Tag Archives: precentral gyrus

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.

Increased Cortical Thickness in Male-to-Female Transsexualism – A Review and a Hypothesis

This study found that male-to-female transsexuals* (MTF) had thicker cortexes than control males did in certain areas of the brain. It is not clear what the differences mean.

We don’t know what caused the difference, we don’t know how the difference affects people, and we don’t know if the difference is related to gender identity, sexual orientation, or some other factor.

That’s the essence of the study; we don’t know.

Back to the study. The cerebral cortex is the outer layer of your brain. It is made up of gray matter which contains mostly neuronal cell bodies,

As with many studies, the difference they found might be related to gender identity or to sexual orientation. The MtF group included 6 people who were male-oriented and 18 who were female-oriented, so 25% of them were attracted to men. It is likely that 95% of the control males were attracted to women.

The authors suggest that:

“future studies need to explore the possibilities that brain anatomy in MTF transsexuals varies depending on whether they are attracted to men, attracted to women, or attracted to both. Ideally, those studies will also include heterosexual / homosexual control men, matched to MTF transsexuals with respect to their sexual orientation.

As with other studies, we don’t know what the practical effects of these differences might be. What does it do to you to have a thicker cortex? What does it mean if your cortex is thicker in the front versus the back of your brain? Does it matter if it is only thicker on the right side?

or as the authors put it:

“further research characterizing the relationships between cerebral micro-structure and macro-structure as well as brain function is clearly necessary before these regional structural differences (and any inherent hemispheric effects) can be precisely interpreted.”

The authors suggest that the thicker cortexes in the trans women (MtFs, i.e. born male) “resemble the direction of previously reported gender-typical pattern among non-transsexuals, such as thicker cortices in women than in men.”

In other words, they are similar to females, at least in terms of having thicker cortexes than males.

Maybe, maybe not.

This study did not look at any female brains, so we can’t know how the brains of the males or trans women would have compared to female controls.

Furthermore, studies have also linked thicker cortexes to obsessive-compulsive disorder (OCD), autism, social anxiety disorder (here and here), and, in children, to generalized anxiety disorder.**

There is even a study that found a small area*** of increased cortical thickness in people with xenomelia (the feeling that one of your limbs is foreign, also known as Body Identity Integretive Disorder).

Cortical thickness has also been linked to a positive trait; valuing religion or spirituality.

Finally, a couple of studies have found that meditation and sports training are associated with increased cortical thickness.

If we just go be the direction of the difference, the thicker cortex found in trans women could be linked to OCD, social anxiety disorder, autism, gender identity, xenomelia, spirituality, or life experiences.

These are not crazy possibilities; this Spanish study found that trans people had more social phobias than the general population. This Dutch study found that children and teens with gender dysphoria had a higher rate of autism spectrum disorder than the general population.

The important question is not is the cortex thicker, it’s where is the cortex thicker?

So where is the cortex thicker and what does it mean?

First of all, although some studies have found that females have thicker cortexes than males, the differences are not always in the same place in the brain.

This study found that females had thicker cortexes in the right inferior parietal and posterior temporal regions even without correcting for total brain volume (i.e. the males had bigger brains, but the females cortexes were thicker in these areas anyway). When they corrected for age and brain volume, females had thicker cortexes in the temporal and parietal lobes.

This study, on the other hand, found that females had thicker cortexes in the frontal, parietal, and occipital lobes.

And this study, found that females had thicker cortexes in the frontal, parietal, and occipital lobes on the left, and mostly the parietal lobe on the right. In the temporal lobe “small regions of the left and right caudal superior temporal gyrus (STG) and the left temporal pole showed significantly greater cortical thickness in women.”

Finally, this study found that females had thicker cortexes in all four lobes of the brain and on both sides, unlike the studies listed above.

This is like doing four studies comparing all the bones in men’s and women’s bodies.

One study finds that women have thicker arm and leg bones.

The next finds that they have thicker spines, ribs, and leg bones.

A third study finds thicker ribs, skulls, and elbows.

A fourth finds that women have thicker bones everywhere.

You wouldn’t be sure what this means about women’s bones. You wouldn’t even be sure if women’s arm bones were thicker than men’s without more studies.

The effects of having thicker bones would be very different if women had thicker ribs as opposed to thicker elbows. Knowing exactly where the bones were thicker would also be important if you were trying to figure out what exactly caused women to have thicker bones.

Going back to our brains, the frontal lobe is in the front, the occipital lobe is in the back.

The frontal lobe is involved in evaluating consequences and choosing the best action to take.

The occipital lobe is the visual processing center of the brain.

