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If your dog looks pleased to see you – it is probably because it loves the particular way you smell.

The odour of a familiar human apparently lingers like perfume in the animal’s brain – where it triggers an instinctive emotional response, research published yesterday reveals.

Our scent acts on a part of the canine brain associated with reward and the strongest reactions are produced by humans that pets know best, say scientists in America.

Gregory Berns, of Emory University in Atlanta, said: ‘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.

'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.

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'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 university’s experiment - the first of its type - involved 12 dogs of various breeds who underwent brain scans while five different scents were placed in front of 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 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.

The findings, which were published in the journal Behavioural Processes, showed that dogs reacted strongest to the scent of a familiar human even when they were not there.

Pets trained as help or therapy dogs showed greater brain activity than the other dogs in the test.

Researchers say the findings could improve the way animals who assist wounded veterans or disabled people are selected.