How the Brain Senses Smells

Scientists have discovered how humans perceive different odors and distinguish them from different odors. It is pretty fascinating.

At NYU Grossman School of Medicine, scientists have been conducting experiments on mice regarding their ability to sense odors through their brain. Researchers have created an electrical signature that resembles an odor that can trigger the olfactory bulb in the brain’s smell processing center even though the odor is nonexistent.

The odor simulating signal is humanmade giving researchers flexibility and the ability to identify signals in brain activity more accurately. Researchers are able to manipulate the time of nerve signalling and identify from there which changes were the most prominent for the mice to detect the “synthetic odor”.

“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.”

This study focuses and centers on the olfactory bulb which is behind the nose in humans and animals. There have been past studies relating to sensing odors have shown that molecules trigger receptor cells in the nose and then travel to nerve bundles in the bulb which then send neurons to the brain cells. However, the timing to bulb activation is very specific and tricky as it is unique to every smell. Each smell triggers a different signal which then goes to the brain’s cortex and signals a response. Similarly, because scents can change over time and mingle with other scents, scientists have had trouble identifying one specific smell amoung neurons in the brain.

Researchers have now designed a newer study based on genetically engineered mice by another lab that allows brain cells to be activated by shining a light on them, which is a technique called optogenetics. They then trained mice to recognize different signals that were generated by light activation. The researchers would reward the mice with water if they had perceived the right odor and pushed a lever. If the mice pushed the liver after the activation, they would not receive any water. This model allowed researchers to change the timing and mix of the light activation which allowed them to see the mouse’s change in behavior.

The results that researchers found were that changing the odor-set during the first activation led to a 30 percent drop in the mouse’s ability to correctly sense an odor and obtain water. On the other hand, changes in the last activation led to a 5 percent decrease in the acurrate odor sensing.

The timing of the activation worked together amazingly, researchers noted. Their tight control in the model and which receptors in the mouse’s brain were activated enabled the team to sift through the variables and identify whcih odor 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.

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