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* To whom correspondence should be addressed. E-mail: kalaskaj{at}physio.umontreal.ca.
We recorded the activity of 132 proximal-arm-related cells in caudal primary motor cortex (M1) of two monkeys while they generated either isometric forces against a rigid handle or limb movements with an inertial load. The forces and movements were in eight directions in a horizontal plane. The isometric force generated at the hand increased in monotonic fashion in the direction of the target force. The force exerted against the load in the movement task was more complex, including a brief initial accelerating force in the direction of movement followed by a transient decelerating force approximately opposite to the direction of movement as the hand approached the target. EMG activity of proximal-arm muscles reflected the task-dependent differences in dynamics, showing directionally tuned ramp-like changes in activity during the isometric task, and a reciprocally tuned triphasic pattern in the movement task. As a result, the apparent instantaneous directionality of the muscle activity, when expressed in hand-centered spatial coordinates, remained relatively stable throughout the isometric ramp but often showed a significant transient shift during the limb movements. Neuronal activity in M1 showed corresponding significant changes in activity pattern and instantaneous directionality between the two tasks at both the single-cell and population levels. This momentary dissociation of discharge from the directional kinematics of hand displacement is evidence that the activity of many single proximal-arm related M1 cells is not coupled only to the direction and speed of hand motion. These results also demonstrate that population-level signals reflecting the dynamics of motor tasks and of the interactions with objects in the environment are readily available from the caudal part of primary motor cortex. This signal could greatly enhance the performance capabilities of neuroprosthetic controllers.
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