Dogs have a long history alongside humans, giving them an amazing ability to read human cues.

Dogs also have an incredible sense of smell, allowing them to detect diseases, such as COVID-19 and lung cancer, in humans just by smell.

Whether the abilities of dogs extend to detect odors associated with psychological states has been much less studied.

When people are stressed, there are hormonal and nervous system changes that alter the types of smells produced by the body.

My colleagues and I wanted to know if dogs can distinguish between scent samples taken from the same person before and after stress.

To do this, we took ideas from the field of biomedical detection dogs (tracking dogs in a laboratory setting) and combined these ideas with techniques used to test dogs’ perception of smells.

Our results have been published in the magazine PLOS One.

To test whether dogs could detect a scent associated with psychological stress, we attached sensors to the study participants to continuously measure their heart rate and blood pressure.

Participants also rated how stressed they felt before and after participating in the task.

Before the task began, participants wiped gauze on the back of their necks, placed it in a sterile glass vial, and exhaled into the vial. We then had the participants perform a quick mental arithmetic task to induce stress in them.

After the task, the participants gave another assessment of their stress and two additional sweat/breath samples.

The total time between collection of the relaxed (pre-task) and stressed (post-task) samples was four minutes, reducing the likelihood that the participants experienced changes other than the onset of stress.

We only included samples in the study if the person reported finding the task stressful and both their heart rate and blood pressure had increased during the task. We presented samples from 36 individuals to the dogs.

The Dog Training Process

The dogs in this study were pets, voluntarily by their owners, who were trained once a week by researchers in a lab using positive reinforcement.

Before formal data collection began, dogs were taught to communicate that they were picking a sample by standing over it and freezing or sitting in front of it for a few seconds – we called this their “alert behavior”.

The dogs then learned a matching game, where they learned to distinguish between samples with known odor differences.

Once it was determined that the dogs had succeeded, they were ready to be tested.

In testing, we instructed the dogs to differentiate between a person’s samples taken before and after the math task.

To teach the dogs what scent to look for during each testing session, they were first shown the subject’s stress sweat/breath sample alongside two “control samples” – clean gauze in glass vials with no sweat or breath.

The dogs were allowed to sniff all three samples and were rewarded for alerting the researchers to the sweat/breath sample.

After 10 exposures, a second breath/sweat sample was added to the set-up: the relaxed sample from the same person.

Here began the test of discrimination, which took place over the next 20 trials. The dogs’ job was to communicate through their alert behavior which sample they considered to be the same as the sample shown to them in the previous 10 trials, i.e. which sample smelled like the stress sample. Because dogs can use other information to make a choice, we’ve included both visual and scent checks.

If these two scents smell the same to the dog, we would expect them to pick either one by chance. If the two scents smelled differently, they could consistently find the scent presented to them first: the stress scent.

Each set of samples from participants was used only once, so the dogs saw samples from a different person during each session.

From the very first time the dogs were exposed to these monsters, they experienced the monsters as different.

The dogs chose the stress sample correctly in 94 percent of the 720 trials, showing that the participants’ psychological experience with the math task had caused their bodies to emit odors in their breath and sweat for the dogs to perceive.

It should be noted that this study does not determine whether the dogs perceived the stress samples as a reflection of a negative emotional state.

It’s likely that in real-life environments, dogs use various contextual cues, such as our body language, voice, or breathing rate, to help them make sense of a situation. However, the results provide solid evidence that scent is also a component that dogs can pick up on.

Establishing that dogs can detect a scent related to human stress provides a deeper understanding of the human-dog relationship and contributes to our understanding of how dogs perceive and interact with human psychological states.

This knowledge may also be useful for training anxiety and PTSD service dogs that are currently trained to respond primarily to visual cues.

Clara Wilson, PhD student, psychology, Queen’s University Belfast

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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