Researchers have found that the sense of smell becomes more active when people are frightened.
Neuroscientists at Rutgers University studying the olfactory - sense of smell - system in mice have discovered that fear reaction can occur at the sensory level, even before the brain has the opportunity to interpret that the odour could mean trouble.
John McGann, associate professor of behavioural and systems neuroscience in the Department of Psychology, and his colleagues, found that neurons in the noses of laboratory animals reacted more strongly to threatening odours before the odour message was sent to the brain.
"What is surprising is that we tend to think of learning as something that only happens deep in the brain after conscious awareness," said McGann.
"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 said.
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's skull.
They found that those mice that received an electric shock simultaneously with a specific odour showed an enhanced response to the smell in the cells in the nose, before the message was delivered to the neurons in the brain.
This new research - which indicates that fearful memories can influence the senses - 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.
"We know that anxiety disorders like PTSD can sometimes be triggered by smell, like the smell of diesel exhaust for a soldier. What this study does is gives us a new way of thinking about how this might happen," McGann said.
In their study, the scientists also discovered a heightened sensitivity to odours in the mice traumatised by shock.
When these mice smelled the odour 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 odour were four times stronger than it actually was.
This created mice whose brains were hypersensitive to the fear-associated odours. Before now, scientists did not think that reward or punishment could influence how the sensory organs process information.
