http://www.sciencedaily.com/releases/2006/06/060616134400.htm
Source: Duke University Medical Center
Posted: June 18, 2006
Researchers Show How Brain Decodes Complex Smells
Duke University Medical Center researchers have discovered how the
brain creates a scent symphony from signals sent by the nose.
Extracellular recordings from a single male-urine-specific neuron in
the main olfactory bulb of a mouse. The responses of the neuron to
repeated presentations of volatiles from male urine (top panel) or
female urine (bottom panel) are shown. The histograms show the summed
responses. This neuron fired robustly in response to male urine
volatiles (the horizontal lines represent individual stimulus
presentations; the tick marks denote firing of the neuron) and showed
almost no response to female urine volatiles. (Image courtesy of Duke
University Medical Center)
In studies in mice, the researchers found that nerve cells in the
brain's olfactory bulb -- the first stop for information from the nose
-- do not perceive complex scent mixtures as single objects, such as
the fragrance of a blooming rose. Instead, these nerve cells, or
neurons, detect the host of chemical compounds that comprise a rose's
perfume. Smarter sections of the brain's olfactory system then
categorize and combine these compounds into a recognizable scent.
According to the researchers, it's as if the brain has to listen to
each musician's melody to hear a symphony.
Humans may rely on the same smell decoding system, because mice and men
have similar brain structures for scent, including an olfactory bulb,
the researchers said.
"We wanted to understand how the brain puts together scent signals to
make an odor picture. We discovered the whole is the sum of its parts,"
said Da Yu Lin, Ph.D., who conducted the research as a graduate student
studying with neurobiologist Lawrence Katz, Ph.D., a Howard Hughes
Medical Institute investigator at Duke. Katz died in November 2005.
The research appears June 16, 2006, in the journal Neuron. The study
was supported by the National Institutes of Health, the Howard Hughes
Medical Institute and the Ruth K. Broad Biomedical Research Foundation.
Scientists have long debated how the brain makes order out of the
hundreds of volatile chemical compounds that assault the nose. Is the
brain's odor code redundant, with single cells responding to multiple
components in the smell of a freshly baked cookie? Or does the brain
process each scent component like a jigsaw puzzle piece, assembling the
signals until it recognizes the picture is a cookie?
To find answers, the Duke researchers exposed mice to different odors
and measured response of neurons across the olfactory bulb with
intrinsic signal imaging. The imaging technique maps brain activity by
detecting changes in reflected light from the brain with a sensitive
camera.
To start, the researchers separated and identified the volatile
compounds in each odor with gas chromatography. "A complex mixture like
urine has at least a hundred separate compounds in it," Lin said. They
analyzed scents as diverse as peanut butter, coffee and fresh bobcat
urine shipped to the laboratory on dry ice.
The researchers then exposed the mice to the original odor and its
individual compounds. "We found that glomeruli, the functional units of
the olfactory bulb, act as detectors for individual compounds," Lin
said. "There are no single detectors for complete smells."
Thus, to distinguish different scents, the brain must integrate the
signals of multiple chemical components into an odor "picture." The
researchers suspect that this integration doesn't happen in the
olfactory bulb. Instead, the bulb likely passes the data to more
advanced brain structures where it is assembled and recognized as a
specific scent.
Understanding how the olfactory system works in mice may also provide
broader insights into human perception, said Stephen Shea, Ph.D., a
Duke University Medical Center research associate who participated in
the study. Perception relies on combining multiple components, whether
the input is smell, sight or sound. Shea suggested that probing the
olfactory system could help scientists better understand, for example,
how the various biological and neurological components underlying
perception formed and evolved.
