Researchers at the University of Vienna found happy expressions activated brain regions linked to reward, while fearful, angry and sad faces did not produce the same response. The findings suggest dogs attach emotional significance to human smiles and distinguish between facial expressions.

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The researchers also found patterns of brain activity indicating dogs can tell the difference between fear, anger and sadness. Senior author Laura Cuaya said the study is the first brain-scan evidence showing happy human faces have emotional significance for dogs. The team plans further research comparing how human and canine brains respond to the same emotional signals.