Two groups of boys exposed prenatally to exogenously administered estrogen and progesterone were studied on several parameters of psychosexual development. Subjects were twenty 6-year-olds and twenty 16-year-olds whose diabetic mothers received these hormones to prevent pregnancy complication. Hormone-exposed boys were compared with same-aged boys whose mothers had not received exogenous hormones and matched for age and socioeconomic class.
Sixteen-year-olds exposed to estrogen and progesterone were rated lower on several variables related to general "masculinity," assertiveness and athletic ability. Six-year-olds exposed to estrogen and progesterone were rated lower on aggressivity and athletic ability. There were two cases of hypospadias among experimental subjects. While it was not possible to rule out influences other than hormonal which may have influenced results, data suggest that prenatal sex hormone levels may influence some aspects of postnatal psychosexual development in boys.
From mygenes.co.nz, a non-peer-reviewed religious site, that nonetheless at least gives its references:Are transssexual brains different?:
The answer is probably yes, but not because of innateness. The altered brain microstructure is probably due to years of repetitive thinking, fantasy and preoccupation with body image.Thereby ignoring the work by Yalom above, and others.
However the results of studies on transsexuals may be better established than those on homosexual people showing somewhat more reproducible differences in the brains, though still with large overlap. This note argues these differences can be explained by preoccupied thinking and imagination alone.
Recently a paper by Savic and Arver (Savic and Arver, 2011) has appeared. Their innovation is to take a study group composed only of male-to-female gynephiles (i.e. those attracted sexually to women). Previous male-to-female studies mixed gynephiles and androphiles. The authors find that the brains in their study group were not feminised. There was no evidence for female brains in a male body; the brains were male-typical. This is contrary to many of the previous research studies on mixed groups, but the study is thorough. They also found that there were differences in the brains of their study groups which were not found in either heterosexual male or female brains. These regions have been identified as those possibly associated with bodily self-perception (they are also enlarged in those who do lots of meditation, focusing partly on body state).
The authors say this is a “highly speculative” interpretation, but it’s possible they are actually underestimating how much support it has. It is very clear that repeated patterns of mental exercise alone, as seen for example in navigation (London taxi drivers) and internet addiction (Maguire et al. 2006; Zhou et al. 2009) changes significantly the microstructure of the brain. Thinking, particularly repeated thinking, changes brain microstructure.
OK, let's look at these papers. First the Macguire paper, London taxi drivers and bus drivers: a structural MRI and neuropsychological analysis. Maguire, E.A., Woollett, K. and Spiers, H.J. (2006) Hippocampus 16, 1091-101.
Licensed London taxi drivers show that humans have a remarkable capacity to acquire and use knowledge of a large complex city to navigate within it. Gray matter volume differences in the hippocampus relative to controls have been reported to accompany this expertise. While these gray matter differences could result from using and updating spatial representations, they might instead be influenced by factors such as self-motion, driving experience, and stress. We examined the contribution of these factors by comparing London taxi drivers with London bus drivers, who were matched for driving experience and levels of stress, but differed in that they follow a constrained set of routes. We found that compared with bus drivers, taxi drivers had greater gray matter volume in mid-posterior hippocampi and less volume in anterior hippocampi. Furthermore, years of navigation experience correlated with hippocampal gray matter volume only in taxi drivers, with right posterior gray matter volume increasing and anterior volume decreasing with more navigation experience. This suggests that spatial knowledge, and not stress, driving, or self-motion, is associated with the pattern of hippocampal gray matter volume in taxi drivers. We then tested for functional differences between the groups and found that the ability to acquire new visuo-spatial information was worse in taxi drivers than in bus drivers. We speculate that a complex spatial representation, which facilitates expert navigation and is associated with greater posterior hippocampal gray matter volume, might come at a cost to new spatial memories and gray matter volume in the anterior hippocampus.OK, so we're talking about the hippocampus, and only the hippocampus. Furyhermore, we're only looking at grey matter distribution within that particular part of the brain.
Now let's look at the Zhou paper : Gray matter abnormalities in Internet addiction: A voxel-based morphometry study. Zhou et al, Eur J Radiol. 2011 Jul;79(1):92-5. :
BACKGROUND:This study aims to investigate brain gray matter density (GMD) changes in adolescents with Internet addiction (IA) using voxel-based morphometry (VBM) analysis on high-resolution T1-weighted structural magnetic resonance images.This time, we're looking at other parts of the brain : the left anterior cingulate cortex, left posterior cingulate cortex, left insula, and left lingual gyrus. And again, only grey matter distribution.
METHODS:Eighteen IA adolescents and 15 age- and gender-matched healthy controls took part in this study. High-resolution T1-weighted magnetic resonance imaging scans were performed on the two groups. VBM analysis was used to compare the GMD between the two groups.
RESULTS:Compared with healthy controls, IA adolescents had lower GMD in the left anterior cingulate cortex, left posterior cingulate cortex, left insula, and left lingual gyrus.
CONCLUSIONS: Our findings suggested that brain structural changes were present in IA adolescents, and this finding may provide a new insight into the pathogenesis of IA.
I've blogged about the Savic paper before, and that post is worth re-visiting.
MtF-TR displayed also singular features and differed from both control groups by having reduced thalamus and putamen volumes and elevated GM volumes in the right insular and inferior frontal cortex and an area covering the right angular gyrus.
I'll add another paper to the mix : Regional gray matter variation in male-to-female transsexualism. by Luders et al Neuroimage. 2009 Jul 15;46(4):904-7.
Results revealed that regional gray matter variation in MTF transsexuals is more similar to the pattern found in men than in women. However, MTF transsexuals show a significantly larger volume of regional gray matter in the right putamen compared to men.
Altogether, females had the largest gray matter volumes in all but two significant clusters, which were located in the left and right putamen. Here, MTF transsexuals had the largest gray matter volumes (see Fig. 1). For the remaining clusters, MTF transsexuals had the smallest gray matter volumes, but their data spectrum largely overlapped with that of males.
OK, let's put this in a table:
|no data||no data||no data||no data||no data||no data||no data||no data|
|no data||-||-||-||no data||no data||no data||no data||no data|
|no data||no data||no data||no data||no data||+||no data||no data||no data|
|no data||no data||no data||no data||no data||no data||+||+||+|
What conclusions can we draw from this, in terms of cause and effect? Well, the London Taxi Driver paper is pretty good evidence of neuroplaticity at work. The ratio of grey matter in the hippocampus changes as the result of repeated actions and thoughts. This is only to be expected, as the hippocampus is both one of the simplest neurological structures, and also known to be associated with laying down long-term memory. Total amount doesn't change, but distribution changes.
Regarding the Japanese
In both these cases, the repetitive physical and mental actions occur for significant periods of the day, totalling half the waking hours or more. It's difficult imagining what repetitive action is involved in being MtoF, male or female for that matter.
In summary, the case for neuroplasticity as regards grey matter distribution is very strong. As regards grey matter total, less strong, but still stronger than I'd thought in certain areas. Regarding the putamen though, very weak or non-existent.