were presented. Thus, the configural properties of facial features, which cause an inversion effect in face recognition in humans and chimpanzees, were thought to be a source of the upright superiority effect in the visual search process. To examine this possibility, various stimuli manipulations were introduced in subsequent experiments. The results clearly show that the configuration of facial features plays a critical role in the upright superiority effect, and strongly suggest similarity in face processing in humans and chimpanzees.