Why faces may be special: Evidence of the inversion effect in chimpanzees

Five chimpanzees were tested on their ability to discriminate faces and aut omobiles presented in both their upright and inverted orientations. The fac e stimuli consisted of 30 black and white photographs, 10 each of unfamilia r chimpanzees (Pan troglodytes), brown capuchins (Cebus apella), and humans (Homo sapiens). Ten black and white photographs of automobiles were also u sed. The stimuli were presented in a sequential matching-to-sample (SMTS) f ormat using a computerized joystick-testing apparatus. Subjects performed b etter on upright than inverted stimuli in all classes. Performance was sign ificantly better on upright than inverted presentations of chimpanzee and h uman faces but not on capuchin monkey faces or automobiles. These data supp ort previous studies in humans that suggest the inversion effect occurs for stimuli for which subjects have developed an expertise. Alternative explan ations for the inversion effect based on the type of spatial frequency cont ained in the stimuli are also discussed. These data are the first to provid e evidence for the inversion effect using several classes of face stimuli i n a great ape species.