One face was positive, expressing happiness or a state of play, and another was negative, expressing anger or aggression.
For humans, a voice that was unfamiliar and in an unknown language to the dog accompanied the image.
A British dog heard the voice of a Portuguese woman, for example.
"Previous studies have indicated that dogs can differentiate between human emotions from cues such as facial expressions, but this is not the same as emotional recognition," researcher Dr. Kun Guo said.
The Huffington Post reported that dogs in the study lingered on images that matched in appearance and sound.
So regardless of whether it was a human or a dog, the pups stayed engaged when the image and sound were both positive or when both were negative.
"Our study shows that dogs have the ability to integrate two different sources of sensory information into a coherent perception of emotion in both humans and dogs," Guo continued. "To do so requires a system of internal categorization of emotional states."
So when you frown at your pup and use a stern tone when it chews on your socks, it's possible it knows you're upset with it.
And when you smile at your pet and use a softer, happier tone, it's possible your dog knows you're happy to be with it.