Sniffing and Smelling
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Sobel, N., Prabhakaran, V., Desmond, J.E., Glover, G.H., Goode, R.L.,
Sullivan, E.V., & Gabrieli, J.D.E. (1998). Sniffing and smelling: Separate
subsystems in the human olfactory cortex. Nature, 391, 282-286.
Overview:
Stinky Research
Recognizing different chemicals in the environment is one of the evolutionarily
oldest senses. Over the millennia, this sense differentiated into olfactory
and gustatory senses. Taste and smell are still very tightly interrelated
- this becomes personally evident when you can't taste anything when you
have a bad cold.
Olfaction is difficult to study. Although research has shown that people
can distinguish as many as 10,000 different odors, we have no way of classifying
these odors. Color, brightness, tone, and loudness - characteristics of
more commonly and thoroughly studied vision and audition - can be classified
according physical scales. How would you categorize the smell of a rose
versus a wood fire versus chlorine? It appears that there may be a number
of different odor receptors, each specialized for particular types of chemicals.
Thus rose-like odors may stimulate one type of receptor and fire-like odors
may stimulate another in the olfactory system, much like bright light and
colors stimulate cones and dim light stimulates rods in the visual system.
Sobel, Prabhakaran, Desmond, Glover, Goode, Sullivan, and Gabrieli (1998)
conducted research that indicates that olfaction is made up of two separate
processes, sniffing and smelling.
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Article Summary
Researchers have determined that the piriform cortex at the base of the
temporal lobe is activated during olfaction in a variety of animals, including
rats, rabbits, dogs, monkeys, and hedgehogs. Sobel, Prabhakaran, Desmond,
Glover, Goode, Sullivan, and Gabrieli (1998) were interested in whether
humans show similar cortical activity during olfaction. They used functional
magnetic resonance imaging (fMRI), which measures blood oxygen levels in
different areas of the brain, and made an interesting discovery about olfactory
processes. Their findings suggest that there are two olfactory processes,
sniffing and smelling.
Sobel et al. first examined brain activation while people sniffed. Sniffing
plain air resulted in activation in expected subcortical areas and in the
piriform cortex at the base of the temporal lobe. This is the same cortical
area activated in many different animals that have been studied using electrophysiological
recordings. Furthermore, the left piriform cortex was more strongly activated
than the right piriform.
Sobel et al. wondered whether this cortical activation was a result
of motor activity associated with sniffing. To examine the role of motor
activity related to sniffing, they scanned people under three conditions,
sniffing with a blocked nose to prevent air flow in the nostrils, puffing
air into the nostrils to simulate the air flow associated with sniffing
but without associated motor activity, and sniffing through partly blocked
nostrils. People who sniffed with blocked noses, thus performing a sniff
but without air flow occurring in the nostrils, showed no piriform activation.
Conversely, people who were scanned while air was puffed into their nostrils,
thus simulating the air flow part of a sniff but without motor activity,
showed strong activation of the piriform cortex. Finally, sniffing with
a partly blocked nose resulted in partial piriform activation. These findings
indicate that piriform activity is not due to the motor activity of sniffing.
To rule out the possibility that the cortical activity is an artifact
of air pressure changes around the nasal passages, and not truly a sensory
process, Sobel conducted two additional studies. In the first, people sniffed
before and after application of a topical anaesthetic. The piriform cortex
was not activated when people sniffed with numb nostrils. Sobel et al.
also scanned a woman with a severed left olfactory nerve. There was no
piriform activation when she sniffed with her left nostril but there was
significant activation when she sniffed through her right nostril. Thus,
nerve conduction appears to be essential for the cortical activation to
occur.
Based on these two sets of findings, Sobel et al. concluded that piriform
activation is due to the sensation of air flow in the nostrils. They speculate
that this sensation may be an attention mechanism. The sensation of air
flow may "prime" the olfactory system for the imminent arrival of some
odor to be identified.
Sobel differentiated sniffing and smelling in an additional set of scans.
They scanned people while they sniffed plain air or an odorant. The odorant
led to additional activation of the right anterior and lateral portions
of the orbital-frontal cortex at the base of the frontal lobe.
Besides shedding light on olfaction, Sobel et al.'s research has some
interesting implications for understanding degenerative diseases. For example,
people who have Parkinson's disease, a gradually debilitating motor disease,
complain about losing their sense of smell. Maybe this symptom isnÕt
some odd artifact of a motor disease but a loss of the ability to sniff.
Olfactory deficits are also associated with schizophrenia; in this case,
frontal lobe damage might lead to the inability to smell even though sniffing
is intact.
Overview | Article Summary
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For Instructors
Links to the Lecture
Although designed for a high school biology course, this Genentech
site outlines interesting activities for exploring the relationship
between olfaction and memory.
Overview | Article Summary
| For Instructors | For Students
For Students
About the Authors
Noam Sobel and Vivek Prabhakaran are neuroscience program graduate students,
John Desmond is a psychology research associate, Gary
Glover is a professor of radiology, Richard Goode is a professor of
surgery, and Edith
Sullivan and John
Gabrieli are professors in the neuroscience program at Stanford University.
Dr. Gabrieli oversees an active cognitive neurosciences research
lab.
About the Journal
Nature publishes a wide variety of
articles in the natural and physical sciences. Check out Nature's science
update for interesting science news.
Links to Life
Learn more about olfaction (and all the other senses) at this detailed
site from the Howard
Hughes Medical Institute. Here's another site
with a thorough coverage of olfaction.
This press
release also includes the fMRI results from the article.
Here is an excellent resource
on Parkinson's disease, a common neurological disease. Recent research
on olfactory and other deficits is shedding light on Parkinson's disease
as well as on the function of the nervous system in general.
Here's an interesting application of research on olfaction; check out
this Web article
on police dog training. |
