Abstract
Common facial expressions of emotion have distinctive patterns of facial muscle movements that are culturally similar among humans, and perceiving these expressions is associated with stereotypical gaze allocation at local facial regions that are characteristic for each expression, such as eyes in angry faces. It is, however, unclear to what extent this ‘universality’ view can be extended to process heterospecific facial expressions, and how ‘social learning’ process contributes to heterospecific expression perception. In this eye-tracking study, we examined face-viewing gaze allocation of human (including dog owners and non-dog owners) and monkey observers while exploring expressive human, chimpanzee, monkey and dog faces (positive, neutral and negative expressions in human and dog faces; neutral and negative expressions in chimpanzee and monkey faces). Human observers showed species- and experience-dependent expression categorization accuracy. Furthermore, both human and monkey observers demonstrated different face-viewing gaze distributions which were also species dependent. Specifically, humans predominately attended at human eyes but animal mouth when judging facial expressions. Monkeys’ gaze distributions in exploring human and monkey faces were qualitatively different from exploring chimpanzee and dog faces. Interestingly, the gaze behaviour of both human and monkey observers were further affected by their prior experience of the viewed species. It seems that facial expression processing is species dependent, and social learning may play a significant role in discriminating even rudimentary types of heterospecific expressions.
Highlights
Because a significant part of emotional expressions are achieved through movements of facial muscles, facial expressions provide crucial visual cues for humans and a range of non-human mammal species to understand other’s emotional state and intention
Recent eye-tracking studies have observed that when categorizing facial expressions, we tend to look more often at local face regions that are most characteristic for each expression category (Eisenbarth and Alpers 2011; Guo 2012, 2013; Guo and Shaw 2015), suggesting that gaze allocation at the eyes, nose and mouth regions could be systematically influenced by the viewed facial expressions
To maximally promote and distinguish between these two processes, we only presented the very basic high-intensity angry and happy conspecific and heterospecific facial expressions, as these expressions are likely to be homologous in morphological features and social functions across the presented species (Leopold and Rhodes 2010; Schirmer and Adolphs 2017), and humans need little learning or experience to recognize high-intensity human happy and angry expressions as suggested by developmental and cross-culture studies (Gao and Maurer 2010; Yan et al 2016)
Summary
Because a significant part of emotional expressions are achieved through movements of facial muscles, facial expressions provide crucial visual cues for humans and a range of non-human mammal species to understand other’s emotional state and intention. As these earlier studies are not designed to compare gaze behaviour in viewing of facial expressions of different categories and different species, we do not know the extent to which the “universality” and “social learning” processes are involved in interspecies emotion perception To further explore these contrasting views in perceiving conspecific and heterospecific facial expressions, in two separate eye-tracking experiments we examined faceviewing gaze allocation of human (including dog owners and non-owners) and monkey participants while exploring human, chimpanzee, monkey and dog faces with positive (happy/playful/appeasement), relaxed (neutral) and negative (angry/threatening) expressions. The “social learning” process would predict an experiencedependent expression categorization accuracy in human viewers (e.g. humans show higher categorization accuracy for human faces than for faces of other species, and dog owners show higher categorization accuracy for dog faces than non-owners), and a species-dependent face-viewing gaze distribution in both human and monkey viewers (e.g. different levels of experience may modify amount of attention directed at the eyes in ‘angry’ human, chimpanzee, monkey and dog faces)
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