ON THE RELATIONS BETWEEN SINGLE-CELL ACTIVITY IN THE MOTOR CORTEX ANDTHE DIRECTION AND MAGNITUDE OF 3-DIMENSIONAL STATIC ISOMETRIC FORCE

We examined the relations between the steady-state frequency of discha rge of cells in the arm area of the motor cortex of the monkey and the direction and magnitude of the three-dimensional static force exerted by the arm on an isometric manipulandum. Data were analyzed from two monkeys (n=188 cells) using stepwise multiple linear regression. In 15 4 of 188 (81.9%) cells the regression model was statistically signific ant (P<0.05). In 121 of 154 (78.6%) cells the direction but not the ma gnitude of force had a statistically significant effect on cell activi ty; in 11 of 154 (7.1%) cells only the magnitude effect was significan t; and in 22 of 154 (14.3%) cells both the direction and magnitude eff ects were significant. The same analysis was used to assess the effect of the direction and magnitude of force on the electromyographic acti vity of 9 muscles of the arm and shoulder girdle. The regression model was statistically significant. For all the muscles studied in 4 of 9 (44.4%) muscles only the direction effect was significant whereas in t he remaining 5 of 9 (55.6%) muscles both the direction and the magnitu de were significant. No muscle studied showed a significant effect of force magnitude alone, These differences in the frequency of occurrenc e of directional and magnitude effects between cells and muscles were statistically significant (P<0.005, chi(2) test). These findings under score the fundamental importance of the direction of force in space fo r both motor cortical cells and proximal muscles and underline the dif ferential relations of the cells and muscles to the direction and magn itude of force. These results indicate that the specification of the m agnitude of three-dimensional force is embedded within the directional signal; this combined direction+magnitude effect was 3.9 times more p revalent in the muscles than in the cells studied. In contrast, the pu re directional effect was 1.8 times more prevalent in the cells than i n the muscles studied. This suggests that the direction of force can b e controlled independently of its magnitude and that this direction si gnal is especially prominent in the motor cortex.