Question: why do some people get phantom limb pain? And how come some people believe that certain parts of their bodies are not their own?

Matthew Longo answered on 11 Mar 2019:

Most people who have a limb amputated will experience phantom sensations of their limb continuing to exist. This is likely because our conscious experience of the body (our ‘body image’) is stored in the brain, and not in the body itself. When a limb is removed, the body image remains, leading to the limb continuing to be experienced, even though the person knows perfectly well that it isn’t actually there.

Unfortunately, some of these people experience severe pain that seems to them to come from their non-existent limb. The reasons for this are not fully clear. One interpretation is that the phantom limb is experienced as being in a painful posture, for example with the fingernails digging into the skin. Another theory is that brain plasticity resulting from the amputation goes wrong somehow, leading to conflicting signals which are interpreted as pain. Finally, some researchers think that sensory signals reaching the brain from the limb stump play an important role. Understanding phantom pain and devising new ways to treat it are important goals for on-going research.

Regarding your second question, there are some patients with damage to the parietal lobe on the right side of their brain who develop a condition known as ‘somatoparaphrenia’, and who deny that one of their limbs belongs to them. Somatoparaphrenia almost invariably occurs in patients who have severe problems with the sense of touch in the affected limb, problems with perceiving where the limb is in space (known as ‘proprioception’), and difficulties in moving the limb. Somatoparaphrenia is thought to be particularly linked with problems in acting with the limb. One interpretation is that the brain has a very strong expectation that when we send a motor command to our limb telling it to move, that the limb ought to move. When after a stroke, the limb doesn’t move when the patient tells it to, the brain may interpret this conflict as a cue that the limb isn’t actually part of the body. The neurologist Oliver Sacks wrote a book about his own similar experience of ‘disowning’ his own leg after he broke it in a ski accident. We all may have somewhat similar experiences if we sit wrong on our foot and it goes to sleep. But, unlike patients with somatoparaphrenia, we may experience a feeling of disownership over our limb, but we don’t *believe* that it’s not part of our body.