http://www.washingtonpost.com/ac2/wp-dyn/A5340-2001Aug27?language=printer

Study Finds Brain Reacts To Sex-Specific Chemicals

By Shankar Vedantam
Washington Post Staff Writer Tuesday, August 28, 2001; Page A01

Certain chemicals similar to the male and female sex hormones trigger
distinctive brain activity when sniffed by the opposite gender, providing
the strongest evidence yet for the existence of human "pheromones,"
scientists reported yesterday.

Brain scans of two dozen volunteers in Sweden found that a part of the
brain involved in regulating sexual behavior lit up when women were exposed
to a substance similar to testosterone, while the same brain area in men
lit up when they were exposed to a substance similar to estrogen.

The research, which convincingly demonstrated that the effect of these
chemicals on the brain is not because of their odor, will be of interest to
romantics, pharmaceutical companies and savants of armpit chemistry.

Although human pheromones have long been embedded as real in the public
imagination, spawning a bustling market of perfumes and potions for suitors
seeking to turn on the opposite sex, scientists have long debated whether
they existed.While those scientific questions persist, the new research
suggests that at least some human behaviors may be subliminally influenced
by invisible chemicals with no obvious odors.

"It's great, it's very exciting and very interesting," said Noam Sobel, a
neuroscientist at the University of California at Berkeley who studies
pheromones.

No visual or auditory signal that he could think of, said Sobel, has ever
been known to produce so sharp a distinction between men and women. "Is it
proof that these are pheromones?" asked Sobel. "No, but it is another block
in the wall and it is a block in the wall that closes up the hole."

While animal studies have shown that the part of the brain activated in the
new study -- the hypothalamus -- is associated with reflexive sexual
responses, it remains unclear whether humans necessarily respond in
similarly predictable ways.

"I'm leaving open the possibility that these and other compounds may be
human pheromones but one should not walk away after reading this paper that
these two compounds are the [only] human pheromones, and that one affects
females and the other affects males," said Charles J. Wysocki, a
neuroscientist at the Monell Chemical Senses Center in Philadelphia.
"There's much much more work that needs to be done."

Besides their role in sex, pheromones are widely involved in regulating
other behaviors in the animal world. Animals, for example, mark territory
using pheromones. Scientists have also found a specific piece of tissue in
the nasal passageways of animals called the vomeronasal organ (VNO), which
is what they use to sense pheromones.

There has been some evidence that humans are similarly influenced by
pheromones. Women living in groups, for example, begin to tend to
menstruate together -- an effect that some scientists believe is caused by
pheromones. But scientists have been unable to identify any specific
chemicals that clearly act like pheromones, or convincingly prove that
humans have active VNOs.

Researchers at the Karolinska Institute in Stockholm exposed a dozen men
and a dozen women to a variety of smells. One was plain air, another
vanilla. The two others were the testosterone- and estrogen-related
compounds. While the subjects breathed the chemicals for a minute, the
researchers conducted scans of their brains.

None of the chemicals seemed to be particularly striking in terms of odor.
Participants rated all about average. Men did not show any striking brain
response to the testosterone-related chemical and women's brains were
likewise blase to the estrogen-related compound.

"One would expect that females would find the female compound more pleasant
but there was no difference," said Ivanka Savic, a neurologist in
Karolinska's department of neuroscience. "There was no [smell] difference
between males and females. This means it is not the smell component that is
responsible for the sex-specific activation. It's something else."

Whatever the something else, it caused an activation in the part of the
brains of both men and women that is involved in regulating sex -- the
hypothalamus, Wysocki said.

Activation of the hypothalamus in monkeys, according to published research,
elicits penile erections in male monkeys and copulatory behavior in
females, said Savic, in a telephone interview.

"In rodents, the females start ovulating as soon as they smell a male
pheromone," she said. "There is no way that pheromones are going to induce
such a reflex in humans because we are so much more complicated."

Among other differences, humans have a much larger frontal cortex than most
animals, and it is the part of the brain that is involved in many higher
cognitive functions, including inhibition and self-control.

Of the two chemicals studied, said Bernard Grosser, chairman of the
department of psychiatry at the University of Utah School of Medicine,
androstadienone, the testosterone-related compound, was shown to reduce
pulse rate, increase temperature and produce relaxation in women in other
studies. No obvious sexual effects were observed.

The estrogen-like compound produced similar, but not identical, effects in
men. Feelings of well-being and relief from tension could be part of
humans' sexual responses, of course, and Grosser pointed out that other
chemicals may also be involved in regulating human sexual behavior.

Savic, who published her research in this week's issue of the journal
Neuron, said that it was interesting that the men and women in her study
chose "animalistic" associations when asked to describe the two chemicals.
Some said it smelled "sweaty" or like "an animal."

"One of my female subjects said this smells like my sister's old sanitary
napkins," said Savic. "To me, that's very illustrative."