SELECTIVE FACILITATION OF RESPONSES TO CORTICAL STIMULATION OF PROXIMAL AND DISTAL ARM MUSCLES BY PRECISION TASKS IN MAN

1. The responses of the first dorsal interosseus (1DI), opponens polli cis (OF), extensor digitorum communis (EDC), brachioradialis (BR), bic eps brachii (BE) and anterior deltoid (AD) muscles to magnetic stimula tion of the motor cortex were recorded during different motor tasks. 2 . Two precision and two power isometric tasks were investigated. The p recision tasks were a pincer grip ('grip') and a thrust against a. tar get with the wrist ('push'). In the former, the prime movers were the intrinsic hand muscles, while the proximal muscles played a postural r ole, In the latter, the prime movers were the proximal muscles. In bot h tasks, force was controlled through visual feedback. The power tasks required encirclement of a cylinder with the fingers ('grasp'), or su staining a weight suspended at wrist level ('load'). 3. Magnetic stimu lation was applied in eight subjects by a coil placed over the vertex at 1.1-1.2 times the motor threshold for the most excitable muscles. T his produced in the prime mover muscles larger motor-evoked responses (MEPs) during grip or push tasks than grasp or load tasks, in spite of similar background EMG; levels. During grip tasks, only one of the tw o prime movers showed task-dependent changes. In the postural muscle A D there was no significant difference between MEPs during grip and gra sp tasks; however, BE responses were larger during grasp than grip tas ks. 4. MEPs simultaneously recorded in the prime movers were plotted a gainst each other. The slope of the regression line for AD versus BB w as larger in push than load tasks, whilst the changes in MEPs of 1DI a nd OP were independent during both grip and grasp tasks. 5. In three s ubjects, MEPs were also elicited by electrical stimulation during grip and grasp tasks. MEP changes tended to parallel those obtained for ma gnetic stimulation, but the increase in size of the electrically evoke d MEPs during the precision task was smaller. 6. In all subjects the m edian and ulnar nerves were stimulated during grip and grasp tasks, an d an H reflex was evoked in the hand muscles of five subjects. In no c ase did the two tasks produce reflexes of different amplitude. 7. The motor response of both proximal and distal muscles can be task depende nt, in spite of the differences in their principal functional role and cortical representation. The modulation is related to the degree of c ontrol requested by the task, and is likely to reflect selective chang es in the excitability of corticospinal neurones.