On the Science of Changing Sex

Brain Maps…

Posted in Brain Sex, Science Criticism by Kay Brown on September 19, 2017

phrenologyOr Searching for the Lost Continent of Atlantis

A recently popular counter argument to evidence of the two type taxonomy that arises in the transgender communities is that transsexuals brains have cross-sexed maps of the body.  It sounds so reasonable doesn’t it?  That is to say, if our bodies are sexually dimorphic with respect to genitalia and secondary sexual characteristics, shouldn’t our brains be as well?  And if the sexes have sexually dimorphic brain mapping of those body parts, could it not be that transsexuals have been “cross-wired”?  And further, couldn’t that explain all transsexuals and maybe even non-gender-dysphoric transgender people?

Trusting “truthiness” gut feelings is how we form false beliefs.  We need to look at this issue objectively, both open-mindedly to see if true, and skeptically to find the flaws in this hypothesis to prove it wrong if it is wrong.  So let’s look at the evidence shall we?

First, this idea seems to have cropped up BEFORE any supporting evidence.  Thus, it may be that when this idea is being explored, it will be subject to strong confirmation bias.  I’ve already written about such an incident in a previous essay when, ten years ago, Vilayanur Subramanian Ramachandran tried to pass off experimentally and statistically flawed data to support this hypothesis by comparing the experience of phantom limb sensations of a small group of control men who had penectomies with post-op MTF transwomen.  The study was roundly and rightly criticized by Anne Lawrence for not showing what it purported to show.  When writing about it, I discussed the theoretical objections to the notion of sexually dimorphic neural maps of the genitalia in the brain,

“First and foremost of which is that the genitalia are not really all that different in quality… and only superficially different in quantity.  Nearly each feature of the external and even of some of the internal structures are homologous.  That is, for each feature found in a male, there is a feature that matches it in the female, which is only different in degree, not in kind.  The most obvious example is the glans of the penis is homologous with the glans of the clitoris.  Inside of the penis, and down into its root inside of the body, is spongy tissue that expands when blood pressure fills it with blood.  Inside of the clitoris and down into its root inside of the body, is spongy tissue that expands when blood pressure fills it with blood.  Quite literally, a penis is a very large clitoris; And a clitoris is a very small penis.  Oh there are differences in how the urethra is routed, but even there, they start in the same place.  In men there are two glands called the Cowper’s, which produce a clear fluid that aids in lubrication during sex.  In women there are two glands called the Bartholin’s which produce a clear fluid that aids in lubrication during sex.  Why are they called two different names?  Finally, the scrotal sac is the same tissue as the labia majora, but have fused together.  Thus, the two sexes, which seem so different to a naive observer, are really very nearly the same to a student of anatomy.  So, given that the two are really very nearly the same, shouldn’t the neural maps be the same?”

I stand by my objections with regard to genitalia being sexually dimorphically represented in the brain.  But could there be other areas that are sexually dimorphically represented in the brain?  There could be, in fact… there SHOULD be.  To be specific, those areas of the body which are not homologous between the sexes should be expressed non-homologously in the brain.  Specifically, the uterus and fallopian tubes.  Further, these areas of female anatomy are not served by the pudendal nerves like the genitalia so may experience quite different representation in the brain.

Consider also the phenomena of neural atrophy.  If the brain is not stimulated by external events… for example someone born blind, the portion of the brain not stimulated does not fully develop while it may also be “repurposed” for another function.  That is to say, it is remapped and recruited by neurologically nearby functions.  So, we would expect to find sexually dimorphic maps of the somatic sensation processing functions associated with organs which are non-homologous.

However, under this analysis, we would NOT expect to find a section of a male-to-female transsexual’s brain waiting for input from non-existent non-homologous female anatomy!  That is the equivalent of searching for the Lost Continent of Atlantis.  You can put it one your paper map of the globe, but that won’t mean that you can find it on the real earth.  It sank into mythology a long time ago.

So, can there be sexually dimorphic brain development involving somatic maps where the opposite happens?  That is, can the brain fail to develop a map for a somatically sexually dimorphic feature that does exist?  This might be possible in theory.

