The first sniff may be all that matters, Duke study says

The first food you smell in the Brodhead Center might be the only food your brain registers. 

A recent study published by Duke neurobiologists suggests that our sense of smell is dependent on the first smell signals they detect. 

“How is it that we can recognize an odor, even though the concentration of the odorant may change dramatically?” said Kevin Franks, assistant professor of neurobiology. “For example, outside a bakery, you can smell the bread. As you walk in, the concentration of the bread smell gets much higher, but you’re still able to recognize it as bread.”

The question of smells in the environment versus their interpretation is not new to sensory neuroscience. Franks noted a similar phenomenon with light—whether in a well-lit or dark room, red still looks red. 

Scents travel from the nose to the olfactory bulb then to the olfactory cortex, the region in the brain where the smell signals are interpreted. Using the mouse's olfactory system, Franks’ lab examined the brain’s response to various concentrations of different odors. 

“In the olfactory bulb, the activity patterns evoked by two different odors were as dissimilar as responses were to one odor at different concentrations,” Franks said. “In the cortex, responses to two different odors were different, but responses to two different concentrations were very similar.” 

The olfactory bulb has receptors that respond to different concentrations of odorants. Higher concentrations activate less specific receptors, while lower concentrations activate very specific receptors that convey more information about the stimulus. 

“The brain takes advantage of timing to selectively pay attention to specific inputs,” Franks said. “Specific inputs are activated more strongly, so those inputs reach the brain earlier.”

Researchers then tried to understand how the brain suppresses the subsequent, less specific inputs. With those fleeting odorants, fewer olfactory bulb cells responded. At higher concentrations, however, more cells responded, but their cortical patterns barely changed. 

“The cortex essentially shuts itself off in response to the earliest inputs,” Franks said. “It’s like a queue of odors trying to get into the brain through a single door. The first one gets in and closes the door behind it.” 

Franks said this research helped researchers understand how the body maintains representation of unfamiliar stimuli. Future studies will go further to investigate how experiences change perceptions, and what the neural structure of memory looks like. 

Ultimately, Franks hopes that lessons learned from smell can be generalized across both species and systems. Other brain areas seem to have similar circuitry but do not have the same definitive inputs as olfaction. By studying the olfactory system, Franks hopes to understand other circuitries with more clinical relevance. 

“We’re predicting that the same circuits we found in the cortex that shut itself off will be similar to circuits in the hippocampus and frontal cortex, where the brain needs to process sequences or patterns that are structured in time,” he said.


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