May 13, 2005  |  By: LOU CHIBBARO JR.  | COMMENTS     

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.

Supports genetic theory
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 ...