The Processing of Human Emotional Faces by Pet and Lab Dogs: Evidence for Lateralization and Experience Effects

Abstract

From all non-human animals dogs are very likely the best decoders of human behavior. In addition to a high sensitivity to human attentive status and to ostensive cues, they are able to distinguish between individual human faces and even between human facial expressions. However, so far little is known about how they process human faces and to what extent this is influenced by experience. Here we present an eye-tracking study with dogs emanating from two different living environments and varying experience with humans: pet and lab dogs. The dogs were shown pictures of familiar and unfamiliar human faces expressing four different emotions. The results, extracted from several different eye-tracking measurements, revealed pronounced differences in the face processing of pet and lab dogs, thus indicating an influence of the amount of exposure to humans. In addition, there was some evidence for the influences of both, the familiarity and the emotional expression of the face, and strong evidence for a left gaze bias. These findings, together with recent evidence for the dog's ability to discriminate human facial expressions, indicate that dogs are sensitive to some emotions expressed in human faces.

Fig 1. Experimental set up.

(a) Lateral view of chin rest device, projection screen and feeding machines seen from the rear of the room during training with geometrical figures and (b) front view of the chin rest device including the eye-tracking camera with IR illuminator.

Fig 2. Examples of the experimental stimuli.

Fig 3. Scheme showing the areas of interest (AoI) for forehead, eyes, face rest, mouth and rest of the picture.

Fig 4. Total fixation time (+/- 95% CI) per trial for pet and lab dogs and subdivided for the emotion type.

Fig 5. Average fixation duration (+/- 95% CI) of the first fixation directed into the face region for pet and lab dogs.

Fig 6. Fixation count (Mean+/-SE) into an area of interest during a trial subdivided by dog type (a) and emotion type (b).

Fig 7. Frequencies [%] of the first fixations into the areas of interest subdivided for the emotion type with the subcategories dog type and familiarity.

Fig 8. Mean fixation start (+/- 95% CI) of the first fixation directed into the areas of interest.

Fig 9. Coordinates of the first fixation made into the face region subdivided for the dog type.

The origin corresponds to the middle of the vertical and horizontal dimension of the face.