How Does a Dog's Brain Respond to the Smell of a Familiar Human?

And what does it tell us about the importance of people to their dogs?

Science on whether our dogs love us, like this happy dog and girl at sunset
Photo: hitmanphoto / Shutterstock

By Zazie Todd, PhD

New fMRI research by Gregory Berns et al (in press) shows that dog’s brains respond differently to the smell of a familiar human compared to an unfamiliar human and other canines – suggesting that certain people are special to their dogs.

The research focussed on a part of the brain called the caudate, which has been much investigated in humans, monkeys and rats. The scientists explain that “caudate activity is correlated with salient, usually rewarding signals that cause the animal to change its behavioural orientation to approach or consume the stimulus.”

Previous research by the team showed that this part of the brain lights up when the dog is given a hand signal that means it will be given a treat, confirming that caudate activation in dogs is connected with rewards.

The results showed that the caudate was activated significantly more in response to the smell of the familiar human than to any of the other smells – even the familiar dog. The scientists say,
“Importantly, the scent of the familiar human was not the handler, meaning that the caudate response differentiated the scent in the absence of the person being present. The caudate activation suggested that not only did the dogs discriminate that scent from the others, they had a positive association with it. This speaks to the power of the dog’s sense of smell, and it provides clues to the importance of humans in dog’s lives.”
Does this mean we can say that dogs love us? It’s certainly the case that when people look at photographs of loved ones, the same part of the brain is activated. But it's hard to interpret the activation on the scan in terms of the dog's subjective experience.

The researchers caution there is another possible explanation in terms of conditioning. It may be that the familiar person had previously given the dog food and so the scent was simply eliciting a conditioned response. The researchers say they think it unlikely it is a conditioned response, because it was typically the handler – not the familiar human – who was responsible for feeding the dog.

The results also showed that the olfactory bulb in the brain was activated by all five smells. This is not surprising but it is useful to know the result is as expected. The canine brain presents a bit of a challenge for fMRI studies – training needs aside – simply because of the great variety of head shapes in dogs. 

12 dogs took part in the study. They had all previously taken part in fMRI research, in which they had to lie absolutely still during the scan. The smells came from swabs taken from the armpit of humans and from the perineal-genital area of dogs.

The scents used in the study were of a familiar human, an unfamiliar human, a familiar dog, an unfamiliar dog, and the dog’s own scent. The familiar human was not the dog’s main caregiver – as that person was present during the scan – but someone else from the household, typically the husband or child of the main caregiver. The familiar dog lived in the same house.

The dogs were trained using positive reinforcement and models of the equipment.  A clicker was used in initial stages of the training, but since the equipment is noisy it would not be heard during the scan itself. The dogs were taught a hand signal that meant they would get a reward, and this was used to replace a clicker in later stages of training. 

The training specific to this study included preparing the dog for a different head-coil than in previous scans, and getting used to having scent-impregnated cotton wool swabs put under the nose while they remained still.

The number of dogs is small, and there are always trade-offs in the statistics used to make sense of fMRI scans. But the results are very intriguing, and we look forward to future research from this team.

The full paper is available (open access) at the link below. Photographs of the dogs who took part are on page 3.  You might also be interested to read about subsequent research from this team on dogs' preferences for food and social interaction

How do you know you have a special place in your dog’s heart?

Zazie Todd, PhD, is the author of Wag: The Science of Making Your Dog Happy. She is the founder of the popular blog Companion Animal Psychology, where she writes about everything from training methods to the human-canine relationship. She also writes a column for Psychology Today and has received the prestigious Captain Haggerty Award for Best Training Article in 2017. Todd lives in Maple Ridge, BC, with her husband and two cats.

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Berns, G., Brooks, A., & Spivak, M. (2014). Scent of the familiar: An fMRI study of canine brain responses to familiar and unfamiliar human and dog odors Behavioural Processes DOI: 10.1016/j.beproc.2014.02.011

If you enjoyed this, you might also like:
Dogs Can Haz BrainScanz and EEG?
Canine Neuroscience

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  1. Open access, bless them !

