Brain changes linked to phantom-limb pain.
A new study suggests that the most severe pain of this type occurs in people in whom, after amputation, an area of the brain that formerly handled sensations from the severed limb undergoes extensive reorganization. Large-scale remodeling of this strip of sensory tissue may somehow alter the neural circuits involved in pain, thus resulting in phantom-limb pain, propose Herta Flor, a psychologist at Humboldt University in Berlin, and her colleagues.
"This is the first evidence that there is a central nervous system correlate of phantom-limb pain," says study coauthor Edward Taub, a psychologist at the University of Alabama at Birmingham. "It's one big piece of the jigsaw puzzle in understanding this phenomenon, but it's not the whole story."
Flor's team recruited 12 men and 1 woman, ranging in age from 27 to 73, who had had an arm amputated at least a year before the study. The scientists measured magnetic responses in participants' brains to light pressure on the intact thumb and pinkie finger, as well as on the left and right sides of either the lower lip or the chin. The researchers then mapped areas of magnetic activity onto a reconstructed image of the somatosensory cortex, a nerve impulse center for various body parts.
Since the left and right hemispheres of the brain regulate opposite sides of the body, stimulating the fingers sparked magnetic responses only in the hemisphere opposite the intact limb. A mirror image of the finger activation sites was then projected onto the hemisphere opposite the amputated arm.
This technique provides a good estimate of the location of somatosensory sites for an absent limb, Taub argues. Prior magnetic imaging work by another research team had found little variation in the location of corresponding parts of the somatosensory cortex in the hemispheres of nonamputated individuals.
Cortical reorganization, evidenced by substantial encroachment of sensory areas for the face into regions previously reserved for the amputated fingers, was most pronounced in the eight participants who suffered from phantom-limb pain, Flor's group reports in the June 8 Nature. The greater the cortical shift, the more phantom-limb pain amputees reported.
The new data support earlier studies that charted cortical reorganization in monkeys after researchers cut nerve impulses to the animals' limbs.
Nervous system damage may trigger a strengthening of connections between somatosensory cells as well as the formation of new ones, Taub suggests. In some cases, an imbalance of pain messages from other brain areas may occur, leading to phantom-limb pain, he theorizes.
Or phantom-limb pain may result from a remapping of somatosensory areas that accidentally infringes on nearby pain centers, contends Vilayanur S. Ramachandran, a neurologist at the University of California, San Diego. Ramachandran and his coworkers reported last year that marked somatosensory reorganization had occurred in two amputees.
Further research with the more precise brain-imaging methods now available is needed to confirm the substantial cortical changes reported in adult amputees, notes Tim P. Pons, a neuroscientist at Wake Forest University's Bowman Gray School of Medicine in Winston-Salem, N.C.