Scientists decode how the brain senses smell

Scientists have further decoded how mammalian brains perceive odors and distinguish one smell from thousands of others. In experiments in mice, New York University (NYU) Grossman School of Medicine researchers have created an electrical signature that is perceived as an odor in the brain’s smell-processing center—the olfactory bulb—even though the odor does not exist. The results are published in the journal Science.

Because the odor-simulating signal was human made, researchers could manipulate the timing and order of related nerve signaling and identify which changes were most important to the ability of mice to accurately identify the synthetic smell.

“Decoding how the brain tells apart odors is complicated, in part, because unlike with other senses such as vision, we do not yet know the most important aspects of individual smells,” said lead investigator Edmund Chong, a doctoral student at NYU Langone Health, in a university press release. “In facial recognition, for example, the brain can recognize people based on visual cues, such as the eyes, even without seeing someone’s nose and ears. But these distinguishing features, as recorded by the brain, have yet to be found for each smell.”

Past studies have shown that airborne molecules linked to scents trigger receptor cells lining the nose to send electric signals to nerve-ending bundles in the olfactory bulb called glomeruli, and then to brain cells (neurons). The timing and order of glomeruli activation are known to be unique to each smell, researchers say, with signals then transmitted to the brain’s cortex, which controls how an animal perceives, reacts to, and remembers a smell. But because scents can vary over time and mingle with others, scientists have until now struggled to precisely track a single smell signature across several types of neurons.

For the new study, the researchers designed experiments based on the availability of mice genetically engineered by another lab so that their brain cells could be activated by shining light on them—a technique called optogenetics. Next, they trained the mice to recognize a signal generated by light activation of six glomeruli—known to resemble a pattern evoked by an odor—by giving them a water reward only when they perceived the correct “odor” and pushed a lever.

Using this model, the researchers changed the timing and mix of activated glomeruli, noting how each change impacted a mouse’s perception as reflected in a behavior: the accuracy with which it acted on the synthetic odor signal to get the reward. The researchers found that changing which of the glomeruli within each odor-defining set was activated first led to as much as a 30% drop in the ability of a mouse to correctly sense an odor signal and obtain water. Changes in the last glomeruli in each set came with as little as a 5% decrease in accurate odor sensing.

“Now that we have a model for breaking down the timing and order of glomeruli activation, we can examine the minimum number and kind of receptors needed by the olfactory bulb to identify a particular smell,” said senior investigator and neurobiologist Dmitry Rinberg, in a press release.