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Eye Tracking

At WKPRC we use eye tracking to study apes’ eye movements while they are watching selected images and videos on a monitor. To keep the animals’ attention focused on the screen, they are given juice to drink during the study. An eye tracker is positioned beneath the screen. This tracks the apes’ eye movements. The eye tracker emits invisible infra-red light that is reflected by the apes’ retinas, enabling the distance and angle of their pupils relative to the screen to be calculated. In this way it is possible to determine exactly where the ape is looking at any given moment in time.

Based on the duration and patterns of their gaze, conclusions can be drawn as to which parts of an image the ape is paying particular attention to. Studies have shown that there are both similarities and differences in the way apes and humans view images. For example, when great apes see photos of fellow species or other species of apes, they – just like humans – direct their attention first to the face and especially to the eyes of the depicted individual (Kano, Call, & Tomonaga, 2012; Kano & Tomonaga, 2009, 2010). However, the gaze of chimpanzees, gorillas and orang-utans soon migrates to the mouth and other parts of the body, whereas bonobos and humans usually spend more time making eye contact with the depicted individual (Kano, Call, & Tomonaga, 2012; Kano, Hirata, & Call, 2015; Kano & Tomonaga, 2010). The differences in social attention between chimpanzees and bonobos, which are very closely related, are particularly interesting. This gives rise to speculation that such differences may be related to the fact that the two species have developed very different social systems. Eye tracking therefore makes it possible to gain deep insights into the perception and attention of the animals and thus get to the bottom of the behaviour of different species.

Using eye tracking, scientists have also discovered that apes, just like humans, not only follow actions in videos with their gaze, but that their gaze can even be one step ahead of the action: Chimpanzees, orang-utans and bonobos at Leipzig Zoo were shown a video in which a hand reached for one of two objects. When they saw a similar video again afterwards, they looked at the object that had previously been grasped, even before the hand moved there (Kano & Call, 2014). This indicated that the apes were anticipating what would happen next in the scene. Predictive gaze behaviour can be used to answer questions about various cognitive abilities of apes, such as long-term memory (how long can chimpanzees and their fellow species remember the last scene of a film?) or the ability to put themselves in the shoes of others (can apes predict what a protagonist is likely to do next?).

As the above examples show, eye tracking is a very versatile tool that allows us better to understand not only what apes are directing their attention towards and how they perceive the world, but also what expectations they have of certain actions. This in turn reveals a great deal about the cognitive abilities of apes and how they differ from those of humans.