Page 95 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Emotions hold the attention of bonobos and humans
individuals lying down on grass, sitting, walking, or cycling with a neutral facial expression (see Table S3 for more information on the composition of the scenes). In total there were 10 unique stimuli per emotional category (5) and per species (2), as well as 100 unique neutral stimuli, as each emotional stimulus was matched with a neutral stimulus. Stimuli consisted of a subset of the validated sets by Kret et al. (2016) and van Berlo & Kret (2021). They were colored pictures with a dimension of 500x430 pixels, matched on luminance level and number of individuals depicted as much as possible.
Calibration
Before commencing testing, we conducted a manual two-point calibration using the
infant calibration procedure in Tobii Studio. We used a relatively small number of 4 reference points because apes tended to look only very briefly at the points. However,
two-point calibrations are often used in great ape research as they are reasonably
sufficient for the research questions asked, and also attainable given the constraints
of working with animals (Hopper et al., 2021b). A small video displaying penguins
(270x155 px) was used for the reference points. Calibrations were repeated until a
sufficient calibration was obtained (i.e., Tobii Studio indicated no large calibration
errors). For each individual, we continued using their first successful calibration
throughout the entire experiment. To make sure that the calibration remained
sufficient over time, we showed bonobos a 9-point grid before the start of each test
session and visually inspected the accuracy of the calibration (see supplements for
more information regarding calibration).
Procedure
Before commencing the experiment, bonobos were familiarized with the setup by showing each individual at least two sets of 10 trials with stimuli of animals and objects. Due to time constraints, once four individuals were able to drink from the setup during most of the practice sessions, we moved on to the experiment. Bonobos then participated in an experiment in which they could freely view socio-emotional and neutral scenes (presented at the same time) of unfamiliar conspecifics and of unfamiliar humans (Figure 2). Because the bonobos were not physically separated from other group members, the progression from trial to trial was manually controlled by the experimenter. This was done to ensure that data would only be collected when bonobos were attending the screen, and not when there were disturbances such as individuals moving away from the setup or individuals being distracted by others.
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