Medics are keen to find concrete physical evidence to help
those children who feel they are trapped in the body of the opposite sex. One key
brain region involved is the BSTc, an area of grey matter. But the region is
too small to scan in a living person so differences have only been picked up at
post-mortem.
Antonio Guillamon's team at the National University of
Distance Education in Madrid, Spain, think they have found a better way to spot
a transsexual brain. In a study due to be published next month, the team ran
MRI scans on the brains of 18 female-to-male transsexual people who'd had no
treatment and compared them with those of 24 males and 19 females.
They found significant differences between male and female
brains in four regions of white matter – and the female-to-male transsexual
people had white matter in these regions that resembled a male brain (Journal
of Psychiatric Research, DOI: 10.1016/j.jpsychires.2010.05.006). "It's the
first time it has been shown that the brains of female-to-male transsexual
people are masculinised," Guillamon says.
In a separate study, the team used the same technique to
compare white matter in 18 male-to-female transsexual people with that in 19
males and 19 females. Surprisingly, in each transsexual person's brain the
structure of the white matter in the four regions was halfway between that of
the males and females (Journal of Psychiatric Research, DOI: 10.1016/j.jpsychires.2010.11.007).
"Their brains are not completely masculinised and not completely
feminised, but they still feel female," says Guillamon.
Guillamon isn't sure whether the four regions are at all
associated with notions of gender, but Ivanka Savic-Berglund at the Karolinska
Institute in Stockholm, Sweden, thinks they might be. One of the four regions –
the superior longitudinal fascicle – is particularly interesting, she says.
"It connects the parietal lobe [involved in sensory processing] and frontal
lobe [involved in planning movement] and may have implications in body
perception."
A 2010 study of 121 transgender people found that 38 per
cent realised they had gender variance by age 5. White matter differences could
provide independent confirmation that such children might benefit from
treatment to delay puberty.
A study by Sean Deoni's team at King's College London
suggests it may soon be possible to look for these differences in such
children. Deoni's team adapted an MRI scanner to be as quiet as possible so it
could be used to monitor the development of white matter in sleeping infants.
Using new image analysis software they could track when and where myelin – the
neuron covering that makes white matter white – was laid down (Journal of
Neuroscience, vol 31, p 784). Although the sample was too small to identify any
gender differences in development, Deoni expects to see differences developing
in the brain "by 2 or 3 years of age".
Guillamon thinks such scans may not help in all cases.
"Research has shown that white matter matures during the first 20 to 30
years of life," he says. "People may experience early or late onset
of transsexuality and we don't know what causes this difference."
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