In amputees, the brain no longer receives information from the lost limb. But the receiving station still works, and without incoming data, it can go haywire.
"We think the brain cells may be firing off randomly," says Tsao. The result: The amputee feels burning, throbbing, spasms or any number of other unpleasant sensations in the absent limb. One of Tsao's patients describes feeling as if a spike were being driven continuously through his foot.
Many researchers, including Tsao, suspect that mirror therapy eases these sensations by creating the illusion of two whole limbs.
Although patients know that the mirror does not reflect reality, other parts of the brain are not so savvy. These sensory input regions accept the mirror's evidence: The amputated limb reappears, provides the brain with sensory data and stops the random firing.
Tsao's research provides more evidence for an increasingly popular idea, that pain, particularly the chronic sort, is more a problem of brain than body.
"The key is not what happens in the body part; it's what the brain perceives is going on," says Oxford University neuroscientist Lorimer Moseley, who studies how the brain senses pain. "Any pain that anyone experiences is because the brain has constructed it."
Moseley expresses some skepticism about Tsao's results and says that larger studies are needed. He noted that mirror therapy sometimes loses effectiveness when used over longer periods.
Scientists believe that mirror treatment might help with other pain conditions. Dr. Eric Altschuler, a researcher at the University of Medicine and Dentistry of New Jersey, has used mirror therapy to help stroke patients. It not only reduces pain, but can restore use of paralyzed limbs.
"Mirror therapy is potentially a tremendous paradigm shift in therapy for many different problems," says Altschuler.
Tsao is starting two more mirror studies. One will examine whether mirrors can help arm amputees, while the other will use MRI scans to find out what happens in the brains of phantom limb pain patients before, during and after mirror treatment.
But even this work will likely leave many questions unanswered.
"Anybody who says they understand the brain," says Altschuler, "is a liar, a fool or both."
david.kohn@baltsun.com
