It turns out you may actually be able to smell fear. A new study found that odors associated with electric shocks in mice trigger a sensory-cell reaction much stronger than for non-fearful odors.

Study researcher John McGann, a neuroscientist at Rutgers University in New Jersey, said the finding was surprising: Sensory neurons are at the very beginning of the circuit that enables the perception of smell, far outside of conscious control, yet they "learn" to tune into scary smells.

"The effects of learning can happen not just on behavior, but on sensory processing," McGann told LiveScience.

Researchers have long known that associative learning changes processing in the brain so that, for example, the ability to differentiate between one type of scent and another becomes more sensitive. But most people tend to think that the sensory nerves that sit in the nose and directly contact the chemicals that make up scents are one-trick ponies. A smell comes along; the neurons duly note it and inform the brain; and the brain does the hard work of remembering what that scent means.

Not so, the new study reveals. The sensory neurons adapt, too.

"The earlier we look the earlier we find influences of learning," McGann said.

Researchers used relatively new techniques to spy on mouse olfactory sensory neurons. They then placed the mice in metal-floored enclosures and pumped in two smells, both simple one-chemical esters. One smelled fruity, the other a bit like nail polish remover. For each mouse, one of the smells presaged an unpleasant electrical shock to the feet from the enclosure floor.

Once the mice learned to associate smell with shock, researchers anesthetized them and exposed them to the smells again, watching under a microscope to see how their olfactory neurons responded compared with unfamiliar smells.

They found that scary smells had a large effect: The neurons released far more neurotransmitters when exposed to the shock-associated scent. It was as if the mice had been exposed to the smell at four times the concentration than they really were, McGann said.

"The circuit became hypersensitive to that odor," he said.

The researchers now want to test if they can reverse the sensory learning by training the mice not to associate the scary smell with a shock anymore. They also plan to "get a little bit more into the nuts and bolts" in the brain to find out how the neurons change their behavior, McGann said.