Page 106 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 4
(Belopolsky et al., 2011). Our data are not fine-grained enough to disentangle the two processes, as eye-tracking is not yet optimized for apes. However, given that bonobos do appear to have an immediate bias towards playful and yawning scenes, but not attend to them longer when given the opportunity to do so, these categories likely elicit a bottom-up attentional process. Future studies could focus on distinguishing between bottom-up and top-down attentional processes, especially now that new eye-tracker models allow for greater sampling rates and are more forgiving in terms head movements (which is important when working with animals).
We expected a similar (but less pronounced) attentional bias pattern when the bonobos viewed emotional scenes involving humans. Although we did not find robust evidence that bonobos looked longer at any of the human emotions compared to neutral scenes, the looking duration pattern was similar to viewing bonobo scenes. Specifically, bonobos seemed to look slightly longer at humans expressing distress. These findings may be explained by human expressions of distress sharing similar morphological action tendencies as bonobo expressions of distress. For instance, a general feature of fearful expressions is the tendency to make oneself small, indicating weakness or submissiveness, and this occurs in humans and many other primates (Kret et al., 2020). Moreover, the scream face of apes shares a lot of morphological similarities with its human equivalent (Parr et al., 2007). Furthermore, the finding that bonobos looked longer to the neutral scenes that accompanied a human sex scene is curious. In the neutral scenes, people wore more clothing, which may provide a salient cue (e.g., due to more variation in patterns) than seeing people without clothes in the sex scenes (Van Renswoude et al., 2019). Finally, as scenes showing bonobos engaged in play or a grooming bout did not hold attention longer than neutral scenes, the human variant of these scenes is likely also not very salient to bonobos.
In experiment 2 with human participants, we found an overall preference for viewing emotional scenes over neutral scenes, and with human emotional scenes receiving slightly more attention than bonobo scenes. Humans showed the most pronounced effect in the distress category, with longer looking durations towards distressed conspecifics compared to the neutral scenes. Moreover, humans also preferentially looked at individuals that were embracing each other, playing, or having sex. An implicit attentional bias for threatening signals has been studied a great deal in humans. Most studies indicate that in highly anxious individuals, attention to negative or threatening stimuli is strongly prioritized (Armstrong & Olatunji, 2012). Results on non-anxious individuals are mixed, showing that an implicit bias towards
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