After experiencing a stroke, a Swiss woman at Geneva University Hospital began experiencing phantom limb for an arm that didn't exist -- and never had. Doctors subjected her to a MRI:
Link via Instapundit
Researchers instructed the woman to move her right hand. As expected, the motor cortex and visual processing areas in the left side of her brain became mobilized.
The same effects were observed to a lesser extent when the woman simply imagined moving her right hand. Imaginary movements of the woman's paralyzed left hand prompted the same activity in the brain, but on the right side.
But when doctors asked her to move her phantom arm, her brain reacted as though the arm really existed and could be moved. In addition, the patient's visual cortex was also activated, indicating the she actually saw the imaginary limb.
And when she was instructed to scratch her cheek, regions of the brain relating to touch were activated.
Link via Instapundit
http://en.wikipedia.org/wiki/Gil_Hamilton