A 70-year-old engineer who has just retired confesses that he has had a life-long urge to have his left arm amputated below the elbow. He has the arm removed and feels much better.
Another man loses his arm in a car accident, but still feel its ghostly presence; this phantom limb is clenched in a painfully awkward position.
A third man, a student of mine, makes a remarkable recovery from a coma, only to become convinced that his mother and father are impostors.
All three case studies are fascinating. Yet as I argue in my new book, The Tell-Tale Brain, they can also teach us a great deal about how the brain does its near-miraculous work.
This is an organ of staggering complexity: a 3lb lump of jelly that can contemplate the meaning of infinity, the idea of God, and even its own existence.
One principle that often crops up is the importance of evolution, especially the notion that a brain mechanism that evolved for one function can be hijacked for another. As the great biologist Theodosius Dobzhansky said, "Nothing in biology makes any sense except in the light of evolution."
Take that 70-year-old engineer who wanted to amputate his arm – a classic case of apotemnophilia [a neurological disorder in which a sane and rational individual wants a healthy limb or limbs to be amputated].
Many far-fetched Freudian ideas have been proposed to explain this condition (including the idea that the amputation stump would resemble a large penis). But the real answer is genetic. It turns out that the right hemisphere of the brain contains a complete representation of your body – a genetically specified "body image".
Our brain-imaging experiments showed that with this particular patient, the hand on this genetic map was missing – so the sensory impulses from the arm had nowhere to go to, and the patient experienced it as "over-present". Similar changes in body images might even explain the desire that transsexuals have to have their penis amputated.
Phantom limbs [the limb has been removed, but the patient feels its presence] are more common – and in some cases, the ghostly arm is paralysed, and fixed in a very painful position.
This also provides an explanation for why Trans people sometimes require surgery, and sometimes do not.
We know that Transsexuality is caused by a partly cross-sexed brain. There's enough evidence from autopsies, fMRI scans and various tests of hearing, sense of smell etc (all are sexually differentiated) to prove the anatomical difference.
The conditions vary though, some parts of the brain are always affected, others may or may not be. If the part determining the "body map" is affected, then surgery will be as necessary as re-attachment would be for a man whose genitalia was severed in an accident. Not to have it would cause severe psychological distress, and an unacceptable risk of suicide.
That trans women do not report the same rate of "phantom penis syndrome" after genital reconstruction surgery as men do after castration as part of anti-cancer treatment provides additional evidence, as does the many reports of "phantom penis syndrome" in pre-operative trans men.
MRI imaging of trans people who require surgery, and those who do not, could well locate more precisely the area of the brain concerned.
There's a few minor errors in the article - in general, genital reconstruction for trans women doesn't involve penile amputation, just re-shaping an outie to an innie. And the brain's neuro-anatomy isn't "genetic" as such, anomalies in the hormonal environment in the womb can re-program development so it doesn't follow the usual pattern as laid down by the genes - but otherwise correct.
It's worth noting that Professor Vilayanur S. Ramachandran has done more research in this precise area than any other scientist in the world, and is the director of the Center for the Brain at the University of California, San Diego.