What makes a smell “bad”? A new study uncovers how emotional associations with odors are wired in the brain, hinting at why some smells can trigger deep aversion — or, potentially, be reprogrammed.
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Smells trigger powerful emotional responses through specialized brain cells. These insights may improve sensory health.
You wouldn’t microwave fish around your worst enemy. The smell lingers in both the kitchen and your memory. It’s a scent few people like, and even fewer associate with anything pleasant.
But what makes our brains label a smell as unpleasant?
A new study from UF Health researchers uncovers how the brain determines when a smell is disliked—or even strongly rejected.
Or as first author and graduate research fellow Sarah Sniffen puts it: “How do odors come to acquire some sort of emotional charge?”
How the brain assigns emotional value to odors
In many ways, our world relies on the power of smell to influence emotions, from perfumes and cooking to the way grocery stores are designed.
“Odors are powerful at driving emotions, and it’s long been thought that the sense of smell is just as powerful, if not more powerful, at driving an emotional response as a picture, a song or any other sensory stimulus,” said senior author Dan Wesson, Ph.D., a professor of pharmacology and therapeutics in the UF College of Medicine and interim director of the Florida Chemical Senses Institute.
But until now, scientists have been unsure how the brain connects the areas that create emotional responses with those that process smell.
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The research team began by focusing on the amygdala, a part of the brain that shapes emotional reactions to sensory input. While all five senses—sound, sight, taste, touch, and smell—interact with the amygdala, the sense of smell reaches it through a more direct pathway.
“This is, in part, what we mean when we say your sense of smell is your most emotional sense,” Sniffen said. “Yes, smells evoke strong, emotional memories, but the brain’s smell centers are more closely connected with emotional centers like the amygdala.”
The research team began by focusing on the amygdala, a part of the brain that shapes emotional reactions to sensory input. While all five senses—sound, sight, taste, touch, and smell—interact with the amygdala, the sense of smell reaches it through a more direct pathway.
“This is, in part, what we mean when we say your sense of smell is your most emotional sense,” Sniffen said. “Yes, smells evoke strong, emotional memories, but the brain’s smell centers are more closely connected with emotional centers like the amygdala.”
Unique brain cells categorize odors
In the study, researchers examined mice, which share important neurochemical similarities with humans. Mice can learn to recognize odors and sort them into positive or negative categories.
By observing their behavior and measuring brain activity, the team identified two genetically distinct types of brain cells that help assign smells to either pleasant or unpleasant emotional responses.
Initially, the team expected that one cell type would generate a positive emotion to an odor, and another would generate a negative emotion. Instead, the brain’s cellular organization gives the cells the capability of doing either.
“It can make an odor positive or negative to you,” Wesson said. “And it all depends upon where that cell type projects in your brain and how it engages with structures in your brain.”
But why is knowing more about how we categorize smells important? Well, for starters, smells — and our reactions to them — are a part of life. Sometimes, however, our reactions to them can be outsized, or take on a negative association so strong it disrupts how we live.
“We’re constantly breathing in and out and that means that we’re constantly receiving olfactory input,” Sniffen said. “For some people, that’s fine, and it doesn’t impact their day-to-day life. They might even think, ‘Oh, odors don’t matter that much.’ But for people who have a heightened response to sensory stimuli, like those with PTSD or anxiety or autism, it’s a really important factor for their day-to-day life.”
Toward clinical applications and emotional healing
In the future, the research could help clinicians adjust for heightened sensory response that some people struggle with in their everyday lives, Wesson added. One example? A patient associating a clinic’s smell with transfusions that made them queasy.
Based upon the receptor systems in these specific brain pathways, the team members believe they might be able to change those associations.
Potentially, medications could suppress some of these pathways’ activity to allow you to overcome stressful and aversive emotional responses.
Conversely, these pathways could be activated to restore enjoyment to things that people might have grown indifferent to — like those who lose their appetite from illness.
“Emotions in part dictate our quality of life, and we’re learning more about how they arise in our brain,” Wesson said. “Understanding more about how our surroundings can impact our feelings can help us become happier, healthier humans.”
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