Reorganization in primary motor cortex of primates with long-standing therapeutic amputations

Intracortical microstimulation was used to investigate the organization of primate primary motor cortex (M1) in three squirrel monkeys and two galagos years after the therapeutic amputation of an injured forelimb or hindlimb. In two squirrel monkeys with forelimb amputation, physiological results we re correlated with the distribution of corticospinal neurons after injectio ns of tracers into the lower cervical segments of the spinal cord. Distribu tions of labeled corticospinal neurons helped identify the locations of the former forelimb cortex in M1. Evoked movements from M1 ipsilateral to the missing limb were not obviously different from M1 of normal controls. Stimu lation in the deefferented part of M1 contralateral to the missing limb eli cited movements of the remaining proximal muscles as well as movements from adjacent body representations in all cases. Stimulation in the deefferente d forelimb cortex evoked shoulder stump, trunk, and orofacial movements, wh ereas stimulation in the deefferented hindlimb cortex evoked hip stump, tru nk, and tail movements. Movements were evoked from all sites in the deprive d cortex, so that there were no unresponsive zones. Minimal levels of curre nt necessary to evoke these movements varied from those in the normal range to those of much higher levels, with the average threshold higher than nor mal. Finally, multiunit recording from the two galagos revealed that the de prived portions of S1 were responsive to touch or taps on the stump and nei ghboring body parts.