Spatial reorganization of cortical motor output maps of stump muscles in human upper-limb amputees.

Spatial changes of the motor cortical representation of the biceps brachii muscle (stump muscle) were studied in ten patients with long-standing amputations at the level of the forearm. Motor output maps were established by focal transcranial magnetic stimulation on a skull surface grid overlying the motor cortex. Characteristics of the motor output map were its spatial extension (number of effective stimulation sites), the maximal response amplitude and the center of gravity (COG) of the spatial distribution of response amplitudes. The extension of the stump muscle motor maps was increased (ratio: 1.5+/-0.3 versus 1.0+/-0.3 in control group; P<0.05) and the stump muscle motor responses were much larger (ratio: 2.6+/-0.6 versus 1.0+/-0.5 in the control group; P<0.05). The COG of the stump muscle map was significantly shifted laterally by, on average, 6.0+/-7.7 mm (range, -3.4-21 mm; P<0.05), either reflecting gross changes of local cortical excitability or structural anatomic reorganization.