SYSTEMATIC CHANGES IN DIRECTIONAL TUNING OF MOTOR CORTEX CELL-ACTIVITY WITH HAND LOCATION IN THE WORKSPACE DURING GENERATION OF STATIC ISOMETRIC FORCES IN CONSTANT SPATIAL DIRECTIONS

We examined the activity of 46 proximal-arm-related cells in the prima ry motor cortex (MI) during a task in which a monkey uses the arm to e xert isometric forces at the hand in constant spatial directions while the hand is in one of nine different spatial locations on a plane. Th e discharge rate of all 46 cells was significantly affected by both ha nd location and by the direction of static force during the final stat ic-force phase of the task. In addition, all cells showed a significan t interaction between force direction and hand location. That is, ther e was a significant modulation in the relationship between cell activi ty and the direction of exerted force as a function of hand location. For many cells, this modulation was expressed in part as a systematic arclike shift in the cell's directional tuning at the different hand l ocations, even though the direction of static force output at the hand remained constant. These effects of hand location in the workspace in dicate that the discharge of single MI cells does not covary exclusive ly with the level and direction of force output at the hand. Sixteen p roximal-arm-related muscles showed similar effects in the task, reflec ting their dependence on various mechanical factors that varied with h and location. The parallel changes found for both MI cell activity and muscle activity for static force production at different hand locatio ns are further evidence that MI contributes to the transformation betw een extrinsic and intrinsic representations of limb movement.