It was previously thought that we become afraid of an odor, such as leaking gas, only after information about a scary scent is processed by our brain.
But neuroscientists at Rutgers University studying the olfactory system in mice have discovered that this fear reaction can occur at the sensory level, even before the brain has the opportunity to interpret that the odor could mean trouble.
John McGann, associate professor of behavioral and systems neuroscience in the Department of Psychology, and his colleagues, report that neurons in the noses of lab animals reacted more strongly to threatening odors before the odor message was sent to the brain.
McGann said that we tend to think of learning as something that only happens deep in the brain after conscious awareness, but now we see how the nervous system can become especially sensitive to threatening stimuli and that fear-learning can affect the signals passing from sensory organs to the brain.
McGann and students Marley Kass and Michelle Rosenthal made this discovery by using light to observe activity in the brains of genetically engineered mice through a window in the mouse' skull.
They found that those mice that received an electric shock simultaneously with a specific odor showed an enhanced response to the smell in the cells in the nose, before the message was delivered to the neurons in the brain.
The scientists also discovered a heightened sensitivity to odors in the mice traumatized by shock.
When these mice smelled the odor associated with the electrical shocks, the amount of neurotransmitter, chemicals that carry communications between nerve cells, released from the olfactory nerve into the brain was as big as if the odor were four times stronger than it actually was.
This new research could help to better understand conditions like Post Traumatic Stress Disorder, in which feelings of anxiety and fear exist even though an individual is no longer in danger.
The study is published in journal Science.