Scientific American Mind-June/July 2007

STEVE LEWIS Getty Images (top); GETTY IMAGES (bottom)
The nose
knows: a
“sniff test”
may detect
Alzheimer’s
disease early.
The Scent of Science
Predisposition for Addiction
Most of us do not give our sense of smell a
passing thought unless there are cookies in
the oven or fl owers in bloom. But scientists are
probing this underappreciated sense to better
comprehend the workings of our brains, from
memory formation to Alzheimer’s disease.
Some of the latest fi ndings:
■ Smell and memory are intimately related—
just think about how suddenly a familiar scent
can whisk you into the past. Now a new study
shows that smell can help the brain encode
memories, too. Volunteers memorized the
locations of several objects while smelling a
rose scent, then some of them were exposed
to the same scent while they slept. Those with
perfumed sleep remembered the locations of
the objects much better than their fragrancefree
peers did, because the scent probably
reactivated memories stored temporarily in
the hippocampus.
■ Neurodegenerative diseases such as
Alzheimer’s and Parkinson’s often damage
the sense of smell fi rst, because there is
more neurological machinery devoted to the
other senses. A new “sniff test” could provide
an early warning for these diseases. People
with a normal sense of smell unconsciously
stop sniff ing as soon as their brain detects an
odor, but those with olfactory damage take
the same large sniff regardless of whether an
odor is present. By measuring the amount of
Many people assume that an addict’s substance abuse is
responsible for the damage done to his or her brain. New
research shows that some of that “damage” may have
been there to begin with.
Chronic drug users have fewer dopamine
D2 receptors than nonusers in the reward
pathways of their brain, which often
makes them less sensitive to natural pleasures
such as food and attractive mates.
Scientists believe this receptor defi cit may
reinforce addiction by causing users to
seek from drugs what they are unable to
get naturally—the “high” caused by a surge of dopamine.
Now Jeffrey Dalley and his colleagues at the University of
Cambridge have shown that some people may be born with
an abnormally low D2 receptor count, predisposing them to
impulsive behavior and drug addiction.
The team compared the brains of six impulsive rats and six
normal rats and then allowed the animals to self-administer
cocaine. The impulsive rats became addicted more quickly
than their nonimpulsive lab mates, and they showed a
air taken in during sniffs, the new test can
reveal a damaged sense of smell before it is
otherwise noticeable.
■ But why do we sniff in the fi rst place? Olfactory
neurons, once thought to respond only to the
chemicals that constitute odors, have now
been shown to activate when air hits the inside
of the nose. The harder we sniff, the more
excited these neurons become, and the better
they are able to detect and decode scents.
—Karen Schrock
signifi cantly lower number of D2 receptors in the ventral
striatum, a brain region associated with reward anticipation
and craving. The researchers found no differences in the
dorsolateral striatum, an area involved in compulsive drugseeking
behavior. A decrease in D2 receptors within this brain
area, according to past fi ndings, is seen
most commonly after habitual drug use.
“This last point is crucial because it
suggests that progressive drug use
produces progressive changes in the
brain,” Dalley says.
The scientists proposed a hypothesis:
some drug addicts are born with a
localized reduction of D2 receptors in the
ventral striatum. This anomaly predisposes them to high
levels of impulsivity, which may lead to their initial
experimentation with drugs. Long-term drug abuse, in turn,
may cause damage in the dorsolateral striatum and other
parts of the brain’s reward pathway, causing addicts to
compulsively seek out drugs.
If the researchers are correct, D2 receptors may one day
be used to identify people at high risk for drug abuse.
—Thania Benios
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