Page 93 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Emotions hold the attention of bonobos and humans
tracker and the webcam, was placed inside a wooden box inside the bonobos’ enclosure (Figure 1).
The front of the wooden box was a 3 mm thick, scratch proof polycarbonate
plate. At mouth’s height, a drinking nozzle was attached to the panel. During the
experiment, bonobos were rewarded with diluted juice (1 part syrup, 5 parts water)
at short intervals (roughly every 5 seconds), and provided through the nozzle. To
minimalize distractions, other bonobos present in the enclosure were rewarded with
the same juice by the caretaker, after they performed a body-part training that is
used for veterinarian purposes. Bonobos were familiar with drinking from the nozzle
because their enclosures were also fitted with these nozzles for drinking water. The
computer and the other screen for the experimenter were located outside of the
enclosure. This second screen displayed Tobii Studio Pro’s Live Viewer, enabling the 4 experimenter to track where bonobos were looking in real time.
Figure 1. Drawing of the setup at primate park Apenheul. Illustration by Brenda de Groot.
Stimuli
Stimuli consisted of emotional and neutral scenes selected from previously validated sets (bonobos: Kret et al., 2016, humans: Kret & van Berlo, 2021; van Berlo et al., 2021). While it is common in psychological research to use isolated facial expressions of
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