Primary and Supplementary Motor cortex implement parallel control solutions for rhythmic and discrete arm movements

Abstract

Arm movements are rhythmic, discrete, or some combination of the two. Conflicting evidence supports each of two possible solutions for how motor cortex controls them: that either it uses the same strategy for controlling rhythmic and discrete movements or different strategies for distinct movement types. Using recurrent neural network modelling and multi-unit recordings during an arm-cycling task, we show that primate motor cortex uses both solutions. Primary motor cortex (M1) dynamics converge to the same limit-cycle when executing both movement types. In contrast, supplementary motor area (SMA) dynamics diverge according to the type of the upcoming movement before reaching a helical spiral. Our results reconcile opposing views on the cortical control of rhythmic and discrete movements by showing that the two solutions are not mutually exclusive but implemented in parallel within motor cortex. Instead, our results challenge us to understand why both solutions are implemented to solve the same problem.