The saying goes “dog is man’s best friend” and now scientists say they have proved it.

Researchers from the University of Mexico discovered that dogs recognised the emotional state of humans by reading visual facial cues.

They claim it is evidence that dogs have a rich social relationship with humans.

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Seven domestic dogs were trained to remain awake, still and unrestrained inside an MRI scanner for the four-month study.

The scientists then showed the dogs pictures of different human facial expressions and images of everyday inanimate objects.

They discovered that brain activity in the dogs dramatically changed when they viewed images of human faces.

“In this case, they presented facial expressions and worked out that basically the same areas of the brain triggered in dogs as it does in humans in terms of reading and understanding facial cues,” said Bradley Smith, an animal behaviourist from the Central Queensland University.

“Whether dogs perceive the same way as humans do, that’s what we don’t know.”

The researchers believe the part of the dogs’ brain that recognises humans could be anatomically and functionally similar to regions found in other species, like humans, monkeys and sheep.

Dr Smith said this study highlighted how unique canines were.

“So a lot of species can read their own species’ facial cues but dogs can actually follow their own and also human,” he said.

The study included five border collies, one labrador retriever and one golden retriever, all recruited through local families.

“So breeds like border collies … and hunting dogs that work alongside people, that have been trained to work with us, they’re the ones that are really good at paying attention and following our cues because that’s what we designed them to do,” Dr Smith said.

Nathan Williams is a dog behaviour specialist based in Queensland and he works with problematic dogs in animal shelters.

He said dogs had evolved over tens of thousands years to detect even the most nuanced changes to expression.

“It’s deeper than that, like a dog can recognise threat in a person better than us when the facial expressions are very subtle.

“They’re so subtle that we can’t read them that well but a dog can recognise it.”

Scientists are keen to try the same experiment on cats in the hope of settling the age old debate of whether dogs or cats are better as pets.

But they do not like their chances of being able to scan a cat in an MRI machine.

-ABC