New research on how dogs perceive smell reveals that canines' brains produce a reward signal more strongly in the presence of familiar humans than they do in the presence of familiar dogs or unfamiliar humans.

The process may be similar to when a person smells the perfume or scent of a person they love; the smell triggers an emotional reaction, though not necessarily a cognitive one.

"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," said study leader Gergory Berns of Emory University.

The study was the first brain-imaging research on dogs responding to biological odors.

"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," Berns said in a news release. "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."

Research collaborator Mark Spivak said that dog's most powerful and important sense is olfaction, their sense of smell.

For their experiment, the researchers used 12 dogs of various breeds. Each of the dogs had been trained to lay perfectly still while undergoing an fMRI scan. As the dogs' brains were being scanned the subjects were presented with different scents. Each of the five scents was sterile and isolated from other odors.

The scent samples came from the test subject itself, an unfamiliar dog, a dog that lived in the same household, a human the dog had never met, and a human that lived in the dog's household. To control the experiment, the researchers made sure the scent of the familiar human was someone other than the dog's handler while the experiment was being conducted. Human scents were collected from armpits, dog scents were collected from genital areas.

"Most of the dog owners and handlers involved in the experiment were women, so most of the familiar human scent donors were their husbands," Berns said. "We requested they not bathe or use deodorant for 24 hours before taking the sample. Nobody was too happy about that."

The fMRI scans indicated that for each of the scents tested elicited a similar response in region of the dogs' brains dedicated to detecting smells. However, the strength of the response varied, with the familiar human eliciting the strongest response, followed by that of familiar dogs.

"The stronger caudate activation suggested that not only did the dogs discriminate the familiar human scent from the others, they had a positive association with it," Berns 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. Whether this is based on food, play, innate genetic predisposition or something else remains an area for future investigation."

Berns and his colleagues published their work in the journal Behavioural Processes.