Page 38 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms
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                                Chapter 2
yawning (e.g., boredom (Burn, 2017) or stress (Maestripieri et al., 1992; Paukner & Anderson, 2006)), but it is a highly contagious behavior that could be a proxy for empathy (but see Massen & Gallup, 2017). Moreover, bonobos responded faster to probes replacing yawning stimuli than other categories in the study by Kret et al. (2016), and therefore we included it in our study.
We matched emotional and neutral scenes on the number of individuals depicted (ranging from one to six), their identity, and by visual inspection of color and luminance. All 12 bonobos in the Apenheul group were present in the familiar stimulus set, and we estimate the presence of 30 unique individuals in the unfamiliar stimuli. Furthermore, the pictures were cropped in such a way that the bonobos’ faces and/or bodies covered most of the stimulus area. Backgrounds of the stimuli either showed a bit of grass or part of a tree, or, when the stimuli were of individuals in their inside enclosure, of a white-grey wall and sometimes a beam (part of the inside construction). All pictures were rated on emotional valence and intensity (arousal) by three primate experts from Apenheul and three primate researchers, who showed high intraclass correlations (ICCvalence = .82, ICCintensity = .87, supplements, Table S2).
Figure 2. Examples of stimuli of all emotional categories used in Experiment 1. An emotional picture was always paired with a neutral picture. The emotional and neutral pictures were of either familiar or unfamiliar individuals.
Procedure
The bonobos were already familiarized with the dot-probe procedure during a previous study (Kret et al., 2016), but did go through a short refamiliarization phase (about 7 sessions per ape spaced across a 2-month period). During this phase,
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