Posted: Dec 18, 2017 12:37 pm
I believe that the brain contains a kind of map of the limbs, but that map can go wrong. When it does so, the person thinks that the affected limb does no belong to them, and, in extreme examples, has the offending limb removed. I have even read of a case where a man's penis was similarly affected, and, sure enough, he had the organ cut off! 
EDIT: Here's an article about brain body maps, and the possible effects of faults in them:
https://www.theguardian.com/science/2005/nov/29/neuroscience.highereducation
Note that it is dated 2005.
EDIT: Here's an article about brain body maps, and the possible effects of faults in them:
https://www.theguardian.com/science/2005/nov/29/neuroscience.highereducation
How the brain builds its image of the body
· 'Pinocchio' principle used to find key region
· Study could lead to treatment for anorexia
Scientists have identified the region of the brain that is responsible for the way people view their bodies. The parietal cortex generates the body image, and disruption of the region's normal functioning could play a role in conditions such as anorexia and body dysmorphic disorder, in which people grossly over- or underestimate their body size, researchers believe
The researchers, led by Henrik Ehrsson, a neuroscientist at University College London, scanned volunteers' brains while carrying out an illusion that made them think their waists were shrinking.
Each of the 17 volunteers had a vibrating device attached to their wrist to stimulate a tendon, which created the false sensation that the wrist joint was flexing. When they placed their hand on any part of the body, the imaginary wrist movement made them think that part of the body was shrinking or growing.
...
Note that it is dated 2005.