Scientists have
further decoded how mammalian brains perceive odors and distinguish one smell
from thousands of others.
In experiments in mice, NYU Grossman School of
Medicine researchers have for the first time 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.
Because the odor-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 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 current study results,
published online in the journal Science on June 18, center on
the olfactory bulb, which is behind the nose in animals and humans. 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
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 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.
If mice pushed the lever after activation of a
different set of glomeruli (simulation of a different odor), 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 odor
signal to get the reward.
Specifically, researchers found that changing which of
the glomeruli within each odor-defining set were activated first led to as much
as a 30 percent 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 percent decrease in accurate odor 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
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.
Rinberg, an associate professor at NYU Langone and its
Neuroscience Institute, says the human nose is known to have some 350 different
kinds of odor 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 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.”
Journal article: https://science.sciencemag.org/content/368/6497/eaba2357
Source: https://myfusimotors.com/2020/06/23/scientists-decode-how-the-brain-senses-smell/
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