Parallel planning through an optimal neural subspace in motor cortex

How do patterns of neural activity in motor cortex contribute to the planning of a movement? A recent theory developed for single movements proposes that motor cortex acts as a dynamical system whose initial state is optimized during the preparatory phase of the movement. This theory makes important yet untested predictions about preparatory dynamics in more complex behavioral settings. Here, we analyzed preparatory activity in non-human primates planning not one, but two movements simultaneously. As predicted by the theory, we found that parallel planning was achieved by adjusting preparatory activity within an optimal subspace to an intermediate state reflecting a tradeoff between the two movements. The theory quantitatively accounted for the relationship between this intermediate state and fluctuations in the animals behavior down at the trial level. These results uncover a simple mechanism for planning multiple movements in parallel, and further point to motor planning as a controlled dynamical process.