So I recently read this paper from V.S Ramachandran's group at the Salk Institute and came to realize how weird the field of cognitive neuroscience can really get. The paper is on Apotemnophilia (phew!! saying it is a task in itself), which yours truly had never heard of before. In any case, the disease is characterized by the desire to amputate one's own limb (Now how about that). These patients are otherwise mentally normal.
Individuals suffering from apotemnophilia always date the desire for amputation since childhood and often term the limb as being over-present or intrusive. Most obtain an amputation and paradoxically report feeling much more 'complete' and happier. Traditional explanations for the disease range from it being sexual paraphilia, related to the phallic resemblance of an amputee's stump (Someone get Freud in here!) or perhaps the mere sight of an amputee is permanently imprinted in the malleable psyche of a child as the "ideal body representation"
In this paper (McGeoch et. al (2009)), they demonstrate that the disease actually has a neurological basis based on the following observations:
1) Sufferers have no other psychological disorders
2) They desire an amputation of a limb at a specific level
3) There is a left sided bias.
The last observation points a finger to one particular area in the right parietal lobe called the superior parietal lobe (SPL) which receives connections from a host of other areas, namely the visual, primary somatosensory, secondary somatosensory, premotor and motor cortices. Additionally, the right parietal cortex is known to play a vital role in constructing body image and damage can lead to various disorders like somatoparaphrenia (denial of ownership of left arm) and others. The autors postulate that the right SPL may contain a hardwired representation of the body and if a particular limb were missing from the representation, the consequence may be a desire for amputation.
Right hemisphere of averaged control brain (a,b) and subject brain (c,d). Right SPL is outlined in black. Images a and c show touch to the left foot of control and subject respectively while b and d show touch to the right foot of control and subject respectively . This subject wanted right below-knee amputation (McGeoch et. al (2009))
This confirms the hypothesis that there is a congenital failure to represent affected limbs in the body image. Since visual and somatosensory inputs are still intact, but there is no corresponding limb representation, the result would be a mismatch that manifests itself as an intrusive and over-present limb.
McGeoch, P.D., Brang, D., Song, T., Lee, R.R., Huang, M., & Ramachandran, V.S. (2009). Apotemnophilia - the neurological basis of a 'psychological' disorder Nature Precedings : 10101/npre.2009.2954.1
Brang, D., McGeoch, P., & Ramachandran, V. (2008). Apotemnophilia: a neurological disorder NeuroReport, 19 (13), 1305-1306 DOI: 10.1097/WNR.0b013e32830abc4d