>Actually, the "phantom limb problem" really is not a problem for neuroscience
>at all. There was a good treatment of this phenomenon in _Scientific
> American_ in 1993 or so. Briefly, what happens is that the somatosensory
>system of the cortex is something like a map: Normal neural activity from a
>limb, for example, arrives in a specific area of cortex. When that area of
>cortex signals other areas of the brain, such as association cortex, the
>activity is "interpreted" as representing stimulation of that limb.
>However, if the limb is no longer there, other neural fibers will begin
>encroaching on the "vacated" space of this cortical map with *their* firing
>being interpreted as sensation in that limb. It is a matter of the brain
>having "learned" that certain inputs in certain areas mean specific things.
>A similar phenomenon is that of referred pain--such as feeling a heart
>attack as sharp pain radiating down the left arm. The heart has few pain
>receptors, but these few synapse in the cervical and thoracic spinal cord
>onto the same spinal cord neurons that relay pain sensation from the left
>arm. The intense firing of the heart's nerve fibers excites these neurons in
>the spinal cord and they relay the activity to areas of the somatosensory
>cortex associated with the left arm.
This same phenomenon applies to the auditory cortex. I have lost a lot of the
use of the nerve cells in my ears. These are the hair cells that vibrate at a
characteristic frequency in response to a sound wave. Since the brain no
longer receives any signal from some of these cells, the brain cells
apparently get bored and start firing at random. This causes me to constantly
hear a ringing in my ears. When I was a child, I thought this was normal and
that everyone had it. It wasn't until it all got worse, and my wife thought I
ignored her terribly that I found out it is not normal. It is just a bored
brain.
glenn
Foundation,Fall and Flood
http://members.gnn.com/GRMorton/dmd.htm