Page 37 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Emotional attention is modulated by familiarity
Stimuli and validation
Stimuli consisted of bonobo pictures collected in different zoos and from the internet.
Stimuli of familiar individuals consisted of pictures of the group living in Apenheul,
and unfamiliar stimuli depicted a small selection of individuals from five different 2 zoos (Cologne, Planckendael, Twycross, Cincinatti, and San Diego Zoo). We only
included clear pictures in our sample (i.e., no pixelations, adequate lighting). In total,
the study included 656 novel and unique pictures (346 of familiar and 310 pictures
of unfamiliar individuals). All pictures were resized to 330 x 400 pixels and showed
either a neutral scene (i.e., individuals sitting or lying down or involved in a non-social
activity, showing a neutral expression) or an emotional scene.
While we currently do not fully understand what bonobo emotions entail, we rely on existing observational work to establish relevant socio-emotional behaviors and expressions that may underlie emotional states. Here, the valence-arousal model by Russel (1980) can be used as a useful guideline. We use socio-emotional scenes of bonobos engaged in play, grooming, or sex (positively valenced), and bonobos showing distress or that were self-scratching (negatively valenced), or yawning (unclear valence) as proxies of emotional states (see Figure 2 and, in the supplements, Table S1). We used similar emotion categories as Kret et al. (2016) (but all novel images), with the exception that we included self-scratching as a new category and left out pant hoot and food, because these did not attract attention over neutral scenes in our previous study.
Play, grooming, and sex are important for establishing or maintaining social bonds (Moscovice et al., 2019; Palagi, 2008; Schroepfer-Walker et al., 2015), and may therefore reflect positively valenced behaviors (Furuichi, 2011). Play scenes involved playful interactions between two bonobos, or an individual playing with objects, and included the relaxed open-mouth (‘play face’) expression (Signe & Van Hooff, 2018). Grooming scenes involved grooming bouts between two or more individuals. Furthermore, sexual scenes displayed two or more individuals copulating, or showing an erection (males) or large genital swelling (females). Scenes showing distressed bonobos included one or more individuals displaying a fear grin that is typically produced by primates in distress (De Waal, 1988; Parr et al., 2007). Self-scratching scenes displayed one or two individuals scratching themselves on the head or body. Self-scratching is indicative of stress in both primates and humans (Troisi, 2002), and by incorporating it as an emotional stimulus, we increased the number of negatively valenced stimuli. Finally, yawning scenes showed one individual with an open mouth, with or without teeth exposure. It is unclear what emotional state may underlie
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