Familiarity and Face-Inversion Effect in Japanese Macaques ( Macaca fuscata ) during the Preferential Looking Task

Abstract Four young laboratory-born Japanese macaques (Macaca fuscata) looked at the photographs of familiar and unfamiliar persons presented at upright and inverted orientations by pressing the lever under the conjugate schedule of sensory reinforcement (successive preferential looking procedure). Three types of photographs were prepared: photographs with persons taken in front view, those taken in back, and those without persons. The monkeys looked longer when the face was upright than inverted only for the pictures containing unfamiliar person with front view. The other types of photographs did not cause inversion effect. Familiarity weakened the face-specific inversion effect in monkeys. This difference may be due to in part the lower preference for familiar faces and the difference in processing mode between familiar and unfamiliar faces.