The reorganization of the motor system in primates after the loss of a limb.

As a result of an extensive search, we were able to obtain a limited number of macaque monkeys, New World squirrel monkeys, and prosimian galagos years after they had received a therapeutic amputation of a forelimb or hindlimb as a treatment for injury. Forelimb or hindlimb regions of primary motor cortex, M1, were stimulated in these primates with microelectrodes using amounts of current just sufficient to evoke a movement. Results were compared to those obtained from normal primates or from M1 contralateral to the intact limb in the same primates. In all cases, more sites in M1 contralateral to the amputated limb evoked movements of the limb stump and muscle in the adjoining shoulder or hip. In two of the macaque monkeys, injections in M1 reveal more widespread intrinsic connections than in normal M1, and in several monkeys and galagos, injections of tracers in muscles of the stump, shoulder, or hip labeled spinal cord motor neurons that normally project to the distal limb. These anatomical results suggest that the functional changes in M1 following amputation are mediated in part by the formation of new connections.