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January 27, 2015

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Brain wiring may reflect gender identity

Jan. 12, 2015
Courtesy of Medical University of Vienna
and World Science staff

The wir­ing be­tween many parts of our brains dif­fers de­pend­ing on wheth­er we think of our­selves as male or fema­le, a study has found. 

The study spe­cif­ic­ally ex­am­ined peo­ple’s self-per­cep­tion as male or fema­le, as dis­tinct from their phys­i­cal form. In fact, it con­clud­ed that in trans­sex­u­al peo­ple—where the self-im­age and the phys­i­cal ap­pear­ance don’t match—the char­ac­ter­is­tics of this brain wir­ing are some­where in be­tween those of more typ­i­cal men and wom­en.

Artist's rendering of connections among nerve cells (courtesy of Medical Univ. of Vienna)


The sci­en­tists be­hind the study took var­i­ous meas­ures of how fast wa­ter mo­le­cules flow in dif­fer­ent parts of the brain, which serves as an in­dica­t­ion of the fi­ne struc­ture of nerve con­nec­tions be­tween brain re­gions. 

These con­nec­tions con­sist of long ex­ten­sions of nerve cells, called ax­ons, which con­nect to oth­er nerve cells in a very in­tri­cate net­work.

The find­ings, by brain re­search­er Ge­org S. Kanz of the Med­i­cal Uni­ver­sity of Vi­en­na and col­leagues, were pub­lished Nov. 12 in the Jour­nal of Neu­ro­sci­ence

The re­search­ers used a brain scan­ning meth­od known as dif­fu­sion-based mag­net­ic res­o­nance to­mog­ra­phy, which ex­am­ines the spread, or dif­fu­sion, of wa­ter in each part of the brain in or­der to re­veal the fi­ne struc­ture of brain con­nec­tions.

While the ex­act rea­sons for the find­ings are un­known, the re­search re­vealed a great­er amount of dif­fu­sion over­all in females than in ma­les, the au­thors said. Trans­sex­u­al peo­ple were found to be some­where in be­tween, with “fema­le-to-ma­le” trans­sex­u­als (phys­ic­ally female peo­ple who con­sid­er them­selves ma­le) slightly more si­m­i­lar to wom­en than their “ma­le-to-fema­le” coun­ter­parts.

The sci­en­tists stud­ied be­tween 21 and 23 peo­ple from each of the four groups.

These re­sults were al­so re­lat­ed to lev­els of the hor­mone tes­tos­ter­one, the in­ves­ti­ga­tors said—but the re­sults weren’t ex­plained by tes­tos­ter­one lev­els, as the results per­sisted af­ter ac­count­ing for those lev­els, and for sex­u­al ori­enta­t­ion. The find­ings, how­ev­er, fit with the hy­poth­e­sis that the “hor­monal en­vi­ron­ment” shortly be­fore and af­ter birth af­fects brain con­nec­tions, they wrote.


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The wiring between many parts of our brains differs depending on whether we think of ourselves as male or female, a study has found. The study specifically examined people’s self-perception as male or female, as distinct from their physical form. In fact, it concluded that in transsexual people—where the self-image and the physical appearance don’t match—the characteristics of this brain wiring are somewhere in between those of more typical men and women. The scientists behind the study took various measures of how fast water molecules flow in different parts of the brain, which serves as an indication of the fine structure of nerve connections between brain regions. These connections consist of long extensions of nerve cells, called axons, which connect to other nerve cells in a very intricate network. The findings, by brain researcher Georg S. Kanz of the University Clinic for Psychiatry and Psychotherapy of the MedUni Vienna and colleagues, were published Nov. 12 in the Journal of Neuroscience. The researchers used a brain scanning method known as diffusion-based magnetic resonance tomography, which examines the spread, or diffusion, of water in each part of the brain in order to reveal the fine structure of brain connections. While the exact reasons for the findings are unknown, the research revealed a greater amount of diffusion overall in females than in males, the authors said. Transsexual people were found to be somewhere in between, with “female-to-male” transsexuals (physically female people who consider themselves male) were slightly more similar to women than “male-to-female” transsexuals (physically male who consider themselves female). The scientists studied between 21 and 23 people from each of the four groups. These results were also related to levels of the hormone testosterone, the investigators said—but the results weren’t explained by testosterone levels, as they persisted after accounting for those levels, and for sexual orientation. The findings, however, fit with the hypothesis that the “hormonal environment” shortly before and after birth affects brain connections, they wrote.