The Importance of Synchrony in the Neural Control of Movement.

Throughout the mammalian cortex, populations of neurons must work together to enact behaviors. While population recordings in motor cortex have revealed many aspects of when neurons fire during behaviors, limitations in causal experiments have made it difficult to identify which features of neural activity directly drive movements and which do not. Here, we explicitly test the principles of neural coding using high temporal precision multiphoton holographic optogenetics in the motor cortex. We show that activation of a small number (50-75) of Layer 2/3 excitatory neurons in the motor cortex is sufficient to drive movements. The efficacy of stimulation-driven movement depends on the state of the local circuit, and, to a lesser extent, the identity of which neurons are stimulated. We test whether evoked activity is acting through a 'rate code' or a 'timing code' by holding the firing rate and cell identities constant while varying the millisecond precise timing of activation. We find an unexpected and strong dependence on inter-cell synchrony when evoking movements. This neural synchrony recruits distinct patterns of recurrent excitation and inhibition. These findings provide evidence that the timing code, more so than the rate code, drives motor output.