Dogs can tell the difference between human happiness, anger, fear and sadness just by looking at facial expressions, according to new research from the University of Vienna.
Researchers and pet owners have long suspected that dogs are unusually good at picking up on human emotions. Brain scans carried out by the Vienna team now show how the animals manage it.
The scientists found that seeing a human smile lights up a unique region of a dog's brain that does not react to negative facial expressions. Negative emotions, by contrast, are processed differently across the entire brain, suggesting a dog's understanding of human feelings is more complex than simply "good" or "bad."
Lead author Dr Raúl Hernández-Pérez, of the University of Vienna, told the Daily Mail: "Dogs are remarkably successful at living and cooperating with us, and emotional cues are an important part of that communication. While we cannot know the subjective emotional experience of dogs, their brain responses can tell us something about the cognitive and affective processes involved."
Training dogs for brain scans
Researchers recruited eight pet dogs that had been specially trained to lie still during fMRI, or functional magnetic resonance imaging, scans. The training meant the dogs could stay awake throughout the process, allowing scientists to show them pictures of smiling faces and watch how their brains responded.
The team found that smiling faces were processed in the temporal cortex and caudate nucleus, two areas of the brain linked to higher cognitive function and reward processing.
Co-author Dr Laura Cuaya, also of the University of Vienna, said: "The activity in the caudate nucleus is particularly interesting. It suggests that happy human faces may have emotional significance for dogs."
Previous studies have shown the caudate nucleus activates when dogs anticipate a food reward, hear praise in a positive tone, see their owners, or even smell a familiar person. In this study the faces shown to the dogs belonged to strangers, suggesting that happy human expressions may carry positive value for dogs even when they do not know the person.
Distinguishing anger, sadness and fear
The first test showed dogs could recognise happiness, but it did not reveal how they process negative emotions. In a follow-up experiment, researchers put 12 dogs through an fMRI scan and showed them pictures of humans who were angry, sad or afraid.
These emotions did not activate a single unique brain region the way happiness did. Instead, whole-brain analysis revealed distinct patterns of activity that separated fear, sadness and anger from one another.
The researchers believe dogs may have evolved this ability to distinguish human facial expressions over tens of thousands of years living alongside people. Dr Hernández-Pérez said: "During domestication, dogs became increasingly exposed to the human social world, a new and highly complex environment that required them to adapt their social skills to interact successfully with people. Dogs that were better able to pick up on human social and emotional cues may have had an advantage when cooperating with humans."
A different evolutionary path from primates
Scientists cannot use the brain scans to determine how dogs subjectively feel, but the findings offer insight into how the ability to perceive emotion develops. When researchers compare humans with primates, they are studying species that share a common evolutionary lineage. Dogs represent a different case entirely.
Dr Hernández-Pérez said: "Dogs and humans are evolutionarily distant species with very different brains, yet both can extract meaningful information from human emotional signals. This gives us an opportunity to study how a very different brain adapted to function in a complex human social environment, and how neural systems can become specialised for navigating social relationships and communicating across species."
Cats fall short by comparison
The findings come after separate research concluded that cats are largely unable to distinguish human emotions from limited cues. Unlike dogs, cats could not tell the difference between laughs, sobs, screams and shouts in that study, reacting to each sound in the same way.
Human voices almost always put the cats studied into a state of heightened alertness, but the emotion behind the noise made no difference to their reaction. The researchers behind that study concluded that cats are not able to distinguish humans' moods from voice alone.