The parietal lobe integrates sensory information; it is also where we have our internal map of our own body.

The temporal lobe is involved in the retention of visual memories, processing sensory input, comprehending language, storing new memories, emotion, and deriving meaning.

So it matters a great deal where exactly women have thicker cortexes than men. In fact, it matters where exactly the difference in cortical thickness is within the lobes of the brain.

In short, we are still figuring out the differences between men and women when it comes to cortical thickness. What exactly are they and what do they mean? This makes it hard to draw any conclusions about whether or not trans women’s brains are like cis women’s.

To further complicate matters, this study found that age affected the sex differences in cortical thickness.

Going back to the original study, where were the trans women’s cortexes thicker than the control males?

In the left hemisphere, their cortexes were thicker in “the orbito-frontal cortex, the middle frontal gyrus, in the vicinity of the central sulcus (near midline), in perisylvian regions (close to the post central gyrus), as well as within the paracentral gyrus and orbito-frontal gyrus (medial surface).”

These are areas within the frontal lobe and the parietal lobe.

In the right hemisphere, their cortexes were thicker “along the post and pre central gyrus (expanding into middle frontal regions), the parietal cortex (near midline), the superior temporal sulcus, the inferior temporal gyrus, as well as within the orbito-frontal, fusiform, and lingual gyrus, and the precuneus (medial surface).”

These areas are within the frontal lobe, the parietal lobe, the temporal lobe, and the orbital lobe.

Although the areas where trans women have thicker cortexes are in all four lobes of the brain, it looks like the total area where they have thicker cortexes is not large (see Figure 1).

It is very hard to compare, but it does not look like the areas where trans women have thicker cortexes than men in this study are the same as the areas where females have thicker cortexes than males in other studies (Figure from the first study of males and females, above).

It doesn’t look like the areas where the trans women’s cortexes were thicker are the same as in the studies that linked increased cortical thickness to OCD, although there might be some overlap with the patients who had social anxiety disorder.**** Perhaps one of the trans women had social anxiety disorder and it affected part of the results.

On the other hand what if the areas of increased cortical thickness in trans women have to do with something specifically about trans women?

I want to stress that we do not know what the areas of increased cortical thickness mean functionally for trans women or cis women or people with OCD or people who meditate. Many areas of the brain are involved in more than one process. In addition, it may be that what we are looking for is a network of areas, not just one area.

We really can’t tell what it means that trans women have thicker cortexes where they do.

Nevertheless, I want to offer up a hypothesis that I hope someone will test.

Looking at the right side of the brain, the areas where trans women’s cortexes are thicker seem to have something to do with the body and perception.

According to my trusty Wikipedia,

“The postcentral gyrus is the location of the primary somatosensory cortex, the main sensory receptive area for the sense of touch. Like other sensory areas, there is a map of sensory space in this location, called the sensory homunculus.”

The precentral gyrus includes the primary motor cortex which works with other areas of the brain to plan and execute movements. It also includes a map of motor areas corresponding to body parts.

The parietal lobe integrates sensory information.

The superior temporal sulcus is involved in the perception of where others are gazing and the perception of biological motion. It may have multisensory processing capabilities.

The inferior temporal gyrus is associated with visual processing and possibly face perception.

The orbitofrontal gyrus is part of the orbitofrontal cortex which is involved in the cognitive processing of decision-making; it doesn’t seem to fit the pattern here.

The fusiform gyrus  is involved in face and body recognition. Furthermore, “Increased neurophysiological activity in the fusiform face area may produce hallucinations of faces.”

The lingual gyrus is linked to processing vision.

The precuneus is part of the parietal lobe. It includes three subdivisions: a sensorimotor region,  a cognitive/associative region, and a visual posterior region. It is involved in sense of self, memory, and motor imagery and coordination.

On the left side of the brain, most of the areas of relative cortical thickness do not seem to be related to the body. However, the paracentral lobule includes the supplementary motor area in the frontal lobe and part of the parietal lobe.

Could it be that the increased cortical thickness on the right side is somehow related to trans women’s negative feelings about their bodies?

Could something in their body maps or perception be causing their dysphoria?

Could their gender dysphoria have changed their brains?

This might be an area for future research.

Future research into gender identity and the brain should definitely include some gay men and lesbian controls so that we can separate out the effects of gender identity and sexual orientation.

It would also be a good idea for future studies to include more information on other conditions that might affect their results such as depression, anxiety, Aspergers syndrome, etc. Although this study says that the subjects were free of psychosis, they do not discuss issues like OCD, etc.

