Tag Archives: putamen

Sex Dimorphism of the Brain in Male-to-Female Transsexuals – Review

This study found that trans women (born male) had brains like men’s, at least in terms of gray and white matter volumes and hemispheric asymmetry.

In a few areas, trans women’s brains were different from both men’s and women’s brains.

The authors suggest that the differences they found between trans women and cis people’s brains are related to body perception.

They conclude:

“The present data do not support the notion that brains of MtF-TR are feminized. The observed changes in MtF-TR bring attention to the networks inferred in processing of body perception.”

The study only looked at trans women who were attracted to women. This is both a strength and a limitation of the study.

It is a strength because it avoids confusion between gender identity and sexual orientation. Many recent studies have compared trans women attracted to men to men attracted to women; if you find a difference between the two groups, you can’t be sure if it is due to sexual orientation or gender identity.

This study, however, compared trans women attracted to women with men attracted to women and women attracted to men.

This is also a limitation because it is extremely unrepresentative of trans women. About half of trans women are attracted to men, much more than in the general population. If we want to understand how gender dysphoria works, we need to look at both groups of trans women.

A study looking at just trans women attracted to women was necessary, but clearly we need some follow-up research. Is this result true for trans women who are attracted to men? How do these results compare to cis gay men’s brains?

You can stop here if you want. You now know the main result of the study.

On to the specifics of the study. What exactly did they find?

Ways trans women’s brains were like men’s brains

1. Total brain tissue volume was smaller in heterosexual women (HeW) than in heterosexual men (HeM)  and gyenephillic male to female transsexuals (MtF-TR).*

(Gyenephillic=attracted to women.)

2. Total intracranial volume was smaller in HeW than in HeM or MtF-TR. There was no difference between the groups in total white matter volume or total gray matter volume when you took into account the total intracranial volume.

3. HeM had a larger gray matter volume than HeW in the lingual gyrus, the cerebellum, right putamen, and left amygdala and perirhinal cortex. HeW had larger gray matter and white matter volumes in the precentral gyrus.

None of these differences were reproduced when comparing HeM to MtF-TR.

4. HeW had larger hippocampi compared to both HeM and MtF-TR, mostly due to the left hippocampus. There was no difference between HeM and MtF-TR.

5. HeM and MtF-TR had rightward assymetries in the brain; HeW did not. Specifically:

a. the volume of the right hemisphere was larger than the left for HeM and MtF-TR, but not for HeW;

b. the volume of the thalamus was significantly larger in the right hemisphere for HeM and MtF-TR, but not for HeW – however, there was no significant differences in the groups’ assymetry indices (volume of right side/volume of left side).

c. the volume of the hippocampus was significantly larger in the right hemisphere for HeM, but not for HeW. The volume was also larger for MtF-TR, but this was not statistically significant – although the p-value was 0.065, so it was close. However, the differences in assymetry indices between MtF-TR and HeW and between HeM and HeW were significant.

Ways trans women’s brains were different from cis people’s brains

1. MtF-TR had larger gray matter volumes than either HeM or HeW in the “right temporo-parietal junction (around the angular gyrus and in the posterior portion of the superior temporal gyrus)**, and right inferior frontal and insular cortex.”

2. MtF-TR had a smaller gray matter volume than either HeM or HeW in the thalamus.

3. MtF-TR had smaller thalamuses and putamens than either HeM or HeW; this was a measurement of the regional structural volume. There was no difference between HeM or HeW.

Areas where they found no differences between the groups

1. There were no group differences in the caudate volume.

2. There was no assymtery in the caudate or putamen.

What does all this mean?

First, it does look like the authors are right; the trans women’s brains were more like men’s than women’s. It is possible that trans women’s brains are like women’s brains in some other way than the ones the authors looked at. Nevertheless, the similarities to men’s brains found in this study are fairly large.

What is perhaps, more interesting, is the ways that the trans women’s brains were different from cis people’s brains, whether they were male or female.

The authors of the study point out that these are new findings and they need to be confirmed with larger studies. “Any interpretation must, therefore, proceed cautiously and can at this point only be highly speculative.”

The authors go on to suggest that their findings might be related to own body perception. As they point out, one of the main symptoms of gender dysphoria is discomfort with your own body. Some studies suggest that the areas of the brain where trans women were different from cis men and women could be part of a network involved in own body perception.

