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Dogs that can predict when their owners are going to have an epileptic seizure may be recognising the 鈥渟mell of fear鈥�.

A small study suggests that a compound in sweat recognised by seizure alert dogs may be the same as one released when people watch scary movies, in this case Stephen King鈥檚 It.

Some animals communicate by releasing hormones that can be smelled, called pheromones, including ones that warn of danger, but it is unclear if human pheromones exist. Some small studies have hinted that we may change our behaviours based on the scent of others 鈥� for instance, dental students perform significantly worse聽when treating mannequins wearing T-shirts from people who were stressed compared with those from people who were calm.

, a neurologist at Denver Health Medical Center in Colorado, came across the idea of fear pheromones because of his work with a charity that trains dogs for people with epilepsy, called .

The dogs are trained to press a speed-dial button on a phone if their owner has a seizure while at home. But many also learn to recognise if their owner is going to have a seizure up to an hour in advance. It is thought that dogs may be able to predict a seizure by smelling a change in the person鈥檚 sweat, triggered by brain changes that eventually develop into a seizure.

In the new study, Maa and his colleagues used four assistance dogs that had been taught to touch the trainer鈥檚 left hand if they smell a scent indicating an imminent seizure, and their right hand if that scent is absent.

The animals were given 90 pads to sniff containing a variety of sweat samples. These included sweat taken while people watched the film It, exercise sweat and samples taken from someone with epilepsy during a seizure and when seizures were absent.

The dogs indicated they could smell a seizure scent with 96 per cent of the seizure samples and 69 per cent of the fear samples. They correctly rejected all the other samples.

The success rate for the fear samples may have been lowered by the fact that two of the 15 volunteers who provided the samples clearly did not find the film scary, as they laughed through most of it, says Maa.

Previous research has narrowed down the field of potential pheromone candidates to three chemicals, with one called

There are several other compounds in 鈥渇ear sweat鈥�, so it isn鈥檛 yet clear if menthone is indeed the key chemical, says at Radboud University in the Netherlands. Before menthone can be considered a pheromone, it needs to be shown that it can change people鈥檚 behaviour, he says.

While fear pheromones played a stronger role in our evolutionary past, humans rely more on reading fearful expressions on people鈥檚 faces, says at Utrecht University in the Netherlands. 鈥淔aces are a book you can read.鈥� However, his group has shown that people are better at recognising fearful faces.