How macaques view familiarity and gaze in conspecific faces

The pattern of visual fixations on an image depends not only on the image content but also on the viewer's disposition and on the function (or pathology) of underlying neural circuitry. For example, human viewers display changes in viewing patterns toward face images that differ in gaze direction or in the viewer's familiarity with the face. Macaques share many face processing abilities with humans, and their neural circuitry is used to understand perception across species, yet their viewing responses to gaze and familiarity of faces is poorly understood. In this study, rhesus macaques passively viewed faces of familiar and unfamiliar conspecifics whose head-and-eye gaze was directed either toward or away from the viewing monkey. The eyes of faces were viewed more than any other feature; furthermore, familiar eyes were viewed more than unfamiliar eyes. In contrast, ears, though not as salient as eyes, were viewed about twice as often for unfamiliar faces as familiar faces. Directed-gaze eyes were fixated earlier, and for a greater proportion of saccades than were the eyes of averted-gaze faces, suggesting that mutual gaze attracts a more immediate and sustained scanning of the eyes. Ears and external features were more salient for averted, as compared with directed gaze. In general, effects were more robust (within and across subjects) for the gaze contrast than for familiarity, perhaps as a consequence of the greater image-based differences for the gaze than the familiarity stimuli used in this study.