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 were correlated with the distribution of corticospinal neurons after injections of tracers into the lower cervical segments of the spinal cord. Distributions 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. Stimulation in the deefferented part of M1 contralateral to the missing limb elicited movements of the remaining proximal muscles as well as movements from adjacent body representations in all cases. Stimulation in the deefferented forelimb cortex evoked shoulder stump, trunk, and orofacial movements, whereas stimulation in the deefferented hindlimb cortex evoked hip stump, trunk, and tail movements. Movements were evoked from all sites in the deprived cortex, so that there were no unresponsive zones. Minimal levels of current necessary to evoke these movements varied from those in the normal range to those of much higher levels, with the average threshold higher than normal. Finally, multiunit recording from the two galagos revealed that the deprived portions of S1 were responsive to touch or taps on the stump and neighboring body parts.