Page 44 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 2
allocated. Indeed, Lewis et al. (2021) showed that bonobos preferentially attend to familiar, high-ranking females rather than unfamiliar females, whereas chimpanzees attend to familiar high-ranking males. We believe the study by Lewis et al. (2021) and ours complement each other in showing that at least for bonobos, seeing familiar individuals brings along a range of potentially relevant social information such as rank and emotional expressions that in turn may modulate attention differently. Therefore, we believe an interesting next step would be to study more closely how familiarity with the expressor modulates attention for emotions across species.
Experiment 2: Bonobos’ attentional bias towards emotions of familiar and unfamiliar humans
Method
Participants and equipment
The bonobos taking part in Experiment 2 were the same as in Experiment 1. Experiment 2 followed directly after Experiment 1, and used the same setup.
Stimuli and validation
Stimuli consisted of isolated emotional and neutral human faces that were either unfamiliar to the bonobos (NimStim Set of Facial Expressions (Tottenham et al., 2009)) or familiar (4 female bonobo animal caretakers that interact with the bonobos on a daily to weekly basis, and 2 female experimenters that trained and tested the bonobos in the past). Emotional expressions consisted of six basic human expressions (Ekman, 1999): anger, fear, happiness, sadness, surprise, and disgust. (Figure 5). Stimuli were in color and sized 330x400 pixels. In total we had 144 stimuli (72 of familiar and 72 of unfamiliar individuals).
While making the photos of the caretakers, the experimenter enacted the facial expressions as an example for the caretakers and instructed them to mimic her. Photos of emotional expressions were taken in the following order: anger, fear, happiness, sadness, surprise, disgust. Each photo of an emotional expression was followed by a photo of a neutral expression to ensure that neutral photos were slightly different from each other. If the experimenter thought a photo was not similar enough to the faces used from the NimStim database, the photo was retaken. Stimuli from familiar individuals were all unique. However, because the NimStim database sometimes
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