How does the brain process complex smells? 

The world around us — even mundane, everyday scenes — contains more sensory information than our brains can interpret all at once. So how do our sensory systems keep us from becoming overwhelmed? 

New research led by the University of Illinois Chicago examined this question in the context of the olfactory system and discovered that while sensory cells in the nose can quickly become saturated by complex scents, the brain is able to turn down the input. The study is published in Nature Communications. 

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With complex smells, “you’re activating all these neurons in the nose, and the brain doesn’t like when that happens; it becomes overwhelmed,” explained Joseph Zak, assistant professor of biological sciences at UIC and lead author on the study. “We discovered that while things scale up in the nose, the cortex can tamp down that activity, which may be an important part of how our brains can focus on what the most important thing is in the moment.” 

Zak and colleagues at UIC and other institutions exposed mice to 85 different odor mixtures, then used a specialized microscope for live brain imaging to watch how neurons in their noses and brains responded. The scents that went into the mixtures were the kind used by perfume and food manufacturers and aren’t generally recognizable on their own, Zak said. But the mixtures were multilayered and complex in the same way the smell of coffee or wine is. 

Previous research had established that the cortex can dampen olfactory input when scents become more highly concentrated, Zak said. But it was not known if the same thing would happen as scents become more complex. 

Additionally, the researchers saw that each mixture they made registered in a unique way in the mouse’s brain, even if one mixture contained very similar individual scents as other mixtures. 

Zak likened this to humans smelling a pepperoni pizza versus an anchovy pizza. Most of the underlying smells are the same — a yeasty bread smell, melted cheese, sauce. “But our brain knows very discretely that pepperoni pizza is not anchovy pizza,” he said. “The perceptual spaces in our brains disentangle those things despite the fact that they’re nearly identical.” 

The researchers also used mathematical modeling of the mice’s neural activity to design a model of what the brain’s olfactory neural circuitry may look like. The model they created looks very similar to the actual architecture of the brain, which suggests that the way the olfactory system is organized has helped shape the physical properties of the brain. 

The study adds to our understanding of how brains efficiently encode complex sensory information, Zak said. 

“We have more information available to us than our brain can handle at any given time,” he said. “And we don’t understand exactly how the brain makes sense of the complex worlds we live in.” 

Zak’s coauthors on the study are Vaibhav Konanur, a postdoctoral research associate at UIC, Gautam Reddy at Princeton University and Venkatesh Murthy at Harvard University. 

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