Of course, as in other studies, the parts of the brain involved in this study have multiple functions. For example, the angular gyrus is also involved in language, math, and memory retrieval. So we can’t be sure exactly what it means that trans women have a larger volume of gray matter in those areas.

On the other hand, people with gender dysphoria don’t have problems with language and math, they have problems with dysphoria about their bodies.

More surprisingly, the authors of this study suggest that people with gender dysphoria may have changed their brains by constantly thinking about their bodies. This is possible, but it seems more likely to me that the problem starts with something in the brain causing people to feel uncomfortable with their bodies.

Here is the author’s argument from their conclusion:

“There is no evidence that this feeling [gender dysphoria] is caused by a general sensory deficit in transsexual persons…Several studies propose that own body perception involves networks in the temporo-parietal, inferior parietal cortex, the inferior frontal, and insular cortex, and their connections with the putamen and thalamus. Thus, theoretically, the experience of dissociation of the self from the body may be a result of failure to integrate complex somatosensory and memory processes executed by these networks.

Such disintegration accords with the present findings and could, perhaps, explain recent observation of poorer parietal cortex activation during a spatial orientation task in MtF-TR compared with male controls.

However, it is difficult to explain how such disintergration can be linked to a dysphoria restricted to the own body’s sex characteristics.

Moreover, even if a link exists, it is uncertain whether the here observed morphometric features in transsexual patients underpin their gender identity or are a consequence of being transsexual.

One highly speculative thought is that the enlargement of the GM volume in the insular and inferior frontal cortex and the superior temporal-angular gyrus could derive from a constant rumination about the own body. Brain tissue enlargement has been detected in response to training, and GM enlargement of the insular cortex has been reported in response to meditation, which involves mental focusing on the own body.”

The authors also point out that it might be that something else is causing both gender dysphoria and changes in neuroanatomy.

They stress that we can’t directly connect changes in gray matter volume to effects on the person.

They point out that they did not look at the hypothalamus, so their findings do not contradict earlier studies of it.

Finally, they call for more research, including research which compares trans women attracted to men to trans women attracted to women, and trans women attracted to men to cis gay men. (Yay!)

“Furthermore, they [the results] were generated exclusively from investigations of nonhomosexual, gynephillic MtF-TR. The issue of possible cerebral difference between gynephillic and androphillic (homosexual) MtF-TR and also between androphillic MtF-TR and homosexual healthy men is of special interest and needs to be addressed separately in future studies. Additional studies of the relationship between brain structure and function in transsexual persons and also extending the material to female to male transsexuals are necessary to more precisely interpret the present observations.”

The bottom line: we need more studies confirming these results. Is there a link between gender dysphoria and the network involved in own body perception? If so, which is the cause and which is the effect? Do these results hold true for trans women who are attracted to men? What about trans men?

There were two results the study did not discuss. It may not mean anything, but I think it is worth mentioning.

1. The volume of the amygdala was larger in HeM than HeF; this fits with other studies of sex differences in the brain. For MtF-TR, their amygdalas seems to have been neither bigger nor smaller than males or females. (Table 3)

2. The subcallosum (BA 24, 32) was larger in HeF than HeM. This fits with a study the authors cite showing that women have larger anterior cingulate gyri – the anterior cingulate cortex includes Brodmann Areas 24 and 32. This area seems to have been neither larger nor smaller in MtF-TR.

According to the authors: “Although sex differences have been described also in the amygdala and cingulate gyrus, these structures were not included in the analysis because the identification of anatomical landmarks in these regions is less reliable, especially when using a 1.5-T scanner.”

In other words, they think there could have been an error in the results. Either there was no sex difference in those areas, or they failed to detect differences between trans and cis women in those areas.

Another possibility is that trans women and cis women were not different in those areas because trans women’s volumes were intermediate between cis men’s and cis women’s.

If this is so, there are many possible explanations. It might have something to do with sex hormones, although it is hard to see why trans women’s brains would have developed like cis men’s in most parts of the brain if they weren’t exposed to sex hormones. You would have to assume that there was something different about how their amygdala and cingulate gyrus responded to sex hormones.

Another possibility is that trans women’s amygdalas were as large as men’s amygdalas, but something made them shrink.

