A Ten-Dollar Cardboard Box Treats Phantom Limb Pain

Most amputees feel their missing limb. About 80 percent feel it in pain — a clenched fist that cannot unclench, a hand caught in a door that never opens, an itch with nothing to scratch. The sensation is real. The neurology is the puzzle.

In 1992, Vilayanur S. Ramachandran, then at the University of California San Diego, published evidence that the somatosensory cortex of an amputee reorganizes after the limb is gone. The strip of brain that used to listen to the right hand begins responding to inputs from the face, because face inputs sit next door on the cortical map. Touching the cheek of an amputee with a Q-tip, Ramachandran could elicit specific tingles in the absent fingers — pinky here, thumb there. The map had moved.

From that, he reasoned that some phantom pain might come from a feedback mismatch: the motor cortex sends a command to clench the missing fist, but receives no proprioceptive confirmation back, leaving the loop locked. He built a small wooden box with a vertical mirror down the center. The patient sat with the intact arm visible on one side of the mirror and the stump tucked into the other. Looking at the reflection, the patient saw two functioning arms. When the intact hand moved, the missing one appeared to move with it.

The trick worked, at least for some. Patients reported the cramped phantom unclenching, the itch fading, the pain dropping. A 2007 randomized trial led by Jack Tsao at Walter Reed Army Medical Center, on military amputees, showed mirror therapy outperformed two control protocols. Mechanism studies since have been mixed; some attribute the effect to mirror neurons, others to plain visual feedback restoring the broken loop. The treatment costs less than ten dollars to build.