Page 58 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 2
were differences between the number of unique individuals in the stimulus sets. Bonobos saw multiple unique individuals in Experiments 1 and 2, and humans saw only one familiar and one unfamiliar individual in Experiment 3. These discrepancies are important to note, but still allow for a qualitative comparison of the results. To the best of our knowledge, this study is the first to look at the modulating effects of familiarity on attention for emotions. Thus, we opted to base our study designs on existing literature on attentional biases for emotional facial expressions in humans (see Van Rooijen et al., (2018) for a review) and emotional scenes in bonobos and chimpanzees (Kret et al., 2016, 2018).
Finally, while our results extend previous findings by Kret et al. (2016), the average difference between emotional and neutral trials in the unfamiliar condition is numerically smaller than the difference reported by Kret and colleagues (i.e,. about 12 ms in our study versus 34 ms in the original study). This is likely due to crucial methodological differences. First, the trials in the original study by Kret et al., (2016) paired emotional or neutral bonobos with control animals (sheep or rabbits). In order to directly test how emotional and neutral scenes compete for attention, in the current study, we chose to present emotional and neutral stimuli within the same trial. Second, Kret et al. (2016) used slightly different categories, i.e., eating and panthoot, but these categories did not elicit an attentional bias and thus were replaced by self- scratching in our study. Third, our design also included stimuli of familiar individuals, which likely attenuated the effect we found for unfamiliar individuals.
To conclude, our study contributes to the understanding of how evolution shaped other-regarding preferences of bonobos and humans by showing that they are deeply ingrained in early social perception and, crucially, are shared between the species. The results also demonstrate that how familiarity modulates emotional attention can differ between species. Importantly, differences in the environments of bonobos and humans may have helped shape the striking differences in how bonobos and humans attend to emotions of familiar and unfamiliar others. It could therefore be interesting for future work to examine the link between emotional attention and familiarity in a wider range of species, progressing our understanding of the origins of the social mind.
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