Primary Motor Cortex Demonstrates Muscle Dynamics and Serves as a Robust Kinetic Input to Brain Machine Interfaces

During movement, signals related to the kinematics and kinetics of movement are mutually correlated, and each is correlated with the discharge of neurons in the primary motor cortex (M1). M1 neurons are characterized by the relationship of discharge to movement direction, which is typically sinusoidal. The point of maximal tuning is called the preferred direction (PD). PDs can change in response to factors like training, attention, altered mechanics, or time passage. I examined whether PD changes could be attributed to measurement noise or limited amounts of data in the absence of external perturbations. I calculated PDs along with confidence bounds for single neurons and found PD changes not significantly different from zero. PDs of neurons in primary motor cortex appear to be stable over behaviorally relevant timescales.;Under changing conditions, I examined the relationship between M1 discharge to kinetmatics and kinetics. When movements are made, kinematic intentions must be transformed into muscle commands. M1 has been implicated in this transformation process. As monkeys made rapid, sequential reaching movements under two different dynamical conditions, I imposed either a clockwise or counter clockwise velocity dependent force field at the hand. The PDs of neural discharge for nearly all neurons rotated in the direction of the applied field. The PD rotation for a given neuron was typically consistent across multiple sessions, with changes occurring nearly instantaneously, reaching a steady state despite ongoing behavioral adaptation. This suggested that most M1 neurons are directly related to the dynamics of muscle activation, not the adaptive changes that occurred within an experimental session.;I furthered this analysis at the population level by using multi-electrode recordings in M1 as input to linear decoders of both endpoint kinematics (position and velocity) and proximal limb EMG during reaching. I tested these decoders across tasks that used fields of different directions or under conditions of altered limb posture. When we used the decoders developed under one task condition to predict the signals recorded from a different task, only the EMG decoders generalized well. These results support the view that M1 discharge is more closely related to kinetic variables like EMG.