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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Reference
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

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