Who Moved My Cheese!?
Researchers Find That "One Sniff Will Do" For Odor
Discrimination |
Rats inhabit a world of smells far beyond
our poor powers to discriminate. Thousands of odors that smell
the same to us, or that we cannot perceive at all, are quickly
recognizable as distinct and meaningful odors to rodents and
other animals in which the Nose Knows. But just how quick?
By measuring the speed of smell, researchers at Cold Spring Harbor
Laboratory have now found that unlike humans, rats can tell two
very similar odors apart with just one sniff. And because it's
not the Nose that Knows, but rather the brain, such studies of
how animals can rapidly and accurately discriminate odors are
revealing vital new information about how the human brain processes
information, guides behavior, and even enables us to be consciously
aware of our own (though less smelly) world, and our own selves.
"We are trying to understand how systems of neurons participate
in the creation of perception, awareness, and behavior," says
Cold Spring Harbor Laboratory neuroscientist Zach Mainen, who
led the new study.
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Click on link above to view movie
of a rat performing a "two alternative" odor discrimination
task. The rat receives a test odor by poking its nose into
the center port. Then, the rat chooses Odor A (green "A")
or Odor B (red "B") as being the same as the test
odor by poking its nose into the Choice Port to its right
(Odor A) or left (Odor B). The rat receives a reward if
it chooses correctly, which it virtually always does in
this simple version of the task. |
By exploring the neural mechanisms by which rodents use odors
to guide their behavior, Mainen and his colleagues hope to
uncover basic principles of brain function that will apply
in many settings, including how our own brains work. But to
get there, they needed to start out by measuring seemingly
strange things such as how many sniffs a rat takes per second.
The answer, according to the new study: about eight sniffs
per second.
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Pattern of nerve cell activity in a rat's brain when
it smells caproic acid (top). The pattern of nerve cell
activity
in a rat's brain is different (bottom) when it smells a different
odor (hexanol). By determining and comparing such patterns,
CSHL researchers are discovering how odors are represented
and assigned distinct identities by the brain. |
Believe it or not, the "eight sniffs per
second" measurement
has helped resolve a hotly debated issue in neuroscience. Researchers
have previously suggested that the brain requires extra time
to distinguish among the millions of different chemical signals
that can be picked up by the nose. The new study, which appears
in the November issue of Nature Neuroscience (advance
online publication date: October 20), overturns this conventional
wisdom
that smell is a slow sense.
"We found that a rat gets a complete sense of an odor with
each sniff. The speed of olfactory perception appears to be limited
by the speed of the breathing cycle rather than by processing
speed in the brain. For humans breathing is relatively slow,
but because rats sniff quite quickly, smell is a fast sense for
them," says Mainen. "Rats probably use a series of
quick olfactory snapshots to solve complex problems like tracking
the source of an odor."
Humans are far more attuned to the visual world, but the computations
our brains carry out are not all that different than in rodents.
According to Mainen, the neural mechanisms that enable rodents
to identify an odor in a single sniff may be similar to those
that help us take in an entire visual scene in a single glance.
Mainen and his colleagues are currently recording electrical
signals from neurons in the brains of rats as they perform the
odor discrimination task (see figures). In this way, the researchers
hope to learn more about information processing in the olfactory
system, and to explore the neural basis of perception, motivation,
decision-making, and other aspects of behavior.
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