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LOU CHIBBARO JR.
Friday, May 13, 2005
Researchers in Sweden have discovered that a section of the brain that may be
linked to sexual arousal responds differently in gay and straight men who were
asked to smell chemical scents extracted from male and female sex hormones.
Swedish neuroscientists report findings showing that the hypothalamus region
of the brain responded in a similar way in a group of gay men and heterosexual
women after both groups sampled odors obtained from male perspiration.
Brain functions of the study subjects were examined through separate positron
emission tomography, or “PET” scans, and magnetic resonance imaging,
or “MRI,” tests.
The same imaging tests given to a sample of heterosexual men showed that their
hypothalamuses did not respond with neurological activity in the same way to
the odor from male perspiration but did respond to a separate odor obtained
from an estrogen-like substance found in women’s urine, according to the
study.
The sample of gay men and heterosexual women, in turn, showed no significant
neurological response in their hypothalamuses when asked to sample the odor
from the female substance, known as EST.
The study was conducted by a team of researchers from the Department of Clinical
Neuroscience & Medicine at Karolinska University Hospital in Stockholm,
Sweden. It was published in the May 10 edition of the Proceedings of the U.S.
National Academy of Sciences.
The test subjects were also asked to smell several “control” substances,
such as lavender oil, cedar oil, and butanol, the study said. These substances
did not trigger any hypothalamic responses during the brain imaging tests, the
study said.
Some scientists familiar with the study said it appears to support the theory,
advanced by previous studies, that link sexual orientation to the genetic makeup
of individuals through mechanisms believed to be acquired at birth rather than
through psychological factors acquired during childhood or adolescence.
The odor obtained from male perspiration used in the Swedish study comes from
a testosterone derivative called AND, the Swedish researchers said in their
study. Some scientists believe AND is a pheromone, a substance shown in animal
studies to trigger sexual responses during mating.
Scientists disagree over whether pheromones play a role in sexual arousal among
humans.
Ivanka Savic, Hans Berglund, and Per Linstrom — the lead researchers
in the Swedish study — conclude that their findings provide new evidence
that pheromones might play a role in human sexuality.
The three researchers also state that their study “further indicates
hypothalamic involvement in physiological processes related to sexual orientation
in humans.”
But they express caution that the different responses in the hypothalamus between
gay men and heterosexual men could be related to “an acquired sensitization
to AND stimuli in the hypothalamus [among gay men] … due to repeated sexual
exposure to men.” They also note that they exposed their research subjects
to far higher concentrations of the hormone-produced odors than would be expected
in normal human interaction, and that the higher concentrations could have played
a role in the hypothalamic responses.
Sandra Witelson, a specialist in brain anatomy with the DeGroote School of
Medicine at McMaster University in Ontario, Canada, told the Associated Press
that the Swedish study “clearly shows a biological involvement in sexual
orientation.”
The Swedish study follows findings in the early 1990s by U.S. researcher Simon
LeVay, which showed that the hypothalamus region of the brain is significantly
larger in heterosexual men than it is in gay men and heterosexual women. LeVay
conducted his research by measuring the size of the hypothalamuses of human
cadavers. LeVay’s study is believed to have provided the first indication
that the hypothalamus may in some way be related to an individual’s sexual
orientation.
The study by the Swedish researchers also follows a study published earlier
this month in the journal Human Genetics by National Institutes of Health researcher
Dean Hamer identifying similarities in the chromosomes of a sample of gay brothers.
Hamer’s latest study is a follow-up to his previous research showing the
existence of clusters of gays within families, a development that suggests,
Hamer and other researchers said, a link between homosexuality and the genetic
makeup of individuals.
Hamer called his findings important because they reinforce the theory that
“sexual orientation is at least partially genetic and that there are many
different genes, not just one or two,” that may be linked to sexual orientation.
According to the report this week in the Proceedings of the National Academy
of Sciences, the Swedish researchers used a total of 36 people as subjects for
their study. The 36 were divided into three groups of 12: gay men, heterosexual
men, and heterosexual women. All 23 were “healthy, un-medicated, right-handed,
and HIV-negative,” the study report says. The report says all 36 were
also within a similar age range and had a similar level of education.
The report said all of the heterosexual men and women scored a “0”
on the Kinsey scale of heterosexuality/homosexuality, while all of the gay men
scored a 6 on the Kinsey scale. The scale, developed by famed sex researcher
Alfred Kinsey, uses a 0 to describe people who say they engage solely in opposite
sex sexual activity throughout their lives. A score of 6 is used to describe
people who engage solely in same-sex sexual activity during their lifetime.
Brian Mustanski, a university of Illinois researcher who worked with Hamer
on the study linking chromosomes to gay siblings, called the study by the Swedish
researchers “very well done” and a significant advance in findings
suggesting a genetic link to sexual orientation.
“What’s so compelling is that the region of the brain [that the
study examined] has been shown to be involved in sexual activity and sexual
orientation,” Mustanski said.
Lou Chibbaro Jr. can be reached at lchibbaro@washblade.com.
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