Scientists have further decoded how the brain distinguishes one smell from thousands of different smells that constantly surround us. Previous studies say that the human brain is capable of distinguishing roughly 1 trillion scents from each other.
In experiments in mice, NYU Grossman School of Medicine researchers have for the first time created an electronic signature that is perceived as an odour in the brain's receptors, called olfactory bulb. This stamp-sized area in the roof of your nasal cavity transmits smell information from the nose to the brain.
Because the odour-simulating signal was humanmade, 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 odours 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, 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 timing and order of glomeruli activation is 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, published online in the journal Science, 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 odour -- by giving them a water reward only when they perceived the correct "odour" and pushed a lever.
If mice pushed the lever after activation of a different set of glomeruli (simulation of a different odour), they received no water. 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 odour signal to get the reward.
Specifically, researchers found that changing which of the glomeruli within each odour-defining set were activated first led to as much as a 30 per cent drop in the ability of a mouse 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 over when, how many, and which receptors and glomeruli were activated in the mice, enabled the team to sift through many variables and identify which odour features stood out.
According to study senior investigator and neurobiologist Dmitry Rinberg, scientists now have a model for breaking down the timing and order of glomeruli activation. They are able to examine the minimum number and kind of receptors needed by the olfactory bulb to identify a particular smell.
Rinberg, an associate professor at NYU Langone and its Neuroscience Institute, says the human nose is known to have some 350 different kinds of odour receptors, while mice, 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," said Rinberg. "This puts us closer to answering the longstanding question in our field of how the brain extracts sensory information to evoke behaviour."
