And Things That Go Bump In The Night…
Phantom Pains… or just a pain in the neck?
Anyone who has had surgery knows that pain is common companion for a while afterward. But, when a significant part of the body is removed, we no longer receive any sort of message from that part… or do we?
In the brain there is a map, a neural representation of the body. In fact, there are several such maps. Each map may have different function than the others. For example, one map is for motor (muscle) control, where-in each muscle is related to a part of the body it effects. Another map is for the sense of body part location, and is usually closely associated with the motor control map; after all, if one is going to control the muscles, one needs to know what needs to be moved to where. Yet another map is about other sensations, including pain. There is even a map of the body which is largely informed by visual information, that is to say, that is kept updated by literally seeing where those parts are in relation to each other.
These maps, if not fed real information, sometimes just make stuff up! When that happens, an individual may “feel” the presence of missing limb or organ.
One of the leading researchers in the field of phantom sensation is renown neurologist and psychophysicist, Vilayanur Subramanian Ramachandran, M.D., Ph.D. He is a professor in the Psychology Department at the University of California, San Diego. This man is no hack. In fact, some of my own work in my chosen professional field is influenced by his work in the visual sciences.
So, it was a surprise that he should couple his work in phantom limb sensation with trying to understand the nature of transsexuality. Dr. Ramachandran hypothesised that transsexuals have a sex atypical somatosensory map of their genitalia. That is to say, that at least some, if not all of the pre-wired neural maps in the neocortex are sexually dimorphic in the region that maps the genitalia. This is not an unreasonable hypothesis on first examination. After all, male and female human beings do have differently shaped genitals, are sexually dimorphic, so why wouldn’t the somatosensory maps be sexually dimorphic to match?
However, there are some theoretical objections to this idea. 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?
We might then object and say, yes, but they are differently sized and configured. But then, one might argue back, that one man’s body isn’t identical to another’s, surely the neural wiring must be flexible enough to learn of these differences and make accommodations?
Finally, it is just these accommodations and plasticity of the neocortex that Ramachandran calls into play in his other work on the genesis of phantom sensation, as the no longer required regions of the original map are recruited and repurposed to processing sensations from nearby functions in the brain. (On a side note, this does not always mean nearby in the sense of external somatic topology… for example, the face and hands are “nearby” in the somatosensory maps, but quite distant externally.)
So, like good scientists, we conduct experiments… Ramachandran reasoned that if there was a significant difference between the putatively sexually dimorphic somatosensory maps of the genitalia between control men and MTF transsexuals, then there may be a difference in their responses to penectomy.
Lacking the ability to recruit men at random to remove their external genitalia, we find groups that have need to do so for other reasons. There are many MTF transsexuals who willingly have their penises removed during Sex Reassignment Surgery (SRS). Dr. Ramachandran reasoned that men whose penises were partially removed because of cancer, but otherwise randomly selected by Dame Fortune, would serve as the controls.
On theoretical grounds, this too has some problems. First, modern SRS is not a simple amputation. In fact, a great deal of effort is taken to maintain as much of the epidermis and their enervation, intact, inverted and invaginated into the neovagina. Only a portion of the external corpus cavernosa, the spongy erectile tissue, is excised. The scrotal tissue is divided and repurposed as the labia majora, essentially reversing the prenatal developmental process. Thus, some of the tissue is in the same place as before, and is in the place expected for a female. And the penile skin, though now inverted, is topologically still in much the same place. This is not the best one-to-one match to the controls. (Ironically, Dr. Ramachandran would have found a better match back in his country of birth, as the traditional surgery for Hijra women, which are essentially the same as the transsexuals of the West, did in fact involve a simple amputation of the penis.)
Thus, there are many reasons to doubt whether there should be, a priori, a sexually dimorphic difference in the somatosensory maps of males and females. There is also the little matter of the fact that there are two kinds of transsexuals, with different etiologies, which may confound the experimental data. And finally, the dramatically different results of penile amputation and modern SRS would lead one to suspect that any differences in response, would be attributable to the differences in surgical result. But… hypothesis are meant to be tested…
So, using data from a 1951 paper on phantom sensations of amputated penises in 12 men and his own research into phantom sensations in 20 MTF transsexuals, he found that six of the 20 MTF transsexuals had phantom sensations while seven of the 12 controls did. Wow, big difference!!!
Wait… not so fast… actually… well… that’s actually a small difference in a small sample size. Let’s imagine that we have two coins, and we have no idea about the nature of statistics and chance. I flip one coin twenty times and it comes up head six times out of twenty, while you flip the other coin twelve times and it comes up heads seven times. So, do we say that my coin is more likely to come up tails the next time, while yours is more likely to come up heads? Ummm…. no.
Random chance alone could give us the same numbers flipping coins as Ramachandran got in his statistics. In fact, Anne Lawrence (gotta love her, she keeps turning up just like that proverbial penny) showed that properly analyzed using not one, but three different and widely accepted statistical tests showed that this small of a sample with this small of a difference in data has no statistical significance. It might be random noise… it might be real… we simply don’t know.
We simply don’t know.
How could we know? First, remove the theoretical problem of comparing apples and oranges (partial penile amputation compared to modern SRS) by collecting data on Hijra’s in India and Pakistan, and carefully note their sexual history and sexual orientation. Second, get a lot more samples of both control and MTF populations. Until then; we simply don’t know.
We simply don’t know.
(Addendum 10/7/2014: Dr. Ramachandran likely chose to examine this topic because he is primarily interested in phantom sensations, but it occurs to me that to explore the concept of sexually dimorphic brain maps, we could, and perhaps should, use pre-operative transsexuals. One could attempt to stimulate various points of genital anatomy and determine if their is any psychophysical response differences between transsexuals and controls. My prediction is that there will be absolutely none, in neither transkids nor autogynephilic transwomen. I also predict that we would also find no difference between FtMs and control females.)
For more essays on trans-brains see Brain Sex.
Lawrence, A. A. (2010). “Transsexuals and nontranssexuals do not differ in prevalence of post-penectomy phantoms: Comment on Ramachandran and McGeoch” (2008), Journal of Consciousness Studies, 17(1/2), 195-196.
Ramachandran, V.S. & McGeoch, P.D. (2007), “Occurrence of phantom genitalia
after gender reassignment surgery”, Medical Hypotheses, 69, pp. 1001–3.
Ramachandran, V.S. & McGeoch, P.D. (2008), “Phantom penises in transsexuals:
Evidence of an innate gender-specific body image in the brain”, Journal of Consciousness Studies, 15(1), pp. 5–16.