Once it was established that the dogs were successful at this, they were ready to be tested.

At the testing, we tasked the dogs with discriminating between a person’s samples taken before and after the arithmetic task.

To teach the dogs what odour they should be looking for in each testing session, they were first shown the person’s stress sweat/breath sample alongside two “control samples” – clean gauze in glass vials with no sweat or breath.

After 10 exposures, a second breath/sweat sample was added to the line-up – the same person’s relaxed sample.

Here began the test of discrimination, which took place over the next 20 trials. It was the dogs’ job to communicate, through their alert behaviour, which sample they perceived as the same as the one shown to them in the previous ten trials, that is, which sample smelled like the stress sample.

If these two odours smell the same to the dog, we would expect them to pick either by chance. If the two odours smell different, they would be able to consistently find the odour first presented to them – the stress odour.

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