PRIMARY MOTOR CORTEX REORGANIZATION IN A LONG-TERM MONKEY AMPUTEE

The primary motor cortex (Mi) was mapped with intracortical microstimu lation (ICMS) in a 15 year-old macaque whose right upper extremity was amputated at the shoulder joint prior to 2 years of age. Movements of the right shoulder girdle and stump were evoked by ICMS throughout th e left M1 upper extremity region. The size of the left M1 upper extrem ity region contralateral to the amputated arm was not appreciably diff erent from the size of the right upper extremity region contralateral to the intact arm. Long stimulus trains and/or higher stimulus current s were needed to evoke detectable movements at significantly more loci in the left than in the right M1 upper extremity region. These observ ations would be consistent with unmasking of a high threshold represen tation of shoulder musculature that normally exists throughout the cen tral core of the upper extremity region, where it underlies a lower th reshold representation of the distal forelimb. Alternatively, invasion of the de-efferented distal forelimb core by surrounding shoulder rep resentation may have occurred. Differences between the limited M1 reor ganization observed in the present study and the more extensive reorga nization of S1 observed in other studies may reflect fundamental diffe rences between M1 and S1, and/or differences in the extent of de-effer entation versus deafferentation.