Studies have linked a smaller amygdala to obsessive-compulsive disorder (OCD), anxiety, PTSD, sociopathy, and early life stress (abuse, neglect, or poverty). [Click through to see the studies.]

Trans people have higher rates of anxiety than other groups, but this study excluded people with any psychiatric disorders.

Trans people also suffer higher rates of abuse and trauma than most people which might have affected their amygdalas.

A final, hypothetical possibility might be that gender dysphoria is in some way related to OCD.

In the case of the cingulate gyrus, we would have to assume that something happened to trans women’s brains to make the volume of their cingulate gyrus increase to be intermediate between cis women and cis men. This is harder to understand, since OCD, anorexia, and body dysmorphic disorder are all correlated with decreases in the size of the anterior cingulate cortex.

However, there is one study suggesting that a large right anterior cingulate “is related to a temperamental disposition to fear and anticipatory worry.” No doubt the experience of being transgender in our society causes people to worry and feel fear; perhaps this changes the brain. Alternatively, it might be that a tendency to worry is somehow linked to developing gender dysphoria.

Original Study:

Sex Dimorphism of the Brain in Male-to-Female Transsexuals by Savic I, Arver S. in Cereb Cortex. 2011 Nov;21(11):2525-33.

A few fun facts:

You can induce out-of-body experiences by stimulating either the temporo-parietal junction or the angular gyrus. (Read more here and here).

The right temporo-parietal junction is also involved in thinking about thoughts. It, or an area close to it, is involved in directing your attention. (Read more here and here.)

The superior temporal gyrus is involved in recognizing your own face, identifying emotions in other people’s faces, and social cognition.

Information about the self may be processed in the right hemisphere; however, not everyone agrees on this theory. (Read more here.)

Increased volume in the left inferior frontal gyrus and right amygdala are associated with worse symptoms in body dysmorphic disorder. The trans women in this study had increased volumes in the right inferior frontal gyrus only. This does, however, suggest that these areas of the brain are important to perceptions of the body. (Read more here.)

*I am using the language of the study now.

** The original text refers to the superior temporal gurus. A cool idea, but probably a typo.

PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects – Brief Review and a cautionary note

This is the abstract of a study which shows that the brains of gay men are similar to the brains of straight women in certain ways while the brains of lesbians are similar to the brains of straight men.

The study looked at hemispheric asymmetry and functional connectivity, two areas where scientists have found differences between men and women.

They scanned the brains of 25 straight men (HeM), 25 straight women (HeW), 20 gay men (HoM), and 20 lesbians (HoW). Fifty of the subjects also participated in a test of blood flow that is used to analyze connections between the right and left amygdalae.

Results (from the abstract):

“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.”

They conclude:

“The present study shows sex-atypical cerebral asymmetry and functional connections in homosexual subjects.”

This study shows the critical importance of controlling for sexual orientation when studying the question of gender identity and the brain.

An extremely high proportion of people with gender dysphoria are primarily attracted to people of their birth sex. In the general population, only about 5% of people are primarily gay or lesbian.

Studies of gender identity and the brain should include both gay and straight control groups.

There is also a need for studies comparing the brains of transgender people who are attracted to people of their birth sex to gay men and lesbian women who are cis. This might be the most fruitful avenue of research into what causes gender dysphoria.

Original Abstract:

PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects by Ivanka Savic and Per Lindström in Proceedings of the National Academy of Sciences of the United States of America, July 8, 2008, vol. 105 no. 27.

Regional gray matter variation in male-to-female transsexualism – Review

This article found that trans women’s brains are more similar to men’s brains than cis women’s brains, at least in terms of the pattern of gray matter variation.

They also found that trans women’s brains had more gray matter in the putamen than both cis men and cis women, although the difference was only significant for cis men.*

The authors found 20 areas of the brain where women had more gray matter than men. The male-to-female transsexuals (trans women) had the smallest volume of gray matter in these areas, but their data spectrum mostly overlapped with the men’s.

In two areas of the brain, the left and right putamen, male-to-female transsexuals had the largest volume of gray matter.**

“…the gray matter volume of this particular structure in the MTF transsexual group was both larger than in males and within the average range of females.”

Gray matter, in case you’re wondering, contains more neuronal cell bodies. White matter is mostly made up of axons that transmit signals within the brain. Previous studies have found that there are sex differences in the distribution of gray matter in the brain.

