Scents of Smell Rooted in Math
Nose knows different odors, no
matter how powerful, thanks to predictable patterns transmitted to brain
By
Jo Craven McGinty in the Wall Street Journal
Crushed coriander seeds burst with a
lemony aroma. Golden turmeric smells like corn cakes. Cardamom gives off a hint
of bitterness. And pulverized cumin seeds smell like moist, peppery earth.
Combine them, and you have the fragrant beginnings of curry.
But how does a nose, bombarded with
odors that arrive in different amounts and combinations, consistently identify
each aroma?
It turns out that it is simpler than
many other neurobiological processes, and can essentially be broken down into a
predictable mathematical pattern.
Odors arrive in small packets—tiny
bouquets of molecules—that are inhaled. Receptor cells inside the nose respond
by producing a series of electrical spikes, which are communicated to the
olfactory bulb in the brain, where the smell is decoded.
“It’s like Morse code,” said Upinder Bhalla,
a professor of neurobiology at the National Centre for Biological Sciences in
Bangalore, India, and lead supervisor of a recent study about the olfactory system that is the first to document the coding is linear. “The
pattern and spacing of the clicks make different letters.”
In this case, the pattern of the
electrical spikes translates to specific smells. But significantly, according
to the study, which was published in the journal Nature Neuroscience, when the
smell is repeated in the same dose, the pattern remains the same. And when the
odor varies in duration, the neurons’ electrical response changes
proportionately.
In other words, the response is
orderly and predictable, rather than chaotic and irregular.
“Mathematically, it can’t be simpler
than this,” said Priyanka Gupta, a graduate student from the National Centre
for Biological Sciences who devised the study and is its lead author. “We can
understand what the brain might be doing.”
Unlocking how the brain handles a
simple process like this can help researchers chip away at how the organ
responds to more-complex events, and it also can inform technology. Knowing how
the brain processes odors, for example, could facilitate the design of
artificial noses to detect bombs.
“We have the hope that if there is a
glimmer of a simple event, we have the chance to understand complex devices
through simple computation,” said Dinu Florin Albeanu, a neuroscientist at Cold Spring Harbor Laboratories and
another co-author of the study. “It’s an entry point.”
Neurons are cells that carry
messages between the brain and other parts of the body, and when they
communicate, they talk to each other simultaneously with electrical pulses that
flit hither and thither. Typically, when a stimulus is introduced—sound, for example—all
hell breaks loose, and the communication becomes chaotic and more difficult to
explain mathematically.
But a study of rats led by Ms. Gupta
in Bangalore and at Cold Spring Harbor Laboratories in New York has
demonstrated that when their brains decode smells in the first stage of the
olfactory system, the process is linear. This allowed the researchers to learn
how the brain responded to a smell once and then reliably predict on average
how it would respond to the same smell in subsequent exposures.
Rats have many more types of
olfactory receptors than humans—1,500 compared with 350—and they rely more
heavily on their sense of smell for survival.
“Animals use their noses in
different ways,” Mr. Bhalla said. “They sniff more when they are interested in
something. There is a time component. The stimulus is patchy. The sample is
patchy.”
Ms. Gupta controlled for these
differences by designing a method to deliver a precise number of molecules of
an odor such as mint or banana to the noses of lab rats at specific points in
time. She documented the spikes in electrical voltage that occurred in the
olfactory bulb in response to the odor. And then she aligned the different
pieces of data.
“The spikes in time are not much
information until we plot when the odor was on or off,” Ms. Gupta said. “We
know very precisely when the spikes happened in time. We align that with when
we turned on the odor.”
The result is the response pattern
associated with a particular smell.
The researchers delivered the odors
to anesthetized rats in a steady stream that, thanks to a tracheotomy, wasn’t
interrupted by exhalation. It is possible that normally functioning, awake
animals experiencing complex odors would respond differently.
Within the field, the results of the
study have been met with surprise, and it is too soon to tell whether others
will attempt to repeat the work or how it may be put to practical use. But if
it holds up, it suggests some things may be easier to accomplish than expected.
“Nonlinear information is much
harder to interpret,” said Venkatesh N. Murthy, a professor of molecular and
cellular biology at Harvard who has read the study. “If you can make everything
more linear, programs and decoding are more simple. The same analogy holds for
the brain.”
For now, at least, the work appears
to put scientists a step closer to understanding what the nose knows.
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