Dogs are excellent students of human behavior. They know when we're about to leave the house. They recognize the suspicious sound of a cheese wrapper opening in the kitchen. And, somehow, they always seem to know when we're getting ready to give them a bath.

But, their interest in us goes beyond our daily routines. Scientists have discovered that dogs can tell the difference between human facial expressions, and they seem to respond to our smiles.

A Smile Is Worth a Thousand Words

At the University of Vienna, eight dogs had a rather unusual assignment: lie perfectly still inside a brain scanner while looking at photographs of strangers. Some of the people were smiling, while others wore neutral expressions.

The dogs were awake throughout the experiment, which made their cooperation rather impressive. Anyone who's tried to persuade a dog to stay still for a photograph will understand.

When the dogs saw smiling faces, activity increased in two areas of their brains. One is involved in processing information, and the other is associated with rewarding experiences. So, a human smile may carry a little extra significance for our canine companions. And, remember, these weren't familiar faces. The dogs were responding to pictures of people they'd never met.

Nobody Likes an Angry Face

Then, the scientists gave 12 dogs a slightly more complicated assignment. Instead of looking at smiles and neutral expressions, they saw people displaying happiness, anger, fear, and sadness.

The dogs' brains responded differently, depending on the expression. Happy faces produced particularly distinctive patterns, but the researchers also found differences in how the dogs responded to fearful faces, compared with angry or sad ones.

The results, published in iScience, suggest that dogs can distinguish between different human emotions just by looking at our faces. Of course, recognizing an expression isn't the same as understanding exactly what someone is feeling. A dog might recognize an angry face without knowing whether we're upset about a parking ticket or the fact that somebody ate our lunch.

Your Dog Has Been Paying Attention

Dogs have lived with humans for thousands of years, giving them plenty of opportunities to become familiar with our behavior.

Facial expressions are only part of the picture. Dogs also notice changes in our voices and body language. And their sense of smell provides information we don't even realize we're giving away. Together, those clues may help to explain why some dogs seem so attentive when their owners are upset.

The Vienna experiments were small, involving eight dogs in one test and twelve in another. And, of course, scientists can't tell what an animal is thinking from a brain scan. Still, the findings offer an interesting glimpse into the remarkable relationship between people and dogs.

The Face That Gets the Best Reaction

It's kind of funny to think that our dogs spend so much time studying us. We're busy trying to figure out whether a wagging tail means excitement or whether that particular bark means somebody's at the door. Meanwhile, they're watching our faces.

And, if a smile really does have special significance to them, that's one more reason to be cheerful when greeting our favorite four-legged friend, even if they've just stolen our seat on the sofa.

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