I don’t think the best approach would be to exclude trans people with other conditions because part of the reality of gender dysphoria is that some people have more than one condition. If we really want to understand what is happening, we need to include that data. However, it might make sense to include some controls who had social anxiety disorder, etc., if only to find where the differences are.

Back, one last time to the study.

The authors’ discussion of their results includes one odd statement:  “In addition, our current findings of significantly thicker cortices in MTF transsexuals than in control men correspond with previous in vivo outcomes revealing that MTF transsexuals show significantly larger gray matter volumes than control men.”

This is an exaggeration of the results of an earlier study that they did on gray matter volume in trans women’s brains.

This earlier study found that trans women’s brains were more like men’s brains in 18 out of 20 areas of the brain. In two areas, the left and right putamen, trans women had more gray matter than both males and females, although within the average range of the females.

Their previous findings could be better summarized as trans women have gray matter volumes that match men’s except in the putamen.

This study did not look at the putamen as it is not in the cortex. The increased gray matter volume in the putamen found in the earlier study is irrelevant to this study.

Actually, at first glance the findings of this study don’t fit well with the findings of the previous study. The cortex is made up of gray matter, so you might expect its thickness to be related to the volume of gray matter. Why do trans women have gray matter volumes that are equal to or less than cis men’s but cortexes that are thicker?

The authors go on to address this question:

Nevertheless, it appears rather surprising that the previous whole-brain approach analysing gray matter did not detect any group differences in cortical regions (e.g., the ones revealed in the current study directed at cortical thickness). It is possible, however, that these two anatomical measurements reflect slightly different aspects on a micro-anatomical level as also suggested by studies investigating direct correlations between cortical thickness and gray matter concentration. Importantly, the current approach also provides an additional dimension of cortical morphology such as its thickness in millimeters, which is not directly captured by voxel-wise analyses of signal intensity changes throughout the brain.

In other words measurements of cortical thickness and gray matter volume may not be the same thing. It is possible that trans women’s gray matter volume is like men’s but their cerebral cortex is thicker in certain areas.

It is still not clear what it means that trans women’s cerebral cortexes are thicker than control males in certain areas.

We don’t know what the difference means functionally.

We don’t know if the difference is related to gender identity, sexual orientation, or some other factor.

We don’t know how the trans women’s cortical thickness would have compared to females since there were no female controls in the study.

We can’t say that trans women’s brains are like females’ brains since both have thicker cortexes than males because a) other factors like social anxiety disorder can make people have thicker cortexes; b) we aren’t sure exactly where females’ cortexes are thicker than males; and c) it looks like trans women and females’ cortexes thicker than males in different places.

And we don’t know if the difference in cortical thickness is related to body perception and sensory integration.

Original Study:

Increased Cortical Thickness in Male-to-Female Transsexualism by Eileen Luders, Francisco J. Sánchez, Duygu Tosun, David W. Shattuck, Christian Gaser, Eric Vilain, and Arthur W. Toga in J Behav Brain Sci. Aug 2012: 2(3): 357-362.

* I am using the language of the study.

** In general, a thicker cortex is probably a good thing; it is associated with meditation and learning. In addition, your cortex thins with age. The volume of the gray matter in your brain also goes down with age or due to illnesses such as depression.

*** the right central sulcus. I thought you’d never ask. And yes, trans women had a thicker central sulcus than control males, but it was on the left side and the overall pattern is different; patients with xenomelia mostly had areas where their cortex was thinner, not thicker. Then again, it might be worth looking further at the central sulcus in trans people’s brains.

**** The study of patients with OCD found that their cortexes were thicker in the right inferior frontal cortex and the right middle temporal gyrus.

This study suggests that the areas of cortical thickness in autism change with age. I don’t see how to compare the data on autism to the data on trans women.

One study of patients with social anxiety disorder found that they had increased cortical thickness in the left inferior temporal cortex. Trans women had increased cortical thickness in the right inferior temporal gyrus – as well as many other areas not found with social anxiety disorder.

Another study of people with social anxiety disorder found that they had increased cortical thickness in two clusters: “One was located in the right middle frontal gyrus extending into superior frontal sulcus, belonging to the dorsolateral prefrontal cortex (DLPFC, Brodmann areas 6/8/9/46). The other covered right superior parietal lobule and angular gyrus and extended in part into right precuneus and inferior parietal lobule (Brodmann areas 7/39/19).”

Trans women also had increased cortical thickness in the right middle frontal region, the right parietal cortex (near the midline), and the precuneus – but they had other areas of increased thickness as well.

Perhaps one of the trans women in the study had social anxiety disorder and it explained some of the areas of increased cortical thickness, but not all.

Or perhaps both conditions involve similar areas of the brain.