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The amygdala (Gr. almond) is a small structure in the medial temporal lobe of the brain, a few inches from either ear.
The left and right amygdalae are not physically identical. Neuroimaging studies suggest that the right amygdala has a larger volume than the left. Additionally, the two are functionally asymmetrical as well.
Recent research points to the circuits, not just the structure. The nerves coursing through the amygdala connect it to a number of important brain centers, including the neocortex and visual cortex.
Specifically, the left amygdala appears to be consistently activated in response to negative, withdrawal-related stimuli. The right amygdala seems to be involved in autonomic responses (heart rate, skin responses, blood pressure, etc.).
Amygdala activation can be driven by one's own emotional experiences. It has long been linked with a person's mental and emotional state, playing a critical role in several emotional behaviors.
These include the evaluation of and response to emotionally salient information, anxiety disorders, depression, and the arousal-related components of drug abuse.
The fact that the amygdala influences diverse emotional responses suggests that it serves an important emotional function. It likely influences perception and arousal responses before higher-level, cognitive analysis of the stimuli.
functional Magnetic Resonance Imaging (fMRI) uses radio waves and a strong magnetic field rather than X-rays to take clear and detailed pictures of internal organs and tissues.
Scientists use this technology to identify brain regions where blood vessels are expanding, chemical changes are taking place, or extra oxygen is being delivered. These are indications that a particular part of the brain is processing information and giving commands to the body.
As a patient performs a particular task, the metabolism will increase in the brain area responsible for that task, changing the signal in the MRI image. So by performing specific tasks that correspond to different functions, scientists can locate the part of the brain that governs that function.
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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.
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.
The scans showed slight asymmetry in the brains of heterosexual males and homosexual females, with the right hemisphere slightly larger than the left.
The right side of the brain in heterosexual males was typically 2% larger than the left.
Lesbians showed a similar asymmetry, with the right hand side of the brain 1% larger than the left.
Scans done on homosexual males and heterosexual females indicated both sides of their brains were the same size.
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.
The overall research is part of a larger effort to identify differences between the male and female brain, in the hope they will shed light on why some mental disorders affect males and females so differently. For example, major depressive disorders are far more common and persistent in females, while autism is around four times more common in boys than girls.
"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.
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 ]
Abstract
Cerebral responses to putative pheromones and objects of sexual attraction were recently found to differ between homo- and heterosexual subjects. Although this observation may merely mirror perceptional differences, it raises the intriguing question as to whether certain sexually dimorphic features in the brain may differ between individuals of the same sex but different sexual orientation. We addressed this issue by studying hemispheric asymmetry and functional connectivity, two parameters that in previous publications have shown specific sex differences. Ninety subjects [25 heterosexual men (HeM) and women (HeW), and 20 homosexual men (HoM) and women (HoW)] were investigated with magnetic resonance volumetry of cerebral and cerebellar hemispheres. Fifty of them also participated in PET measurements of cerebral blood flow, used for analyses of functional connections from the right and left amygdalae. HeM and HoW showed a rightward cerebral asymmetry, whereas volumes of the cerebral hemispheres were symmetrical in HoM and HeW. No cerebellar asymmetries were found. Homosexual subjects also showed sex-atypical amygdala connections. In HoM, as in HeW, the connections were more widespread from the left amygdala; in HoW and HeM, on the other hand, from the right amygdala. Furthermore, in HoM and HeW the connections were primarily displayed with the contralateral amygdala and the anterior cingulate, in HeM and HoW with the caudate, putamen, and the prefrontal cortex. The present study shows sex-atypical cerebral asymmetry and functional connections in homosexual subjects. The results cannot be primarily ascribed to learned effects, and they suggest a linkage to neurobiological entities.
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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. 
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