Persistent hand movement representations in the brains of amputees

Reilly et al . (2006) report that each type of voluntary movement of a phantom hand evokes a distinct pattern of activity of stump muscles in patients with longstanding upper extremity amputations, even when the amputations are above-elbow and the stump muscles are not those ordinarily used in hand movement. This variety of patterns in stump muscle activity was absent in one shoulder-level amputee who reported that he could not move his phantom limb voluntarily. When pressure palsy of the nerves of the stump was induced in three below-elbow amputees by an inflated cuff spanning the elbow, the stump muscle activity decreased, and the patients reported reduced amplitude or loss of voluntary phantom movement. The authors conclude that reorganization of motor cortex following amputation ‘reflects the fact that preserved hand movement representations retarget stump muscles to express themselves’.This interpretation requires clarification. First, we need to know what is meant by ‘hand movement representation’ in the reorganized cortex. We generally use that term to denote the set of neurons in motor cortex that gives rise to a hand movement when activated either voluntarily or electrically. But in the reorganized cortex of these amputees there are no longer any such neurons. Second, the authors' view that retargeting …