Scientists—and dog lovers—have long known that dogs are remarkably good at interpreting our laughs, frowns, and tears. But the biology behind this ability has remained largely unexplored until now.
In a study published in the Cell Press journal iScience on August 7, researchers peered inside the brains of dogs as they viewed images of various human expressions. The brain scans revealed that dogs process smiles in a unique brain region and, for the first time, provided evidence that dogs can distinguish between negative facial expressions, including anger, sadness, and fear.
"Humans are really good at picking up small details on the face, and so are dogs," says first author Raúl Hernández-Pérez from the University of Vienna. "This makes dogs very interesting. By studying their brains, we can understand what adaptations have developed to support their remarkable social and emotional understanding of humans."
The researchers first focused on happiness, an expression that previous studies suggest holds particular significance for dogs. They scanned eight dogs while showing them images of strangers with either happy or neutral expressions. The happy faces lit up the temporal cortex and caudate nucleus in the dogs' brains—regions associated with higher cognitive and reward processing, respectively.
"The activity in the caudate nucleus is particularly interesting," says Laura Cuaya, also of the University of Vienna, who began the research at the National Autonomous University of Mexico and later expanded it through a collaboration at Eötvös Loránd University in Hungary. "It suggests that happy human faces may have emotional significance for dogs."
The team then asked whether the same brain regions respond to other emotions. They showed images of people displaying happiness, anger, fear, and sadness to 12 dogs and analyzed their brain activity using machine learning. They found that the dogs did not react in the same to these negative emotions; the temporal cortex-caudate nucleus network responded uniquely to happiness, allowing researchers to easily distinguish brain reactions to smiles from all three negative expressions.
But the canine brain was not merely sorting the world into good and bad, the team found. A whole-brain analysis revealed distinct patterns of activity that separated fear from anger and sadness, providing the first evidence that dogs can distinguish between specific negative facial expressions.
The researchers say that this ability does not mean that dogs experience emotions exactly as humans do. Also, a still photo of a human face doesn't tell the whole story; in real life, dogs watch human body language, listen to nuances in our voices, and detect information from our scent.
"When we compare humans with primates, we are looking at abilities that may have come from a common ancestor," says senior author Laura Cuaya of the University of Vienna. "With dogs, the story is totally different. They followed a very different evolutionary path, yet through domestication, they developed the social abilities needed to understand and cooperate with us."
In this study, the canine participants were all pets from loving families, notes the team. Free-ranging dogs, such as those that live on the street, navigate a very different social world and may process human cues differently.
Next, the researchers plan to adopt a more dog-centric perspective to understand how dogs integrate these brain signals to make sense of the humans around them. Their future work will also compare how human and dog brains process the same emotional signals.
"I would like people to see dogs as emotional beings," says Cuaya. "They have evolved with us to understand our expressions and cooperate with us, and recognizing that can change how we communicate with and care for them."
