Re: What causes phantom limbs?

       In a recent article in the neuroscience journal Brain (1998, vol.
121, pp. 1603-1630)Drs. Ramachandran and Hirstein suggested that phantom
limb sensations and pain is the result of the brain's confusion among five
different types of input:

1) "Stump neuromas" or the bundles of nerves that collect at a point of
   amputation (this was the reigning theory 20 years ago)
2) Remapping of the regions of the brain receiving touch, pain, and
   temperature input from the limb to other parts of the body.
3) Problems in monitoring the results of motor commands to the limb (the
   muscles of the limb no longer tell the brain the limb is moving or where
   the limb is)
4) A "primordial genetically determined image" of the body tells you that
   there is a limb there whether there is one or not.
5) Sensory memories of painful sensation (due to disease or amputation) in
   the original limb are carried over to the phantom limb.

   The hottest area of research about phantom limb pain is in the remapping
theory.  This says that phantom limb sensations and pain represents neural
plasticity gone haywire.  Neural plasticity refers to the ability of the
brain to change its wiring.  When a limb is amputated its sensory input to
the brain is lost.  Without any feeling coming from that limb the brain
tries harder and harder to get some kind of input from that limb.  At the
same time, the regions of the brain that used to receive their input from
the lost limb begin to get new input from other regions.  Since the brain
is now hypersensitive to any input these new inputs are overwhelming and
can be perceived as pain.