How chimpanzees look at pictures: a comparative eye-tracking study

Abstract

Surprisingly little is known about the eye movements of chimpanzees, despite the potential contribution of such knowledge to comparative cognition studies. Here, we present the first examination of eye tracking in chimpanzees. We recorded the eye movements of chimpanzees as they viewed naturalistic pictures containing a full-body image of a chimpanzee, a human or another mammal; results were compared with those from humans. We found a striking similarity in viewing patterns between the two species. Both chimpanzees and humans looked at the animal figures for longer than at the background and at the face region for longer than at other parts of the body. The face region was detected at first sight by both species when they were shown pictures of chimpanzees and of humans. However, the eye movements of chimpanzees also exhibited distinct differences from those of humans; the former shifted the fixation location more quickly and more broadly than the latter. In addition, the average duration of fixation on the face region was shorter in chimpanzees than in humans. Overall, our results clearly demonstrate the eyemovement strategies common to the two primate species and also suggest several notable differences manifested during the observation of pictures of scenes and body forms.

Humans and chimpanzees attend differently to goal-directed actions

“…But species differences in viewing faces may be more apparent in tasks using dynamic objectdirected actions of others than in tasks that require observers to merely look at still images. Second, although our data on species difference in the grand average of fixation durations are comparable to those of a previous studies (200-300 ms in chimpanzees and 200-700 ms in human adults) 22,27 , our results showed that the fixation durations of chimpanzees differ according to the target of fixations. When fixations to faces were considered, the average fixation duration was shorter in chimpanzees than in humans (for example, 229 ms in chimpanzees and 672 ms in human adults in Experiment 1), but the duration of fixation to the object did not differ between chimpanzees and humans (for example, 490 ms in chimpanzees and 579 ms in human adults in Experiment 1).…”

“…The present results offer new species differences; first, the degree of species difference gauged by the proportion of fixation to faces is larger in our study than the previous study in which participants looked at still photographs containing the whole body of human and non-human animals, although strict comparison is not possible because of methodological differences 22 . But species differences in viewing faces may be more apparent in tasks using dynamic objectdirected actions of others than in tasks that require observers to merely look at still images.…”

“…In general, humans pay attention to other individuals' faces longer (ratio of looking time and fixation duration) and more frequently (number of fixations) than do chimpanzees across all situations, irrespective of goal-directed or non-goal-directed actions. Previous eye-tracking studies have found that chimpanzees pay less attention, although significantly higher than random scanning of a whole picture, to photographed faces, and that chimpanzees move their eyes more rapidly than human adults 22,27 .…”

“…When young nursery-reared chimpanzees are exposed to a novel object, they exhibit gaze alternation between this object and the face of their primary caregiver, a phenomenon similar to human social referencing 17 . Recent eye-tracking studies have illustrated that chimpanzees and macaques are attracted to the face and eye regions of both human and non-human animals 22,23 . Chimpanzees look at the face region longer than at other parts of a body when they are presented with various still photographs depicting human and non-human animals, although the degree to which they look at the faces is somewhat lower than in the case of human adults 22 .…”

“…Recent eye-tracking studies have illustrated that chimpanzees and macaques are attracted to the face and eye regions of both human and non-human animals 22,23 . Chimpanzees look at the face region longer than at other parts of a body when they are presented with various still photographs depicting human and non-human animals, although the degree to which they look at the faces is somewhat lower than in the case of human adults 22 . However, these findings on social referencing and saliency of the face region do not explain how non-human primates might use referential information for understanding others' actions.…”