Reflections of a Healthy Limb

Magicians have long exploited a mirror's knack for tricking the mind's eye. Now researchers are getting in on the act. In the current issue of The Lancet, a team from the University of California, San Diego, reports that stroke patients can benefit from a mirror that causes their brains to mistake a healthy hand for a paralyzed one.

Stroke victims often experience partial paralysis on one side of the body. After seeing mirrors help some amputees control phantom limb pain by making it look like they were relaxing the lost limb, neuroscientist Eric Altschuler and his colleagues decided to try a similar approach on paralysis. They recruited nine stroke patients and had them sit down with either a mirror or a clear plastic sheet placed perpendicular to their body, on their paralyzed side. Patients were asked to put their impaired hand behind the mirror or the sheet. Looking sideways in the mirror, the image of the healthy hand seemed to "substitute" for the hidden hand; the plastic had no such effect. The researchers asked patients in both groups to make a number of symmetrical movements with both hands at the same time, while looking in the mirror or through the plastic. After 4 weeks, the groups switched apparatuses.

Two neurologists who were unfamiliar with the patients or their regimen scored their progress by looking at videotapes of the impaired limbs in 2-week intervals for 8 weeks. One of them said four patients improved significantly on the mirror treatment, another said seven benefited from the trick. Both agreed that only one patient improved on the control treatment.

Altschuler isn't sure why the treatment works. Stroke victims often lose their proprioception--their intuition of where a limb is--and perhaps mirror therapy provides visual feedback that can substitute for this normal sense, he says. "Just like watching a dance instructor perform dance steps can help you learn to perform them properly, the mirror is showing the patient how to move the impaired limb." One of the treatment's strengths is that the patients love it, he says. "It doesn't matter how great a treatment is if no one does it." Indeed, nobody had thought to use mirrors in this clever way, says physical therapist Alison McKenzie of Chapman University in Orange, California. "Patients report a profound sense of awe," because it feels like their paralyzed limb is working. "It's not an immediate cure, but it's a cue that progress is possible."