![]() ( 2022) found that bonobos’ performance on an emotional Stroop task was affected by seeing play faces, also considered a positively valenced social signal (Palagi, 2008). In line with these findings on prioritized processing of emotional signals, other works suggest a sensitivity to emotions in bonobos. ( 2016) observed that bonobos ( Pan paniscus) have an attentional bias towards (mainly positive) emotional cues such as scenes involving sexual behavior, yawning, and grooming. Moreover, this bias towards emotions is likely not uniquely human, and findings suggest it is at least shared with bonobos ( Pan paniscus), our closest living relatives. Additionally, pupil size changes temporally align with the moment individuals make a decision about the nature of a social signal, suggesting a link between pupil changes and emotion recognition (Oliva & Anikin, 2018). Conversely, pupil constriction is associated with fatigue and boredom (Lowenstein et al., 1963). Furthermore, the dilation of the pupils may reflect a variety of socially relevant signals, such as social interest and attention, surprise or uncertainty, and arousal in combination with several emotions (Bradshaw, 1967 Hess, 1975 Lavín et al., 2014). For instance, in several studies, participants showed pupil dilation in response to viewing stimuli with emotionally relevant content (Lester & Gatto, 1990 Muris et al., 2001 Kret, Roelofs et al., 2013, Kret, Stekelenburg et al., 2013), and this phenomenon already occurs in infancy (Geangu et al., 2011 Fawcett et al., 2017 Aktar et al., 2020). ![]() Although one primary function of the pupil is to dilate or constrict in response to light, it can also reflect autonomic responses originating from within the individual. ![]() Moreover, when zooming in on the eyes, the pupils provide a subtle window into an individual’s emotional state. ![]() Eyes not only enable an individual to see the world around them but also serve as a reference point for other individuals (Silk et al., 2008). In the current study, we therefore investigated whether humans and bonobos have an attentional bias towards conspecifics’ pupil size in a dot-probe task.Įye contact may provide a powerful mode of sharing emotions through subtle expressions. Moreover, given that emotional expressions also receive prioritized attention in some other, non-human primates (Kret et al., 2016 Lacreuse et al., 2013 Parr et al., 2013), an important question is whether a potential bias towards pupil size changes is uniquely human or a derived trait shared with other species. However, whether an attentional bias towards emotional cues extends to these subtle signals is still unknown. Several studies suggest that subtle signals, like changes in pupil size, may also play an important communicative role (Kret, 2015). One trait that enables individuals to respond appropriately is an attentional bias towards emotional signals (Phelps et al., 2006 Schupp et al., 2003 Vuilleumier, 2005 van Rooijen et al., 2017). Due to methodological differences between the two experiments, more research is needed before drawing a conclusion regarding bonobos.įor social animals such as humans and bonobos, it is essential to estimate a group member’s emotional state accurately and quickly (Darwin, 1872 LeDoux, 1995 Kret et al., 2020). These results suggest that the attentional bias towards emotions in humans can be extended to a subtle unconsciously produced signal, namely changes in pupil size. Our analysis did not show a significant effect in bonobos. In humans, our analysis showed a small but significant attentional bias towards dilated pupils compared to intermediate-sized pupils and intermediate-sized pupils when compared to small pupils. We used linear mixed multilevel models to examine the effect of pupil size on reaction times. Four female bonobos (6 to 17 years old) completed the dot-probe task presented via a touch screen. A total of 150 human participants (141 female), with a mean age of 19.13 (ranging from 18 to 32 years old), completed an online dot-probe task. Therefore, the current study investigated whether attention is biased towards pupils of differing sizes in humans and bonobos. While an attentional bias towards more obvious emotional expressions has been shown, it is unclear whether this also extends to a more subtle implicit cue, like changes in pupil size. Pupil size can impact social judgment and foster trust and social support, and is automatically mimicked, suggesting a communicative role. Although many emotional signals are explicit and overt, implicit cues such as pupil size also contain emotional information for observers. Previous work has established that humans have an attentional bias towards emotional signals, and there is some evidence that this phenomenon is shared with bonobos, our closest relatives.
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