The authors describe the putamen as being “feminized” in MTF transsexuals. That might be, but it might also be that their putamens are simply different from cis people’s for some other reason.

In addition, the putamen has more gray matter in women than in men, but the trans women’s putamens had more gray matter than either, although the difference between trans women and cis women was probably not significant.**

The authors conclude:

“Overall, our study provides evidence that MTF transsexuals possess regional gray matter volumes mostly consistent with control males. However, the putamen was found to be “feminized” in MTF transsexuals….”

“Taken together, these findings lend support to the hypothesis that specific neuroanatomical features are associated with transsexual identity, where the particular role of the putamen requires investigation in future studies.”

The study results also support the idea that trans women’s brains are more similar to men’s brains than to cis women’s brains. Most of the time when men’s and women’s brains differ, the trans women’s brains were like men’s.

In addition, the authors briefly mention a few areas where women’s and men’s brains were similar, but trans women’s brains were different from cis women’s (see details of study below).

On the other hand, we definitely need more studies of the putamen and gender dysphoria.

As the authors point out, we do not know if the differences in the putamen are the cause or result of gender dysphoria – or if the differences are caused by another factor that also causes gender dysphoria.

It is also possible that the observed differences are caused by sexual orientation, not gender identity. The authors explain that their sample included 6 male-oriented people (25%) and 18 female-oriented people (75%). They did not know the sexual orientation of their control groups, but it is likely that 95% of the males were attracted to females and 95% of the females were attracted to males.

Their sample also included more left-handed people than the control groups. It is possible that handedness affects the size of the putamen in some way.

Another huge issue is that we have no flipping idea what the results mean. The putamen is an area of the brain that is believed to be involved in many different functions including motor skills, memory, and processing sensory information. If it is involved in gender dysphoria, we need studies to figure out how.

Hopefully we will see some studies confirming this result, this time with a control group that includes gay men and lesbians. The study should also control for handedness.

Then we will need more studies looking at exactly what is going on.

Original Article:

Regional gray matter variation in male-to-female transsexualism by Luders E, Sánchez FJ, Gaser C, Toga AW, Narr KL, Hamilton LS, Vilain E. in Neuroimage. 2009 Jul 15;46(4):904-7.

Some details of the study:

The authors looked at 24 trans women recruited through the community organizations and professionals who serve trans people. Their average age was 43 (range 23-72). They were genetic males (they had the SRY gene),*** they were free of psychoses, and they passed a physical and neurological exam. 76% of them were right-handed, compared to 90% or more of the controls.

None of them were on hormones, although they all intended to take them.

More about the results:

“females had more gray matter than males in large portions of the brain… Similarly, females had more gray matter than MTF transsexuals… Although the differences between females and MTF transsexuals did partly overlap with the difference between females and males…, they were spatially more extended, and also evident in a few regions where females and males did not differ… There was no region where females had significantly less gray matter than males… or MTF transsexuals… Similarly, there was no region where MTF transsexuals had significantly less gray matter than males… MTF transsexuals, however, showed significantly more gray matter than males in the right putamen… MTF transsexuals also showed significantly more gray matter than males in the left putamen when findings were not corrected for multiple comparisons (p<0.001, maps not shown).”

I am intrigued by the mention of a few regions where females and males did not differ but females had more gray matter than MTF transsexuals. I wish the authors had discussed these areas. Perhaps they would shed some additional light on gender dysphoria.

*The box plot data on the trans women’s putamen looks pretty different from the cis women’s data. Their median value is higher and their range seems to be much bigger and go up higher. However, the difference is probably not significant since the authors say elsewhere that there was no area where females had significantly less gray matter than trans women.

** It may be that the difference was only statistically significant in the right putamen. Elsewhere in the study the authors say that trans women had significantly more gray matter than males in the left putamen only when the findings were not corrected for multiple comparisons. Because brain scans involve collecting so many data points, the chances of finding correlations by chance are much greater. Thus you have to make corrections. On the other hand, in this case, the other half of the putamen was different at a stastically significant level.

*** Zoe Brain has pointed out that someone could have the SRY gene, but still have unusual chromosomes, if they had Kleinfelter’s syndrome (47xxy) or mosaicism (46xx/46xy). Her interpretation of this study is quite different from mine, but well worth reading.