Know how a whiff of certain odours can take you back in time, either to a great memory or bad one? It turns out emotion plays an even bigger role with the nose, and that your sense of smell actually can sharpen when something bad happens.
Northwestern University researchers proved the surprising connection by giving volunteers electric shocks while they sniffed novel odours. The discovery, reported in Friday's edition of the journal Science, helps explain how our senses can steer us clear of danger.
More intriguing, it could shed light on disorders such as post-traumatic stress syndrome.
"This is an incredibly unique study," said Dr David Zald, a Vanderbilt University neuroscientist who studies how the brain handles sensory and emotional learning. "We're talking about a change in our perceptual abilities based on emotional learning."
Scientists long have known of a strong link between the sense of smell and emotion. A certain perfume or scent of baking pie, for instance, can raise memories of a long-dead loved one. Conversely, a whiff of diesel fuel might trigger a flashback for a soldier suffering PTSD.
Could an emotionally charged situation make that initial cue be perceived more strongly in the first place?
How the study was done
The research team recruited 12 healthy young adults to find out. Volunteers repeatedly smelled sets of laboratory chemicals with odours distinctly different from ones in everyday life.
An "oily grassy" smell is the best description that lead researcher Wen Li, a Northwestern postdoctoral fellow in neuroscience, could give.
Two of the bottles in a set contained the same substance and the third had a mirror image of it, meaning its odour normally would be indistinguishable. By chance, the volunteers correctly guessed the odd odour about one-third of the time.
Then Li gave the volunteers mild electric shocks while they smelled just the odd chemical. In later smell tests, they could correctly pick out the odd odour 70 percent of the time.
MRI scans showed the improvement was more than coincidence. There were changes in how the brain's main olfactory region stored the odour information, essentially better imprinting the shock-linked scent so it could be distinguished more quickly from a similar odour.
What the findings mean
In other words, the brain seems to have a mechanism to sniff out threats.
That almost is certainly a survival trait evolved to help humans rapidly and subconsciously pick a dangerous odour from the sea of scents constantly surrounding us, Li said.
Today, that might mean someone who has been through a kitchen fire can tell immediately if a whiff of smoke has that greasy undertone or simply comes from the fireplace.
But the MRI scans found the brain's emotional regions did not better discriminate among the different odours, Li noted. That discrepancy between brain regions is where anxiety disorders may come in.
If someone's olfactory region does not distinguish a dangerous odour signal from a similar one, the brain's emotional fight-or-flight region can overreact. Researchers say that is a theory not yet tested.
Sense of smell should not be ignored
For now, Northwestern neuroscientist Jay Gottfried, the study's senior author, says the work illuminates a sense that society too often gives short shrift.
"People really dismiss the sense of smell," said Gottfried, who researches "how the brain can put together perceptions of hundreds of thousands of different smells. ... Work like this really says that the human sense of smell has much more capacity than people usually give it credit for."