Viewing heterospecific facial expressions: an eye-tracking study of human and monkey viewers

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.

Human and monkey infant attention to dynamic social and nonsocial stimuli

“…We found that human infants looked longer and had more fixations on average than macaque infants, suggesting that human infants may have more sustained attention than macaque infants during the first year of life. Previous studies comparing human and macaque social attention (e.g., Damon et al., 2017; Guo, Li, Yan, & Li, 2019) have not directly compared species, as in the present study. In addition, we found that macaque infants spent proportionately more time looking to the social video compared to human infants, indicating that macaques’ relative interest in social stimuli may be greater than human infants’ relative interest in social stimuli, across the first year.…”

Bodily emotional expressions are a primary source of information for dogs, but not for humans

“…faces and bodies) is driven by task-relevant or situation-related diagnostic information contained in local image regions rather than AOI sizes or low-level local image saliency, such as local luminance or colour contrast (e.g. Guo et al 2019). Hence a 'uniform looking strategy' is not applicable in social attention research.…”

“…Mouth stretch-Action Unit 27 (AU27), where dogs open their mouths wide, and Cheek raiser-AU6, where humans contract the muscle around the eyes to pull the cheeks upwards as part of "happy" faces; Caeiro et al 2017b). When exploring different categories of human and dog facial expressions, human viewers gaze more frequently and for a longer time at the eyes of expressive human faces, but longer at the mouth of expressive dog faces (Guo et al 2019) or equally long at the eyes and mouth of expressive dog faces (Correia-Caeiro et al 2020). The lack of commonality in facial expressions and face-viewing gaze allocation between these two species questions the degree to which humans and dogs can appropriately interpret each other's emotional state based on facial expression alone.…”

“…(5) Do dogs and humans visually inspect emotionally expressive individuals (dog and human) in the same way? Although there is no study of human gaze allocation at the full body of dogs, based on previous research focusing on facial expressions, we predicted that human gaze would be affected by both the viewed species and emotional expressions (Guo et al 2019;Correia-Caeiro et al 2020) and that both age and sex would modulate gaze patterns at least at human bodies (Pollux et al 2016). We also predicted that the dog gaze would be affected by the viewed expressions and species since they are able to discriminate and recognise (at least some) prototypical facial expressions (Barber et al 2016;Correia-Caeiro et al 2020).…”

Experience has a limited effect on humans’ ability to predict the outcome of social interactions in children, dogs and macaques

“…Finally, to gain a deeper understanding of the processes involved, it would be crucial to determine whether certain body parts of the interacting individuals in the dyad are more salient than others, and how this could influence our ability to read emotions and predict animals’ behaviours (see 45 ). In addition, it could be of special interest within the framework of animal injuries prevention to control whether some body parts are more salient and thereby kept longer in the focus of attention than others, depending on the species observed or even the experience and the age of participants 81 , 82 .…”