MONDAY, June 16 (HealthDay News) - MRI and PET scan studies are showing remarkable similarities between the brains of gay men and straight women, and between those of lesbians and straight men.

For example, the brains of straight men and of gay women share certain common features: both are slightly asymmetric, with the right hemisphere larger than the left, say the Swedish researchers.

On the other hand, the brains of gay men and straight women are both symmetrical.

Similar trends emerged when scientists tracked connectivity in the amygdala, the region of the brain involved in emotional learning and in activating the fight-or-flight response. They noted strong similarities between gay men and straight women, and lesbians and straight men.

The findings are published in the current issue of the Proceedings of the National Academy of Sciences.

"This is a very interesting study demonstrating a possible neurobiological relationship in brain size between gay men and straight women," said Paul Sanberg, distinguished professor of neurosurgery and director of the University of South Florida Center for Aging and Brain Repair in Tampa.

"I do think this is pointing to some type of neurobiological underpinning [to sexual orientation]," added Keith A. Young, associate professor of psychiatry and behavioral science at the Texas A&M Health Science Center College of Medicine in Waco. He was not involved in the study.

"It's hard to know if that's related to genes, or what might happen in the womb -- I think those are the two primary options," said Young, who is also co-director of the Central Texas Veterans' Health Care System Neuropsychiatry Research Program in Temple. "How do those affect early brain development, and how might either genes or exposure to hormones in the womb change the trajectory of the development of emotional processing centers?"

The neurobiology of sexual orientation remains a controversial topic. Some research suggests that the brain activity of homosexual individuals in areas unrelated to sex mirror brain activity in straight individuals of the opposite sex. And certain psychological studies have revealed differences in how men and women use the brain's different hemispheres for verbal tasks, for example.

For this study, researchers from the Karolinska Institute in Stockholm first performed magnetic resonance imaging (MRI) on 90 participants -- 25 heterosexual men and an equal number of heterosexual women, plus 20 homosexual men and 20 homosexual women.

The right hemispheres of straight men and gay women were found to be bigger than the left, while the respective volume of these two cerebral hemispheres were about the same in gay men and straight women.

Fifty of the original participants also underwent positron emission tomography (PET) measurements of blood flow to the brain, designed to analyze connections between the right and left amygdalas. PET scans were performed both while the participants were resting and while they were smelling unscented air.

Here again, lesbians appeared to react more like straight men, while gay men were more like straight women.

According to the authors, fight-or-flight reactions -- controlled in part by the amygdala -- are more common in men than in women.

The differences, which were not related to sexual attraction, could be due to environmental effects, genetics or the influence of sex hormones, the study authors stated.

Although some sex differences in brain size are visible at birth, the brain continues to develop as the child matures, meaning that environmental factors could also play a role.

The current wisdom regarding genetics and sexual orientation posits that genes may play a role in male homosexuality but not in female homosexuality, the researchers said.

Experts have also speculated that exposure to sex hormones prenatally influence sexual preference. Male rhesus monkeys have more androgen (a male sex hormone) receptors in the right side of the brain, while females have an equal distribution of receptors for the hormone.

And brain asymmetry in male rats is established by early exposure to androgens. Symmetry in female brain hemispheres can be reversed by removing the ovaries soon after birth.

This study does little to clarify the reasons behind the differences, the researchers added, although it certainly does add to the debate.

More information

There's more on how the brain works at The Franklin Institute.