On the relations between single cell activity in the motor cortex and the direction and magnitude of three-dimensional dynamic isometric force.

The role of the motor cortex in the control of both the direction and magnitude of dynamic force, when both are allowed to vary in 3D, is not known. We recorded the activity of 504 cells in the motor cortex of two monkeys during a behavioral task in which the subjects used a manipulandum to vary both the direction and magnitude of isometric force in 3D space. The majority (86%) of cells active in the task related to the direction, a tiny number (2.5%) to the magnitude, and a moderate number (11.5%) to both the direction and magnitude of dynamic force output. Finally, we compared neural activity in the same population of neurons during dynamic and static force output and found that the relations to direction and magnitude were very similar in both epochs. Our results indicate that during dynamic force production, cells in the motor cortex are primarily concerned with specifying the direction of force. The magnitude signal is not prominent in motor cortex neurons, and in general, magnitude and direction of force are specified together. Furthermore, the data suggest that the control of static and dynamic motor systems is based, to a great extent, on a common control process.