Voluntary movement initiation is not coupled to optimal sensorimotor oscillatory phases

Successful initiation of a voluntary movement requires transmission of descending motor commands from the primary motor cortex (M1) to the spinal cord and effector muscles. M1 activity alternates between brief excitatory and inhibitory brain states in the form of oscillatory phases that correlate with single neuron spiking rates and population-level neuronal activity. The influence of these brief brain states on fundamental motor behaviors, like movement initiation, is not known. Here, we asked if voluntary movement initiation occurs during specific oscillatory phases of sensorimotor rhythms using a combination of transcranial magnetic stimulation (TMS), electroencephalography (EEG), and behavioral testing. To address this, we empirically determined the time point at which M1 released the motor command required to produce a simple finger movement during a self-paced movement initiation task. We then probabilistically modeled the oscillatory phase at this time point in the mu (8-12 Hz) and beta (13-30 Hz) ranges, and at each frequency between 8 and 50 Hz, determining each subject9s preferred movement initiation phase for each frequency. After pooling the identified phases across subjects, we identified no significant clustering of preferred movement initiation phases within mu or beta frequencies, or at any other frequency between 8 and 50 Hz. These results demonstrate that movements were not preferentially initiated during optimal oscillatory phases at any frequency. Thus, we conclude that initiation of self-paced voluntary movements is not strongly coupled to optimal sensorimotor oscillatory phases in the healthy human brain. It remains to be determined if more complex aspects of motor behavior like action selection occur during optimal oscillatory phases.