A dog watching its owner smile may be doing much more than simply looking at a familiar face. Inside its brain, that smile appears to trigger a distinct response, one that researchers can tell apart from the reactions produced by anger, fear or sadness.
Scientists have long known that dogs are unusually good at reading people. They respond to our voices, gestures and facial expressions, and often seem able to tell when their owners are happy or upset. But researchers have known much less about what happens inside a dog’s brain when it sees these emotions.
A new study, published on 7 August in iScience, offers a closer look. By scanning dogs’ brains as they viewed photographs of human faces, researchers found that dogs process happy expressions in a distinctive way. They also found, for the first time, evidence from brain activity that dogs can distinguish between different negative human expressions.
“Humans are really good at picking up small details on the face, and so are dogs,” says Raúl Hernández-Pérez of the University of Vienna and first author of the study.
“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 because earlier studies suggested that human smiles may have special importance for dogs.
They scanned the brains of eight pet dogs while showing them photographs of strangers displaying either happy or neutral expressions. When the dogs saw happy faces, two areas of their brains became particularly active: the temporal cortex, involved in processing complex information, and the caudate nucleus, which is linked to reward and motivation.
For Laura Cuaya of the University of Vienna, who began the research at the National Autonomous University of Mexico and later expanded it through a collaboration with Eötvös Loránd University in Hungary, the response in the reward-related area was especially striking.
“The activity in the caudate nucleus is particularly interesting,” Cuaya says. “It suggests that happy human faces may have emotional significance for dogs.”
But the researchers wanted to know whether dogs simply separated positive expressions from negative ones, or whether their brains could tell different negative emotions apart.
In another experiment, 12 dogs were shown photographs of people displaying happiness, anger, fear or sadness. The researchers then used computer analysis to search for differences in the patterns of brain activity produced by each expression.
Smiles stood out clearly.
The network involving the temporal cortex and caudate nucleus responded differently to happiness than it did to all three negative expressions. Based on those signals, the researchers could readily distinguish the dogs’ brain responses to happy faces from their responses to angry, fearful or sad ones.
Yet the results suggest the dogs were doing more than dividing human faces into simply “good” and “bad”.
When the scientists examined activity across the entire brain, they found different patterns associated with fear, anger and sadness. According to the researchers, this provides the first brain-based evidence that dogs can distinguish between specific negative human facial expressions.
That does not necessarily mean a dog understands sadness or anger in the same way a person does, the researchers caution. Nor does it prove that dogs themselves experience exactly the same emotions.
The experiments also relied on still photographs, while real-life communication is much richer. Dogs can watch a person’s posture and movements, hear changes in their voice and even gather information through smell.
Another limitation is that the dogs in the study were pets living with families. Their long experience interacting closely with people could influence how their brains respond to human faces. Free-ranging or street dogs, which have very different relationships with people, might respond differently.
Still, the findings could help researchers understand how thousands of years of living alongside humans have shaped dogs’ social abilities.
Unlike humans and other primates, dogs did not inherit these skills from a recent common ancestor with us. Instead, their ancestors followed a very different evolutionary path before domestication brought dogs and humans into increasingly close relationships.
“Through domestication, they developed the social abilities needed to understand and cooperate with us,” Cuaya says.
The researchers now want to investigate how dogs combine facial expressions with other signals when deciding what a person is feeling or likely to do. They also hope to compare human and canine brains responding to the same emotional information.
For Cuaya, the research also carries a simple message for people who live with dogs.
“I would like people to see dogs as emotional beings,” she says. “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.”
