Scientist Elaborate How the Brain Senses Smell

Illustration of brain with olfactory bulb highlighted (stock image).
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Scientists have further decoded how mammalian brains perceive odors and distinguish one smell from thousands of others.

NYU Grossman School of Medicine researchers have conducted experiments on mice to understand how the brain distinguishes smell. The team created an electrical signature, which is perceived as an odor in the brain’s smell-processing center, the olfactory bulb, even though the odor does not exist.

The odor, which is human-made could manipulate the timing and order of related nerve signaling and subsequently identify the changes, which were most significant to the mice to accurately differentiate 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,” says study lead investigator Edmund Chong, MS, a doctoral student at NYU Langone Health. “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,” says Chong. “But these distinguishing features, as recorded by the brain, have yet to be found for each smell.”

The results of the experiment were published in the journal Science on the 18th of June. Previous studies have confirmed that airborne molecules linked to scents trigger receptor cells lining the nose to send electric signals to nerve-ending bundles in the bulb called glomeruli, and then to brain cells (neurons).

The timing and order of glomeruli activation is known to be unique to each smell, researchers state, signals, which then are transmitted to the brain’s cortex, is responsible for how an animal detects and recalls smell. Since scents tend to vary over time and fuse with other scents, scientists have struggled to precisely identify a single smell signature across several types of neurons.

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

If the mice hypothetically pushed the lever after activation of a different set of glomeruli (simulation of a different odour), they received no water. With the aid of this novel model the team of researchers were able to change the timing and mix of activated glomeruli, noting how each change impacted a mouse’s perception as reflected in a behaviour: the accuracy at which the mice acted on the synthetic odor signal to get the reward.

Researchers later discovered that change of the glomeruli within each odour-defining set activated led to as much as a 30 percent drop in the mouse’s ability to correctly sense an odour signal and obtain water. Changes in the last glomeruli in each set came with as little as a 5 percent decrease in accurate odour sensing.

The timing of the glomeruli activations worked together “like the notes in a melody,” say the researchers, with delays or interruptions in the early “notes” degrading accuracy. Tight control in their model with accurate information on which receptors and glomeruli were activated in the mice, enabled the team to sift through many variables and identify which odour features stood out.

“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,” says study senior investigator and neurobiologist Dmitry Rinberg, PhD.

Rinberg, an associate professor at NYU Langone and its Neuroscience Institute, states that the human nose is known to have about 350 different kinds of odor receptors, while mice, on the other hand, whose sense of smell is far more specialized, have more than 1,200.

“Our results identify for the first time a code for how the brain converts sensory information into perception of something, in this case an odour,” adds Rinberg. “This puts us closer to answering the longstanding question in our field of how the brain extracts sensory information to evoke behaviour.”

Funding support for the study was provided by National Institutes of Health grant R01 NS109961.

In addition to Chong and Rinberg, other NYU researchers involved in this study are Christopher Wilson, PhD; and Shy Shoham, PhD. Other study co-investigators include Monica Moroni, PhD; and Stefano Panzeri, PhD, at the Instituto Italiano di Tecnologia, in Rovereto, Italy.

Source: NYU Langone Health / NYU School of Medicine