Scientists - and dog lovers - have long known that dogs are remarkably good at reading human laughs, frowns, and tears. But the biology behind this ability has remained largely unexplored, until now. In a current study published in the Cell Press journal iScience, researchers from the University of Vienna peered inside the brains of dogs as they viewed images of various expressions. The brain scans revealed that dogs process human smiles in a unique way and, for the first time, provided evidence that dogs can distinguish between certain negative facial expressions.

"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."

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. Happy faces lit up the temporal cortex and extended into the caudate nucleus, brain regions associated with higher cognitive and reward processing, respectively.

Happy human faces may have emotional significance for dogs

"The activity in the caudate nucleus is particularly interesting," says Cuaya, who began the research at the National Autonomous University of Mexico and later expanded it through a collaboration at Eötvös Loránd University in Hungary before moving to the University of Vienna in Austria. "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. The answer was no. The temporal cortex-caudate nucleus network responded uniquely to happiness, allowing researchers to tell smiles apart from all three negative expressions.

Dogs can distinguish various emotions in human faces

But the canine brain was not merely sorting the world into good and bad. 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.

"Humans are really good at picking up small details on the face, so are dogs," says first author Raúl Hernández-Pérez, also 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."

Yet the findings don't mean dogs experience emotions exactly as humans do. Also, the canine participants were all pets from loving families. Free-ranging dogs, such as those that live on the street, navigate a very different social world, and may process human cues differently. Outside the scanner, a dog doesn’t simply read a still face; it watches our body language, listens to nuances in our voices, and detects information from our scent.

Next, the team plans to adopt a more dog-centric perspective to understand how dogs integrate these 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."

Summary:

• For the first time ever psychologists from the University of Vienna provided evidence that dogs can read emotions in human facial expressions.
• The researchers analyzed dogs' brain scans using machine learning.
• Happy human faces lit up the temporal cortex and extended into the caudate nucleus, brain regions associated with higher cognitive and reward processing, respectively.
• A whole-brain analysis revealed distinct patterns of activity in dogs’ brains that separated fear from anger and sadness.

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About the University of Vienna:
At the University of Vienna, curiosity has been the core principle of academic life for more than 650 years. For over 650 years the University of Vienna has stood for education, research and innovation. Today, it is ranked among the top 100 and thus the top four per cent of all universities worldwide and is globally connected. With degree programmes covering over 180 disciplines, and more than 10,000 employees we are one of the largest academic institutions in Europe. Here, people from a broad spectrum of disciplines come together to carry out research at the highest level and develop solutions for current and future challenges. Its students and graduates develop reflected and sustainable solutions to complex challenges using innovative spirit and curiosity.

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In the video, researcher Magdalena Boch from the University of Vienna explains what dogs’ brains can also reveal about human social behaviour.