Discrete cortical control during quiet stance revealed by desynchronization and rebound of beta oscillations
Postural sway during quiet stance often exhibits a repetition of micro fall and the subsequent micro recovery. The classical view -that the quiet bipedal stance is stabilized by the ankle joint stiffness- has been challenged by paradoxical non-spring-like behaviors of calf muscles: gastrocnemius muscles are shortened (contract) and then stretched (relax), respectively, during the micro fall and the micro recovery. Here, we examined EEG (electroencephalogram) based brain activity during quiet stance, and identified desynchronization and synchronization of beta oscillations that were associated, respectively, with the micro fall and the micro recovery. Based on a widely accepted scenario for beta-band desynchronization during movement and post-movement rebound in the control of discrete voluntary movement, our results reveal that the beta rebound can be considered as a manifestation of stop command to punctuate the motor control for every fall-recovery cycle. Namely, cortical interventions to the automatic postural control are discrete, rather than continuous modulations. The finding is highly compatible with the intermittent control model, rather than the stiffness control model.