Monkey and human face perception: inversion effects for human faces but not for monkey faces or scenes.

Three rhesus monkeys and two groups of 10 human subjects judged upright or inverted pictures as same or different. The pictures were black and white pairs of human faces, monkey faces, or scenes. The monkeys were trained with sets of 50 pictures and were tested with other sets of 36 pictures from each category. The groups of 10 human subjects were tested with the same pictures used to test monkeys. Both monkeys and humans showed large performance decrements to inverted human faces relative to upright human faces but neither species showed inversion effects for monkey faces or scenes. A second test with both monkeys and humans showed the same pattern of results with a different set of human-face pictures that varied more in sex (female as well as male), facial hair, eyeglasses, haircut, view angle, and background than those of the first test. The results indicate similar face-processing mechanisms in monkeys and humans despite experiential and evolutionary differences.