PARTIAL INACTIVATION OF THE PRIMARY MOTOR CORTEX HAND AREA - EFFECTS ON INDIVIDUATED FINGER MOVEMENTS

After large lesions of the primary motor cortex (M1), voluntary moveme nts of affected body parts are weak and slow. In addition, the relativ e independence of moving one body part without others is lost; attempt s at individuated movements of a given body part are accompanied by ex cessive, unintended motion of contiguous body parts. The effects of pa rtial inactivation of the M1 hand area are comparatively unknown, howe ver. If the M1 hand area contains the somatotopically ordered finger r epresentations implied by the classic homunculus or simiusculus, then partial inactivation might produce weakness, slowness, and loss of ind ependence of one or two adjacent digits without affecting other digits . But if control of each finger movement is distributed in the M1 hand area as many studies suggest, then partial inactivation might produce dissociation of weakness, slowness, and relative independence of move ment, and which fingers movements are impaired might be unrelated to t he location of the inactivation along the central sulcus. To investiga te the motoric deficits resulting from partial inactivation of the M1 hand area, we therefore made single intracortical injections of muscim ol as trained monkeys performed visually cued, individuated flexion-ex tension movements of the fingers and wrist. We found little if any evi dence that which finger movements were impaired after each injection w as related to the injection location along the central sulcus. Unimpai red fingers could be flanked on both sides by impaired fingers, and th e flexion movements of a given finger could be unaffected even though the extension movements were impaired, or vice versa. Partial inactiva tion also could produce dissociated weakness and slowness versus loss of independence in a given finger movement. These findings suggest tha t control of each individuated finger movement is distributed widely i n the M1 hand area.