Dogs’ Brains Distinguish Between Human Smiles and Negative Expressions

Dogs have long been known for reading human laughs, frowns, and tears, but the biology behind that ability had remained largely unexplored. A study published in iScience used brain scans to examine how dogs process human facial expressions, revealing that dogs handle smiles in a distinct brain region and, for the first time, showing evidence that dogs can tell apart different negative 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 began with happiness, scanning eight dogs as they viewed images of strangers showing either happy or neutral expressions. Happy faces activated the temporal cortex and caudate nucleus, regions tied to higher cognitive processing and reward, respectively. “The activity in the caudate nucleus is particularly interesting,” says senior author Laura Cuaya. “It suggests that happy human faces may have emotional significance for dogs.”

The team then tested whether the same regions responded to other emotions, showing 12 dogs images of happiness, anger, fear, and sadness and analyzing the results with machine learning. The temporal cortex-caudate nucleus network responded uniquely to happiness, making it easy to distinguish from the three negative expressions. A separate whole-brain analysis showed the canine brain wasn’t just sorting faces into good and bad—distinct patterns separated fear from anger and sadness, offering the first evidence that dogs can distinguish between specific negative expressions.

The researchers caution that this doesn’t mean dogs experience emotions exactly as humans do, and that a still photo only tells part of the story, since dogs also read body language, vocal tone, and scent in real life. Cuaya notes that unlike primates, who may share emotional processing with humans through common ancestry, dogs “followed a very different evolutionary path, yet through domestication, they developed the social abilities needed to understand and cooperate with us.” All the dogs studied were pets from loving homes; free-ranging dogs may process human cues differently. Next, the team plans to study how dogs integrate these signals and compare human and dog brain processing of the same emotions.