The familiar scent of its owner lingers like perfume in a dog's brain, US scientists claim.
They have pinpointed an area of the canine brain that responds more strongly to the scent of familiar humans that it does to the odour of other humans or even familiar dogs.
Study leader Gregory Berns, of Emory University, in Atlanta, Georgia, said: "When humans smell the perfume or cologne of someone they love, they may have an immediate, emotional reaction that's not necessarily cognitive.
"Our experiment may be showing the same process in dogs. But since dogs are so much more olfactory than humans, their responses would likely be even more powerful than the ones we might have."
"It's one thing when you come home and your dog sees you and jumps on you and licks you and knows that good things are about to happen. In our experiment, however, the scent donors were not physically present.
"That means the canine brain responses were being triggered by something distant in space and time. It shows that dogs' brains have these mental representations of us that persist when we're not there."
The experiment - the first of its type - involved 12 dogs of various breeds who underwent brain scans while five different scents who placed before them.
The scent samples came from the subject itself, a dog the subject had never met, a dog that lived in the subject's household, a human the dog had never met, and a human that lived in the subject's household.
The familiar human scent samples were taken from someone else from the house other than the handlers during the experiment, so that none of the scent donors were physically present.
The study has shown that "dogs' brains have these mental representations of us that persist when we're not there." Photo / Thinkstock
The results showed that all five scents elicited a similar response in parts of the dogs' brains involved in detecting smells. Responses were significantly stronger for the scents of familiar humans, followed by that of familiar dogs.
Dr Berns said: "This suggested that not only did the dogs discriminate the familiar human scent from the others, they had a positive association with it.
"While we might expect that dogs should be highly tuned to the smell of other dogs, it seems that the 'reward response' is reserved for their humans. Whether this is based on food, play, innate genetic predisposition or something else remains an area for future investigation."
The researchers said an interesting twist in their experiment showed that dogs which had received training as service/therapy dogs showed greater brain activity for the scent of a familiar human compared with the other dogs.
Dr Berns said the research would be useful in helping to source dogs that help wounded veterans or those trained to help disabled people. The findings are published in the science journal Behavioural Processes.