How do rhesus monkeys (Macaca mulatta) scan faces in a visual paired comparison task?

July 9, 2003


When novel and familiar faces are viewed si- multaneously, humans and monkeys show a preference for looking at the novel face. The facial features attended to in familiar and novel faces, were determined by analyz- ing the visual exploration patterns, or scanpaths, of four monkeys performing a visual paired comparison task. In this task, the viewer was first familiarized with an image and then it was presented simultaneously with a novel and the familiar image. A looking preference for the novel im- age indicated that the viewer recognized the familiar im- age and hence differentiates between the familiar and the novel images. Scanpaths and relative looking preference were compared for four types of images: (1) familiar and novel objects, (2) familiar and novel monkey faces with neutral expressions, (3) familiar and novel inverted mon- key faces, and (4) faces from the same monkey with dif- ferent facial expressions. Looking time was significantly longer for the novel face, whether it was neutral, express- ing an emotion, or inverted. Monkeys did not show a pref- erence, or an aversion, for looking at aggressive or affilia- tive facial expressions. The analysis of scanpaths indi- cated that the eyes were the most explored facial feature in all faces. When faces expressed emotions such as a fear grimace, then monkeys scanned features of the face, which contributed to the uniqueness of the expression. Inverted facial images were scanned similarly to upright images. Precise measurement of eye movements during the visual paired comparison task, allowed a novel and more quanti- tative assessment of the perceptual processes involved the spontaneous visual exploration of faces and facial expres- sions. These studies indicate that non-human primates carry out the visual analysis of complex images such as faces in a characteristic and quantifiable manner.