22 November 2008
| Isolating Comparable Brain Details In Straight Women And Gay Men |
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| Science - Neuroscience | |||
| Written by TS-Si News Service | |||
| Tuesday, 17 June 2008 17:00 | |||
Stockholm, Sweden. Research into the origins and activation of sexual orientation have become more rigorous with the advent of neuro-based tools and quantitative measurements. Such efforts surpass social studies that depend on everyday observation but ignore the underlying neurobiological mechanisms that underlie the observed behavior.
There are no known causal connections between Harry Benjamin Syndrome (HBS) and sexual orientation, but many social studies are based on an unproven association between the two. Quantitative research into sexual orientation has many benefits. There is potential relevance for people born with HBS since the isolation of causative factors for sexual orientation could eliminate unwarranted associations and support more focused attention to the HBS condition.
PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects. Ivanka Savic and Per Lindström. PNAS doi: 10.1073 / pnas.0801566105). [ Supporting Information PDF ]
A new study from from Sweden's Karolinska Institute (Stockholm Brain Institute) explores the biological basis for sexual orientation. Neuroscientists have discovered striking similarities between the brains of gay males and straight females, offering additional evidence that our neural circuitry includes hardwiring for sexual orientation.
 The current work, led by the neurobiologist Ivanka Savic, builds on the previous research that identified differences in spatial and verbal abilities related to sex and sexual orientation. Tests have found gay males and straight females fare better at certain language tasks, while heterosexual males and lesbians tend to have better spatial awareness. The researchers published their findings in the Proceedings of the National Academy of Sciences (PNAS).
A number of previous studies have looked at the roles genetic, biological and environmental factors play in sexual orientation. However, there has been fragmented evidence that any one factor plays a decisive role. Many scientists believe both nature and nurture play a part.
The new scans reveal homosexual males and heterosexual females have symmetrical brains, with the right and left hemispheres almost exactly the same size. Conversely, lesbians and straight men have asymmetrical brains, with the right hemisphere significantly larger than the left.
The findings suggest that biological factors influencing sexual orientation - such as exposure to testosterone in the womb — may also shape the brain's anatomy.
Savic and her colleague Per Linström took MRI brain scans of 90 volunteers who had been divided into four groups of similar ages according to whether they were male, female, heterosexual or homosexual.
Covariations with the respective amygdala seed region in hetero- and homosexual subjects. The Sokoloff scale indicates T values.
Clusters detected at T = 3.0 are superimposed on the standard MR image of the brain.
Image courtesy of the National Academy of Sciences (PNAS).
The scans showed slight asymmetry in the brains of heterosexual males and homosexual females, with the right hemisphere slightly larger than the left.
The results could explain a University of London study earlier this year that found gay males and straight females share a poor sense of direction compared with heterosexual males, and were more likely to navigate using landmarks alone.
The right hand side of the brain dominates spatial capabilities, so may be slightly more developed in heterosexual males and lesbians. An earlier study by the same team found gay males and straight females outperformed lesbians and straight males at tasks designed to test verbal fluency.
Speaking of the current study, "The observations cannot be easily attributed to perception or behavior," the researchers from Sweden's Karolinska Institute wrote. "These observations motivate more extensive investigations of larger study groups and prompt for a better understanding of the neurobiology of homosexuality," they wrote.
The current study does not indicate whether the differences in brain shape are inherited or due to exposure to hormones such as testosterone in the womb and if they are responsible for sexual orientation. "Whether they may relate to processes laid down during the fetal or postnatal development is an open question."
Savic's team has yet to confirm whether the differences in brain shape are responsible for sexual orientation, or are a consequence of it. To find out, they have begun another study to investigate brain symmetry in newborn babies, to see if it can be used to predict their future sexual orientation.
"These differences might be laid down during brain development in the womb, or they could happen after birth, though it could very likely be a combination of the two," said Savic.
The scientists used positron emission tomography (PET) to look at brain wiring in a smaller group of volunteers, finding that certain brain circuits linked to emotional responses were the same in gay males and straight females. They documented characteristics in the amygdala — an area of the brain responsible for emotion, mood and anxiety — that are shared by homosexual males and heterosexual females.
The amygdala [cf. sidebar] is a key area for emotional responses, such as the "fight-or-flight" response. The investigators found it was wired in a similar fashion in gay males and heterosexual females as well as lesbians and heterosexual males. Brain scans also showed the same symmetry among lesbians and heterosexual males. "There's a well known uneven sex distribution in the number of psychiatric disorders and trying to understand sex differences, and differences in orientation, may give you a hint of the mechanism underlying these diseases," said Savic.
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| Last Updated on Wednesday, 18 June 2008 15:03 |
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The TS-Si News Service is a collaboration of TS-Si staff, contributors, and corresponding institutions. Contents do not necessarily convey official positions of TS-Si, its partners, or affiliates