Eye tracking as a window into great ape emotions

The Comparative Psychology and Affective Neuroscience (CoPAN) lab at Leiden University, led by Prof. Dr. Mariska Kret, focuses on the psychological and neurophysiological underpinnings of emotion across species. Their scientific toolbox combines behavioral observations, non-invasive psychophysiological measures such as heart-rate variability, and touchscreen and eye tracking tasks. Specifically, the lab uses eye tracking to elucidate whether emotions implicitly attract attention, and if so, which ones and whose.

Studying emotion in non-human species requires creativity when choosing the right methodology, as many of the experiments done with humans are not feasible with animals you cannot speak with. Eye tracking allows for non-invasively studying the mechanisms underlying socio-emotional cognition in great apes that are otherwise unattainable. The lab leverages eye tracking to measure attention biases in human and nonhuman great apes. By applying similar study designs across species, they can directly compare results to shed light on when, how, and why emotions evolved.

Currently, the lab is testing chimpanzees, orangutans, and bonobos in different zoos across the Netherlands and Belgium. Participation is voluntary, and they do not restrain the apes in any way. The researchers provide a juice reward through a fixed nozzle during testing to ensure the participant’s head stays stable and their eyes stay in the right position for the eye tracker, avoiding any need for training.

The rewards and challenges of great ape research

Kia Radovanović and Karlijn van Heijst, PhD candidates, share how running eye tracking studies with great apes poses many different practical challenges. They note that maintaining ethical standards always comes first when designing their experiments:

First, we have to find a way to safely build eye tracking setups into the apes’ enclosure in a location that is convenient for us and where we’re not in the way for zoo staff. This requires close collaboration with the zoos we work in, to make sure the setup works for all parties involved (including the great apes!). Our comfort during testing gets lower priority, so we often have to do with camping chairs, or just a convenient ledge next to the enclosure.

Second, because our great ape participants can choose whether they come to the eye tracking setup during our sessions, all sessions are different. Some days, they are all very interested and motivated and we can gather lots of data, some days they do not show up at all, and most days are somewhere in between.

Lastly, we of course cannot give the apes any verbal instructions to look at quite uninteresting stimuli, like a dot or a cross. Therefore we use short videos or animations to attract their attention to the screen for calibration and fixation.

Overall, eye tracking is an incredibly useful and unique tool within our field, as it opens up many possibilities with species we can have limited interaction with. However, eye tracking with great apes requires a lot of patience and perseverance, in addition to flexibility and creativity. Despite these challenges, the insights gained are invaluable—not only do they enhance our understanding of nonhuman emotions and cognition, but they also help us better grasp the evolutionary roots of our own emotional lives.

Tips for unrestrained eye tracking with great apes without the use of a separate testing chamber:

• When choosing a location for your eye tracking setup, consider the apes’ perspective: Can they sit comfortably in front of it? Can they easily get away when there is social tension or a conflict arises? It is for example best to not place the setup in a corner or a tunnel.

• Try many different stimuli for calibration and fixation, interest will likely differ between individuals (and days!)

Publications

van Berlo, E., Roth, T. S., Kim, Y., & Kret, M. E. (2024). Selective and prolonged attention to emotional scenes in humans and bonobos. Proceedings of the Royal Society B: Biological Sciences, 291(2028), 20240433.