Motor cortex single-neuron and population contributions to compensation for multiple dynamic force fields

To elucidate how primary motor cortex (M1) neurons contribute to the performance of a broad range of different and even incompatible motor skills, we trained 2 monkeys to perform single-degree-of-freedom elbow flexion/extension movements that could be perturbed by a variety of externally-generated force fields. Fields were presented in a pseudo-random sequence of trial blocks. Different computer monitor background colors signalled the nature of the force field throughout each block. There were five different force fields: null field without perturbing torque (N), assistive (V-) and resistive (V+) viscous fields proportional to velocity, a resistive elastic force field (E+) proportional to position and a resistive visco-elastic field (VE) that was the linear combination of V+ and E+. After the monkeys were extensively trained in the 5 field conditions, neural recordings were subsequently made in M1 contralateral to the trained arm. Many caudal M1 neurons altered their activity systematically across most or all of the force fields in a manner that was appropriate to contribute to the compensation for each of the fields. The net activity of the entire sample population likewise provided a predictive signal about the differences in the time course of the external forces encountered during the movements across all force conditions. The neurons showed a broad range of sensitivities to the different fields and there was little evidence of a modular structure by which subsets of M1 neurons were preferentially activated during movements in specific fields or combinations of fields.