Discrimination of human and dog faces and inversion responses in domestic dogs (Canis familiaris)

Although domestic dogs can respond to many facial cues displayed by other dogs and humans, it remains unclear whether they can differentiate individual dogs or humans based on facial cues alone and, if so, whether they would demonstrate the face inversion effect, a behavioural hallmark commonly used in primates to differentiate face processing from object processing. In this study we first established the applicability of the Visual Paired Comparison (VPC or preferential looking) procedure for dogs using a simple object discrimination task with 2D pictures. The animals demonstrated a clear looking preference for novel objects when simultaneously presented with prior-exposed familiar objects. We then adopted this VPC procedure to assess their face discrimination and inversion responses. Dogs showed a deviation from random behaviour, indicating discrimination capability when inspecting upright dog faces, human faces and object images; but the pattern of viewing preference was dependent upon image category. They directed longer viewing time at novel (vs. familiar) human faces and objects, but not at dog faces, instead, a longer viewing time at familiar (vs. novel) dog faces was observed. No significant looking preference was detected for inverted images regardless of image category. Our results indicate that domestic dogs can use facial cues alone to differentiate individual dogs and humans, and that they exhibit a non-specific inversion response. In addition, the discrimination response by dogs of human and dog faces appears to differ with the type of face involved

Reading faces: differential lateral gaze bias in processing canine and human facial expressions in dogs and 4-year-old children

Sensitivity to the emotions of others provides clear biological advantages. However, in the case of heterospecific relationships, such as that existing between dogs and humans, there are additional challenges since some elements of the expression of emotions are species-specific. Given that faces provide important visual cues for communicating emotional state in both humans and dogs, and that processing of emotions is subject to brain lateralisation, we investigated lateral gaze bias in adult dogs when presented with pictures of expressive human and dog faces. Our analysis revealed clear differences in laterality of eye movements in dogs towards conspecific faces according to the emotional valence of the expressions. Differences were also found towards human faces, but to a lesser extent. For comparative purpose, a similar experiment was also run with 4-year-old children and it was observed that they showed differential processing of facial expressions compared to dogs, suggesting a species-dependent engagement of the right or left hemisphere in processing emotions

Children's total look.

<p>Mean Lateralisation Index (LI) and standard error (SE) of children's <i>Total look</i> for each image category (dog faces, human faces, and objects) and each emotional valence (negative, neutral, positive). *Significant deviation from chance (0) (2-tailed 1-sample <i>t</i>-test, *<i>p</i><0.05).</p

Dogs' total look.

<p>Mean Lateralisation Index (LI) and standard error (SE) of dogs' <i>Total look</i> for each image category (dog faces, human faces, objects) and each emotional valence (negative, neutral, positive) *Significant deviation from chance (0) (2-tailed 1-sample <i>t</i>-test, *<i>p</i><0.05; **<i>p</i><0.01; ***<i>p</i><0.001). ^# Significant differences between valence of emotions (Post-hoc tests within ANOVA; ^#<i>p</i><0.05; ^##<i>p</i><0.01; ^###<i>p</i><0.001).</p

Children' first look.

<p>Mean Lateralisation Index (LI) and standard error (SE) of children's <i>First look</i> for each image category (dog faces, human faces, and objects) and each emotional valence (negative, neutral, positive). *Significant deviation from chance (0) (2-tailed Wilcoxon signed rank tests, * <i>p</i><0.05; ** <i>p</i><0.01).</p

Children's standard and mirror sessions.

<p>Children' scores (mean±SE) and difference (P value) between standard and mirror sessions regarding both LI (First look) (2-tailed Wilcoxon Signed Rank tests) and LI (Total Look) (2-tailed paired <i>t</i>-tests), for each image category.</p