> Hmm... this spatial map thingamajigger wouldn't have anything to do with the "phantom limb" sensation that amputees have?

Yes, exactly. Ronald Melzack, one of my professors, showed that if one administers an anaesthetic nerve block to a normal unamputated person, s/he will often report a phantom body part or limb. This phantom implies that the brain and central nervous system produce the phantom in response to lack of normal sensory inputs from skin, muscles, and joints.

The part of the brain involved in creating the phantom is called the sensory cortex. If you've taken a general psych course, you'll recall the sensory cortex carries a rough map (schema) of the body, sometimes called a homunculus ("little man"). Each body part in the spatial schema map is wired to its corresponding portion of the real anatomy. When a limb is lost, the corresponding part in the brain cortex rewires its circuitry to make up for the signals it's no longer receiving from the missing limb. It re-routes the old pathways to new areas in the schema. This rewiring and growth of new neuronal cells -- i.e. neural plasticity in adult brain tissue -- is called "cortical reorganization" of the somatosensory cortex.

Another pathway critically involved with this reorganization is based in the parietal lobe. The parietal lobe keeps track of when a feeling is coming from a part of your own body. It tells the cortex that the perceptions of sensation are from the "self." This neurosignature tells the brain that your arm is YOUR arm, not someone else's (Melzack 1992). This sensation of self is vital to our well-being: it could be another reason why after limb amputation, the brain rewiring creates -- more likely than not, at first -- a phantom "replacement" for the missing part of one's self. Riddoch (1941) suggested that the phantom fingers of Admiral Lord Nelson's amputated arm offered him solace, as he felt the ghost fingers were "a direct proof of the existence of the soul." Disturbances to the intrinsic sense of self can be devastating. Oliver Sacks, I think, mentions a patient with lesions to the parietal lobe, who failed to recognize his own leg as "his" -- he would grab it, and throw it out of bed, because the leg felt alien like someone else was invading his private and personal space.

Wolfspirit