Dogs are exceptionally skilled at reading human emotions—from smiles and laughter to frowns and tears. Scientists and dog owners have long recognized this ability, yet the biological mechanisms that make it possible remain largely unknown.
A study published on August 7 in the Cell Press journal iScience offers a closer look at what happens in a dog’s brain when it sees various human facial expressions. Researchers found that smiles trigger specific activity in certain areas of the dog’s brain. They also discovered the first evidence that dogs can distinguish between several negative expressions, including anger, sadness, and fear.
“Humans are very good at noticing subtle facial details, and so are dogs,” says lead author Raúl Hernández-Pérez of the University of Vienna. “That makes dogs very interesting. By studying their brains, we can understand the adaptations that have evolved to support their remarkable social and emotional understanding of humans.”
How dog brains respond to human smiles
Scientists began by studying happiness, as previous research had shown that positive human expressions can be particularly significant for dogs.
They scanned the brains of eight dogs while the animals viewed photographs of unfamiliar people with either happy or neutral facial expressions. When the dogs saw happy faces, activity increased in the temporal cortex and the caudate nucleus. These brain regions are associated with higher-level cognitive processing and reward, respectively.
“The activity in the caudate nucleus is particularly interesting,” says Laura Cuaya, also of the University of Vienna, who initiated the research at the National Autonomous University of Mexico and later expanded it through collaborations at Eötvös Loránd University in Hungary. “It suggests that happy human faces may hold emotional significance for dogs.” Next, the scientists wanted to determine whether these same brain regions responded differently to other emotions. In a second experiment, 12 dogs were shown images of people expressing happiness, anger, fear, and sadness. The researchers used machine learning to analyze the resulting patterns of brain activity.
The dogs did not respond to all negative expressions in the same way. Activity in the temporal cortex and caudate nucleus network was particularly pronounced when the dogs viewed happy faces, allowing researchers to relatively easily distinguish reactions to smiles from reactions to anger, fear, and sadness.
Dogs Distinguish Between Negative Emotions
The results also showed that dogs do more than simply categorize human expressions as positive or negative.
Whole-brain analysis revealed distinct patterns of neural activity associated with fear compared to anger and sadness. According to the researchers, this provides the first evidence that dogs can differentiate between specific negative human facial expressions.
This does not necessarily mean that dogs experience emotions in the same way humans do. The researchers also note that a static image captures only a small fraction of real-world interaction. Dogs typically perceive humans through a multitude of simultaneous signals, including facial expressions, body language, tone of voice, and scent.
“When we compare humans to primates, we look at abilities that might have belonged to a common ancestor,” says senior author Laura Cuaya of the University of Vienna. “With dogs, the story is completely different. They followed a totally different evolutionary path but, through domestication, developed the social skills needed to understand and cooperate with us.”
Domestication May Have Influenced Dogs’ Social Skills
All the dogs in the study were family pets. The researchers note that free-ranging dogs—including those living on the streets—experience a vastly different social environment and may interpret human signals differently. Future studies will take a more dog-centric approach to examine how dogs integrate different signals when trying to understand the people around them. Researchers also plan to compare how dogs and humans perceive the same emotional information.
“I would like people to view dogs as emotional beings,” says Cuaya. “They evolved alongside us to understand our expressions and cooperate with us, and recognizing this could change how we interact with and care for them.”
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