Consider breast tissue.  This is superficially sexually dimorphic after puberty, but largely homologous.  But we have evidence from studies in mice that certain nerves leading to the milk glands begin to form in both sexes, but later atrophy in males in utero.  I haven’t been able to find data on humans regarding the same phenomena.  Mice, being rodents, are close cousins of primates, and thus humans.  But evolution does not always conserve every detail.  So we may or may not have the same phenomena.  However, lets for the moment entertain such a notion.  This would suggest that males would fail to develop brain functions that respond to the sensation of milk gland fullness, fail to develop the needed sensory map for the signals from an infant needing to nurse, and fail to send the needed signals back from other unconscious functions to “let down” the milk to an awaiting baby.  This let down signal is triggered by the sight, sound, and feel of a baby wanting to nurse.  It is thought that originally, only the sensations on the nipple bring about ‘let down’, but soon a mother learns by association the sight and sounds (baby hunger cry) that precede nursing.  It is theoretically possible that we could find the location of this somatic sensorium map and how it feeds the let down function in the human brain and see if it is a) sexually dimorphic and b) anomalous in transsexuals.

body2bmapping2bon2bthe2bbrain2b_new2bscientist2bhomunculusIt is also possible, though I’m not totally convinced, that the maps that allow one to experience touch on the nipples as erotic are also sexually dimorphic.  Interestingly we have discovered that the neural map on the neocortex between the genitalia and the nipples are contiguous and overlapping.  But it turns out, that the very same areas also map for the penis and nipples in males.  Thus, the maps are all in the same place on the sensory cortex.  Both men and women have reported that nipple simulation adds to sexual arousal.  This suggests that this is NOT very sexually dimorphic and is homologous between the sexes.

However, hypothesis were meant to be tested and there is a new paper from Case, et al. that deals with FtM transmen and the possibility of anomalous neurological findings regarding somatic representation.

But before I discus that aspect of the study, I have to share a pet peeve of mine that this paper is guilty of.  It peeves me when I see paper after paper by authors making reference to earlier papers that have clearly been shown to not support a given thesis, especially if those earlier refuted papers are their own.  For example, Swaab’s later papers keep referencing his earlier one regarding transsexuals and BSTc as though that study still had any validity regarding transsexual etiology.  As a reminder, it was Swaab himself that proved it didn’t… but you would never know that from his later papers which keep referring to it as though it did.  In this new paper that also includes Ramachandran as a co-author it references his earlier paper regarding phantom penises as though it supported the notion that MTF transwomen experience fewer of them than control men with penectomies.  But as I mentioned earlier, Lawrence demolished that paper showing that it showed no such thing, not passing even the simplest statistical ‘sniff test’ while I showed not only theoretical problems with the notion but that his purported controls did not qualify as such.  I can forgive not having read my blog, but not of ignoring Lawrence’s reply published in the same journal as the original paper.  My pet peeve is that authors of papers, when they make these references without also referencing those later papers that cast their conclusions into doubt, are guilty of the worst sin of bad science, cherry picking.

Further, the Case paper references xenomelia and observes that this may be similar to transgender, but ascribes it to somatic mapping issues while failing to note that we have another name for xenomelia, “apotemnophilia”, the erotic desire to be an amputee and how that desire arises out of an Erotic Target Location Error (ETLE) for the primary erotic target of amputees.  The authors thus sweep the well documented erotic motivations of both amputation “wannabees” and of autogynephilic transwomen under the rug in order to further their thesis of transgender as a brain mapping issue alone.

But for the moment, lets put these transgressions aside and look at the actual study.  Actually… not much to say about it.  They noted that FtM’s seem to have a reduced somatic awareness of their pre-top-surgery chests as shown by functional brain scans.  And although the authors offer a nod to the notion that their higher level conscious aversion to their breasts, i.e. somatic gender dysphoria, might mean that they repress awareness of touch sensations that announced that they have breats, they bend over backwards to posit that the direction of causality is reversed.

Ummm… No.

That would mean that non-transmen would also have to have less awareness of their chests… and that has never been noted to happen.

All in all, this paper has interesting details on how psychophysical experiments can be conducted using brain scanning, a topic that is very much of interest to me as one whose career has been in applied psychophysics.  It also discloses sexually dimorphic differences in white matter distribution in parts of the brain in which the FtM subjects differed from female controls, thus adding to the growing pool of data that show that gynephilic transmen are, like androphilic transwomen, gender atypical in brain development.  But it does not show any convincing data for a somato-sensory brain map issue as being causitive of transsexuality.

Further Reading:

Essay on phantom penises

Essay on xenomelia / apotemnophilia and its relationship to autogynephilia


Case, et al., “Altered White Matter and Sensory Response to Bodily Sensation in Female-to-Male Transgender Individuals” (2017) Archives of Sexual Behavior

Fun Reading:


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