    Armpit scent is a slightly strange choice though - armpits are well established to have a specific chemistry different to the rest of the body, and they are out of reach of dogs most of the time. It would be fascinating to see this repeated with scent from the hands (or that other notorious target for dog sniffs, the crotch !!), and with muzzle odour from dogs - nose to nose sniffing being far more common than butt sniffing between dogs that already know one another.

    What is a bit of a conundrum is that the primary caregiver was present, but the dogs still lit up to another familiar human who was just a member of the family - could this reflect "you're back, you're back, I've missed you so so much" response to anyone coming home ?, to be followed in short order by "Ok you're here now, so what", which I suspect is a hard-wired atavistic response to the return of a missing pack member.

    So many questions !

  2. In reading this blog article I am more convinced than ever that my dogs and I have a deep connection. I feel very strongly that dogs know who love them and they know a good person when they smell one. It's just like the saying, "If my dog doesn't like maybe I shouldn't either."
    In regards to the experiment, my two dogs act in a similar way when we leave them with a dog sitter. Even though the person we trust our dogs with is kind and affectionate the dogs still do not show the same feelings towards the sitter than with us. I believe it has to do with how smells produce emotional responses. My dogs love us so they trust and are familiar with our safe smell. The dogs associate my family with safety and love. The sitter has a different smell and is not associated with the same emotions for the dogs.
    I also feel the sense of smell for our animals serves as important social function. Dogs not only use smell to be social with humans but other dogs, When dogs smell one another's backside it is a way of saying hello. A greeting, almost like dogs shaking hands. Male dogs also use sense of smell to indicate rather a female dog is in heat.
    In conclusion, this experiment is a wonderful testimony to the love story between humans and dogs. I personally feel my dogs can sense when I am sick, angry, lonely, and worried. Dogs have a great way of telling you they love you by helping you when you are feeling all those emotions by just being present. I am so glad that even in my absence, my dogs still know I love them. I even make it point to leave the blanket that I sleep with out for them when I leave town so they know I am still near.

  3. Dogs are funny creatures, and since their sense of sight isn't as great as humans they rely on sense of smell. Their noses are much more powerful than ours, which would explain their ability to differentiate humans by scent. The first thing my dog does when I arrive home from my friends house he sniffs my shoes for a good while because he smells her dog and her scent as well. Most humans don't have the ability to pick up on that stuff as quickly as dogs, but eventually will with family members and loved ones.
    Sense of smell can be associated with an event, food, people, places, and objects. The smell of a cologne can bring back memories and emotions of your father, or ex-boyfriend, so it would make sense that when a dog smells a familiar human they too feel emotions and are able to recall memories.

  4. I was very interested in this article because I have three dogs. I wonder how they would respond to this experiment because whenever they see a stranger they bark and growl, however the only time they do this is when myself or any of my family members are around. When are dogs are around a stranger and we are not around they do not do this. I believe that dogs do love their owners because they try to protect us from strange people and other strange animals. The book states that humans don't use their sense of smell because we chose not to, If we relied on our sense of smell as dogs do perhaps we ourselves would act as dogs do.
    -Stephanie Kirk

  5. I was interested in this article because I have three dogs. And I believe that dogs love their owners because my dogs will attempt to protect me from strangers whenever myself or my family is near. But if my family is absent my dogs are fairly well behaved, But of course they are still skeptical of the stranger. This makes me wonder how they would react to this experiment. The book states that humans have lost many of their senses of smell because they chose not to use some of them. I wonder if humans where more in tune with their sense of smell, would we act as dogs do?

  6. I believe I have a special place in my pets heart. And not just because I'm a sucker for their cute faces and give them treats haha. Olfaction converts physical energy into a complex pattern of brain activity. And i believe that when we spend time with our pets either playing ball or even just simply petting them that these hold a place in their memory. The setting where these events take place may have an affect on them too. So when we pet them first thing in the morning maybe they remember the smell of the fresh brewed coffee and every morning they wake up to you petting them and the smell of your coffee waiting for you. So the smell of the coffee and your fresh morning breathe triggers a positive memory and therefore bonding between you and your pet.


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