In terms of evolution, smell is one the oldest senses.
Most animals, from invertebrates to humans, have olfaction as a primal influence. The brain subconsciously processes the steady stream of scent molecules wafting under our noses.
Though odors, whether the aroma of moldy cheese or the sweet smell of fresh baked bread, are known to stir the emotions, how they wield their influence biologically on the emotional centers of the human brain, evoking passion or disgust, has been unknown.
Now, researchers using powerful brain imaging technologies are unraveling some of the mystery. They are revealing how anxiety or stress can rewire the brain, linking centers of emotion and olfactory processing, to make typically benign smells malodorous.
A team at the University of Wisconsin-Madison Waisman Center reports that the brains of human subjects who experience anxiety induced by disturbing pictures and text of things like car crashes and war turn neutral odors to offensive ones, fueling a feedback loop that could heighten distress and lead to clinical issues like anxiety and depression.
“After anxiety induction, neutral smells become clearly negative,” explains team leader Wen Li. “People experiencing an increase in anxiety show a decrease in the perceived pleasantness of odors. It becomes more negative as anxiety increases.”
Using behavioral techniques and functional magnetic resonance imaging (fMRI), Li’s group looked at the brains of a dozen human subjects with induced anxiety as they processed known neutral odors.
Functional MRI enables clinicians and researchers to observe the working brain in action. Before entering the MRI where screens cycle through a series of disturbing pictures and text, subjects were exposed to and rated a panel of neutral smells.
During the experiment, the researchers observed that two distinct and typically independent circuits of the brain, one dedicated to olfactory processing, the other to emotion, become finely intertwined under conditions of anxiety.
After anxiety induction and the imaging process, subjects were asked again to rate the panel of neutral smells, most assigning negative responses to smells they previously rated as neutral.
Networking Emotion and Odor
“In typical odor processing, it is usually just the olfactory system that gets activated,” says Li. “But when a person becomes anxious, the emotional system becomes part of the olfactory processing stream.”
Even though the two systems of the brain are located next to each other, under normal circumstances there is limited crosstalk between the two. But under conditions of induced anxiety, the team saw the surfacing of a unified network cutting across the two systems.
“We encounter anxiety and as a result we experience the world more negatively,” said Li. “The environment smells bad in the context of anxiety. It can become a vicious cycle, making one more susceptible to a clinical state of anxiety as the effects accumulate. It can potentially lead to a higher level of emotional disturbances with rising ambient sensory stress.”
Elizabeth A. Krusemark, Lucas R. Novak, Darren R. Gitelman, and Wen Li
When the Sense of Smell Meets Emotion: Anxiety-State-Dependent Olfactory Processing and Neural Circuitry Adaptation
The Journal of Neuroscience, 25 September 2013, 33(39):15324-15332;doi:10.1523/JNEUROSCI.1835-13.2013
Image: Rowena Dugdale, Wellcome Images