Seeing the world in Grey and White…
It is exciting to see that neuroimaging science is getting to the point where we needn’t wait until subjects are dead before we can examine their brains in better detail. Exciting recent developments include a pair of papers from Spain in which Diffusion Tensor Imaging (DTI), a form of MRI, was used to compare FtM and MTF transsexuals to control men and women. The results were that transsexual brains exhibit white matter features, that are known to be sexually dimorphic, mid-way between male and female morphologies. The studies are doubly interesting, because the subjects had not yet begun exogenous Hormone Replacement Therapy (HRT) eliminating the possibility that the features were the result of activating effects of HRT.
So, does this mean that we have proven that transsexuals have “intersexed brains”?
Yes… and at the same time… No.
First, in the case of Female-to-Male transsexuals, it has always been noted that the majority were both gynephilic and masculine behaving since early childhood. So, these results are hardly surprising. However, the exact same thing can be said for butch lesbians, who do not necessarily identify as men. So, if we were to image the brains of a population of such butch lesbians, would we see the same masculinized neurological features? Would we see that transmen were more masculinized than butch lesbians, who would in turn be more masculinized than average lesbians? Also, if we imaged the brains of a population of gay identified transmen (“transfags”), would we then see that they do not share such masculinized features? I’m betting we would, and eagerly await the results of such imaging.
Which brings us back to the Male-To-Female subjects in the Spanish DTI study. As Lawrence has pointed out, different cultures have varying prevalence of non-homosexual transsexuality. Spain is one of those cultures where very few of the MTF transsexuals are non-exclusively-androphilic, with only 9% of the MTF transsexuals being non-exclusively-androphilic. In this case, as reported in the paper, there were no such non-exclusively-androphilic subjects. All were described as “early onset” and androphilic. Thus, this study only tells us about one type of transsexual.
Sadly, there wasn’t a gay male control group. It would have been interesting to see if the MTF transsexuals exhibited the same or more feminization (non-masculinization) of these sexually dimorphic brain structures. One hypothesis regarding exclusively androphilic transsexuals is that they are the extreme end of a range of feminine (hypo-masculine) homosexual males, who find that their innate femininity sufficient that living life as women is a better rational as well as emotional choice. (That is to say, that they are so feminine, like women, that they are women at heart.) I eagerly await DTI imaging studies that can test this hypothesis.
Now that we have DTI data on the HSTS population, we must gather data on the non-HSTS population to see if they do or do not exhibit such partially feminized (hypo-masculinized) features. Once again, we have no data that would show, conclusively, one way or the other, that both types of MTF transsexuals share neurologic features that would serve as the basis of a unifying etiology.
It’s not much of a limb to go out on, given the very strong data that shows that there are two types with two separate etiologies, but I’d be willing to bet that that when we do have such DTI images of gynephilic MTF transsexuals, that they will be no different than typical straight men, at least with respect to these particular sexually dimorphic features.
Which brings us to another study (Luders) looking at grey matter instead of white sexually dimorphic brain structure differences between MTF transsexuals and control men and women, this time one that suggests that for a very localized spot, they found a feature in MTF transsexuals that is different and possibly ‘more feminine’ than male controls. However, globally, the MTF brains were shown to more like men, and if anything, perhaps more masculine than control men, as they showed less grey matter then men, who show less grey matter then women. But one spot, the right putamen, is definately different in MTF transsexuals than in control men, showing more grey matter, in fact, more grey matter than the control women.
Sadly, as the researchers themselves point out, they may have included both exclusively androphilic and non-exclusively androphilic transsexuals without analyzing them separately; Of the 24 subjects, six self-reported to be androphilic and 18 reported to be gynephilic. It is heartening that the researchers recognize that in the future, they need to make the distinction and analyze the two types separately. Further, as these TS folk are pre-HRT, their ages may be used as a statistical proxy for our purposes, as MTF transkids usually (>90%) begin transition and HRT before age 25: the mean age was 47 years old, standard deviation of 13 years, with the range from 23 to 72. We know from Lawrence’s re-sorting from the Smith data set that a fair number of MTF transsexuals who self-report being androphilic are not exclusively so, thus it is likely that less than six of the subjects are exclusively androphilic. A little math will show that that with a normal distribution, only one of the 24 subjects would have been expected to have been 25 or younger, so this 23 year old may be the only one. Therefor, as Luders et al did find a statistically significant signal, we might infer that it is more likely that it came from the non-exclusively androphilic type, and thus likely also autogynephilic.
While it may be tempting to declare that this feature found in the right putamen proves that MTF transsexuals, in-fact, that gynephilic MTF transsexuals, have a part of their brains that is femininized, this conclusion should not be drawn from this data, at this time. Although it may be a marker of transsexual neurology, it may not be from a feminization of this region, especially given that it shows more grey matter than both control men and women. It may be a marker of an unusual neurological development altogether unrelated to sexual dimorphism. It may in fact be a marker for autogynephilia, or proneness to erotic target location errors. In fact, such a marker is expected to be found. We need further studies.
First, we need to compare the two types of transsexuals to test if this is a marker of a unifying neurological feature of transsexuality. Second, we need to compare both of them to individuals who are sexually aroused at the thought of becoming amputees, to test the hypothesis that this may be a marker of proneness to erotic target location errors.
But, in the mean time, the world is not so much to be understood as black and white, but shades of grey and white.
For more essays on trans-brains see Brain Sex.
A good link to explore the first two papers further:
You may wish to read a new blog entry on an additional paper on this topic.
Rametti G, Carrillo B, Gómez-Gil E, Junque C, Segovia S, Gomez A, Guillamon A., “White matter microstructure in female to male transsexuals before cross-sex hormonal treatment. A diffusion tensor imaging study.”
Rametti G, Carrillo B, Gómez-Gil E, Junque C, Zubiarre-Elorza L, Segovia S, Gomez A, Guillamon A., “The microstructure of white matter in male to female transsexuals before cross-sex hormonal treatment. A DTI study.”
Luders E, Sánchez FJ, Gaser C, Toga AW, Narr KL, Hamilton LS, Vilain E., “Regional gray matter variation in male-to-female transsexualism.”