Visual perception of emotion cues in dogs: a critical review of methodologies

Abstract

Comparative studies of human-dog cognition have grown exponentially since the 2000's, but the focus on how dogs look at us (as well as other dogs) as social partners is a more recent phenomenon despite its importance to human-dog interactions. Here, we briefly summarise the current state of research in visual perception of emotion cues in dogs and why this area is important; we then critically review its most commonly used methods, by discussing conceptual and methodological challenges and associated limitations in depth; finally, we suggest some possible solutions and recommend best practice for future research. Typically, most studies in this field have concentrated on facial emotional cues, with full body information rarely considered. There are many challenges in the way studies are conceptually designed (e.g., use of non-naturalistic stimuli) and the way researchers incorporate biases (e.g., anthropomorphism) into experimental designs, which may lead to problematic conclusions. However, technological and scientific advances offer the opportunity to gather much more valid, objective, and systematic data in this rapidly expanding field of study. Solving conceptual and methodological challenges in the field of emotion perception research in dogs will not only be beneficial in improving research in dog-human interactions, but also within the comparative psychology area, in which dogs are an important model species to study evolutionary processes.

Keywords: Bodily expressions; Emotion cues; Facial expressions; Human–dog relationship; Methodology; Visual perception.

Fig. 1

Comparison of articles published until 2000 and from 2001 till 2020 available on GoogleScholar, searched using the keywords “dog cognition”, “dog perception”, or “dog emotion”. The same search using the terms “cat cognition”, “cat perception”, or “cat emotion” was used for comparison purposes in the same periods. The explosion of studies in these areas is particularly evident since the turn of the millennium for dogs: up to the year 2000, GoogleScholar displays only 10 results when searching, for example, for “dog cognition”, but in the next 20 years period (2001–2020) 720 results appear; “dog perception” returns 47 results pre-2000 and 170 studies since, while “dog emotion” returns 7 results pre-2000 and 86 results since. This represents a 72, 3.6, and 12.2 times increase for these research topics in dogs, respectively compared with only 3.4, 1.2, and tenfold increase for cats

Fig. 2

Facial landmarks in dogs and humans (adapted from DogFACS and HumanFACS, respectively: Ekman et al. ; Waller et al. 2013). The FACS systems are anatomically-based, standardised and objective methods of facial coding that avoid subjective labelling (e.g., "smile"). The position of facial landmarks in both species is arranged differently due to the variation in anatomical features such as skull shape, fat deposits, and hair coverage. For example, dogs do not have a forehead or eyebrows (anatomical features unique to humans) and instead have a frontal region and browridges. Pictures by Mouse23 from Pixabay.com (2021) and by Natalie Heathcoat from Unsplash.com (2021), free for commercial use

Fig. 3

Example of differences between characteristic facial cues of emotion in a human and dog (Ekman et al. ; Waller et al. 2013) in equivalent emotional contexts (Correia-Caeiro et al. ; Ekman et al. 1994). Fearful facial expressions in humans tend to include eyes wide open (AU5) and lip corners stretched horizontally (AU20) while dog fearful facial expressions tend to include panting (AD126). Happy facial expressions in humans tend to include the wrinkling around the eyes (AU6), while in dog happy facial expressions tend to include wide open mouths (AU27). AD: Action Descriptor, AU: Action Unit, AD126: Panting, AU5: Upper Lid Raise, AU6: Cheek Raise, AU20: Lip Stretch, AU27: Mouth Stretch. Dog images modified from Caeiro et al. (2017); Images by users Pexels and 2,843,603 from Pixabay.com (2021), free for commercial use, and by Sifis Kavroudakis from Youtube.com (2021)

Fig. 4

Examples of various experimental setups and equipment that can be used to investigate perception of emotion cues in dogs (pictures selected may not be from studies on perception of emotion cues as they are for illustrative purposes only). Experimental setups from: A Correia-Caeiro et al. (2020, 2021), B Barber et al. (2016), C Kis et al. (2017), D Ogura et al. (2020), E Faragó et al. (2010), F Lind et al. (2017), G Muller et al. (2015), H Albuquerque et al. (2021). Image 4-B and 4-G courtesy of Ludwig Huber. 1: Owner sitting behind or next to the dog, 2: Dog participant, 3: Frame for free-range of motion for the eye-tracker, 4: Eye-tracker camera, 5: Infrared camera, 6: Back-projected stimuli, 7: Experimenter facing away from the dog, 8: Eye-tracker target for eye triangulation, 9: LCD display, 10: Chin-rest, 11: Canvas with front-projected stimuli, 12: Speaker, 13: Grey board to pin stimuli, 14: Separator between stimuli pair, 15: Paper printed stimuli, 16: Touchscreen, 17: Owner involved in the task, 18: Experimenter performing emotional displays for the task

Fig. 5

Top left: Laptop screen displaying a coloured image in the visible spectrum; Bottom left: thermal image of same laptop screen after 5 min—the thermal differential is associated with the keyboard and screen base; Top right: LED monitor displaying a coloured image in the visible spectrum; Bottom right: thermal image of same LED monitor after 5 min, highlighting thermal gradient associated with different colours. Image courtesy of Tim Simon

Fig. 6

Examples of stimuli used in experiments aimed at investigating dog perception of facial expressions, with emotion labels and AOIs selected by the respective authors. A—Areas of Interest—AOIs labelled as “eyes”, “midface”, “mouth”, and “whole face”, adapted from Somppi et al. (2016), B—AOIs labelled as “forehead”, “eyes”, “mouth”, and “face rest”, adapted from Barber et al. (2016)