University of Vienna researchers led by Dr Laura Cuaya have scanned dog brains to demonstrate how canine neural networks process human happy, sad, angry and fearful facial expressions.
The study, published on Monday in the scientific journal iScience, showed that viewing happy human faces triggers increased activity in canine brain areas associated with reward processing and higher cognitive function.
Cuaya explained that while human social skills shared with primates stem from common evolutionary ancestors, dogs developed their remarkable social abilities through thousands of years of domestic cooperation with humans.
Brain Scans and Neural Activity
In the first phase of the experiment, researchers scanned the brains of eight dogs, comprising six Border Collies, one Labrador Retriever and one Golden Retriever, while presenting them with photographs of strangers displaying either happy or neutral expressions.
Scans revealed that images of happy faces significantly increased neural activity within the temporal cortex and the caudate nucleus. Cuaya noted that this response suggests happy human faces carry intrinsic emotional significance for dogs.
In neuroscience, the temporal cortex is responsible for sensory processing and visual recognition, while the caudate nucleus is a key component of the basal ganglia involved in reward systems and learning across mammalian species. Functional magnetic resonance imaging, or MRI, measures these neural responses by detecting subtle changes in cerebral blood flow.
Researchers subsequently expanded the study to determine whether the same brain structures responded to a wider spectrum of human emotions. They monitored 12 dogs exposed to pictures of individuals feeling happy, angry, fearful or sad, using machine learning algorithms to analyze neural patterns.
Although the canine brain did not react to angry, fearful or sad expressions in the same manner as it did to happiness, a full-brain analysis revealed distinct patterns of neural activity when the dogs viewed faces expressing negative emotions. First author Raúl Hernández-Pérez emphasized that dogs share a human-like sensitivity to subtle facial details, making their neural adaptations particularly valuable for studying social evolution.
Reward Processing and Domestic Evolution
Cuaya and Hernández-Pérez, who are partners in both private and professional life, conducted the research alongside their two personal dogs, Odin and Kun-kun, who participated in the trials.
Hernández-Pérez told CNN that discovering the involvement of the caudate nucleus in processing happy faces was a pleasant surprise. He noted that in dogs, caudate activity has traditionally been linked to food rewards, as well as social rewards such as hearing praise or smelling a familiar person.
He added that although scientists cannot directly access the subjective experience of dogs, the findings indicate that viewing happy human faces, even those of strangers, engages positive emotional processing.
The research team emphasized that emotional processing in dogs extends beyond static visual images. Canines naturally integrate facial cues with body language, vocal tones and olfactory signals. The authors also noted that all participating dogs lived in loving household environments, noting that street dogs might process human cues differently.
Canine domestication began tens of thousands of years ago, leading to distinct physical and behavioral traits across modern breeds. Border Collies were originally bred as working herders along the Anglo-Scottish border, while Labradors and Golden Retrievers were developed as retrieval hunting dogs, with all three breeds recognized for high trainability and human social engagement.
Scanner Training and Research Methods
Hernández-Pérez told CNN that candidate dogs had to be specially selected and trained for scanner participation. Selection criteria required medium-sized, healthy dogs without behavioral issues such as severe anxiety from loud noises.
The training regimen initially taught dogs to remain completely motionless for one or two seconds before gradually increasing duration. Hernández-Pérez explained that one of the most challenging aspects was communicating to the dog that doing nothing was the actual task. Scientists then introduced the position, equipment and loud sounds of the MRI scanner gradually, ensuring all dogs participated voluntarily and could exit at any point.
The research involved international collaboration across several institutions. Cuaya initiated the research project at the National Autonomous University of Mexico in Mexico City, expanded it through a partnership with Eötvös Loránd University in Hungary, and completed the work after moving to the University of Vienna in Austria.
Study Limitations and Next Steps
The authors identified several limitations in their published paper. The sample size was small, and the majority of participants were Border Collies, a breed widely recognized for its exceptional responsiveness to human social signals.
Researchers noted that Border Collies may possess a heightened capacity to interpret facial expressions, emphasizing that further studies are needed to determine whether brain and behavioral responses vary across different breeds.
The research team plans to adopt a more dog-centered approach in future studies to investigate how canines utilize human emotional signals. Hernández-Pérez told CNN that reflecting on how attentively dogs monitor human emotions should encourage people to better recognize how dogs express their own feelings, highlighting emotion as a bridge between species.
