Page 57 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms
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                                Emotional attention is modulated by familiarity
The results of our experiments should be viewed in light of the study’s limitations.
The most pertinent one is the disparity between our bonobo and human sample size. Unfortunately, comparative studies often suffer from low sample sizes due to the
limited access to individuals and the major efforts and resources that are necessary 2 to conduct non-invasive experiments with animals. Indeed, a recent overview of touchscreen-based studies with great apes in zoos shows an average sample size of
four (Egelkamp & Ross, 2019). That said, the findings are still valuable for understanding our own evolutionary roots and great apes’ socio-cognitive competencies (similar to how findings on one or two patients with unique brain lesions have been crucial for understanding the neuroscientific foundations of emotion recognition (e.g., Adolphs et al., 1994)). In our study, we partly replicate earlier findings by Kret et al. (bonobos: 2016, chimpanzees: 2018), showing that this type of work is fruitful and can lead to reliable results. We also report individual means in the hopes that these data can eventually lead to combined datasets for future examinations of great ape (social) cognition.
A second limitation is that we were only able to test female bonobos. For ethical reasons, we did not separate individuals from the group while testing, thus it was difficult for the three potential male subjects to get tested (because the females were eager to participate and did not allow the males behind the screen). Nevertheless, this makes it difficult to generalize our findings. For instance, it is possible that there are sex differences in attentional biases for emotions, and in humans, there is some evidence for this idea (for a review, see Kret & De Gelder, 2012). Nevertheless, we did not find any effect of sex of the participant performing the dot-probe and sex of the individual on the stimuli in Experiment3 (Table S11.2). We recently also conducted a dot-probe study involving human emotional scenes (rather than faces), and found no sex differences in attentional bias towards emotional scenes (Kret & Van Berlo, 2021). Yet, two primate studies did show that sex can impact attention allocation (Lewis et al., 2021; Schino et al., 2020). It therefore remains possible there are sex differences in bonobos’ attention for emotions (particularly in relation to familiarity) that we could not capture in our study.
Another limitation in our study involves the differences between the configuration of the stimuli used for bonobos (emotional and neutral scenes in Experiment 1, facial expressions in Experiment 2) and humans (also facial expressions in Experiment 3). Moreover, there were discrepancies between the emotional categories used in the experiments, as we used socio-emotional categories for our bonobo stimuli, and basic human emotional expressions for our human stimulus set. Finally, there
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