Time varying encoding of grasping type and force in the primate motor cortex

Abstract

The primary motor cortex (M1) is strongly engaged by movement planning and execution. However, the role of M1 activity in voluntary grasping is still not completely understood. Here we analyze recordings of M1 neurons during the execution of a delayed reach-to-grasp task, where monkeys had to actively grasp an object with either a side or a precision grip, and then pull it with a low or high amount of force. Single cell and neural populations analyses showed that grip type was robustly and specifically encoded by a large population of neurons, while force level was weakly and transiently encoded within mixed-selective neurons that also encoded grip type. Notably, the grip type was stably decoded from motor cortical populations during the preparation and execution epochs of the task. Our results are consistent with the idea that planning and performing specific grasping movements are high-level skills that strongly engage M1 neurons, while the execution of pulling force might be prominently encoded at lower stages of the motor system.

Significance statement Grasping behavior requires precise motor coordination exerted by multiple brain areas, including the primary motor cortex (M1), but the exact role of M1 in grasping preparation and execution remains elusive. Here, we analyzed the neural activity of M1 while two monkeys performed a delayed reach-to-grasp task. We found that two parameters of grasping: grip type and pulling-force level were encoded in the activity of single neurons and the neural population, although with important differences. While grip coding was stronger and more temporally stable, force encoding was weaker and short lived. Our results suggest that grip planning and execution is a high-level neural process that takes place independently of force control in M1.