Corticopostural functional and effective connectivity reveal cortical control of postural sway velocity during quiet standing

Background Despite a large body of evidence showing the involvement of the sensorimotor cortex in postural control, its exact role remains unclear. Models of postural control outcomes suggested that the velocity of the center of pressure is a crucial parameter to maintain balance. Inspired by corticokinematic coherence, we hypothesized that cortical oscillations and the velocity of the center of pressure (CoP) would synchronize and that this synchronization would increase with postural task difficulty during quiet standing. Methods We compared the magnitude of coherence and Granger causality computed between brain oscillations recorded with electroencephalography and the center of pressure velocity in the Delta and Theta frequency bands obtained from 23 participants performing four quiet standing tasks with various levels of difficulty. The effect of postural task difficulty and information flow direction were tested with a linear mixed model while non-parametric correlations were computed between coherence magnitude and postural performance measured by 95% confidence ellipse area and mean center of pressure velocity. Results We found significant coherence between the Cz EEG electrode and CoP velocity in the Delta and Theta frequency bands. This EEG-CoP velocity coherence significantly increased with task difficulty in the Delta (F = 18.8, p < 0.001) and Theta (F = 7.83, p < 0.001) bands. Granger causality significantly increased with task difficulty (F = 12.5, p < 0.001) and was higher in the efferent than afferent direction (F = 78, p < 0.001). The 95% confidence ellipse area was correlated to coherence magnitude in the most difficult condition. Participants showing significant Granger causality in the afferent direction showed more stable postural outcomes. Conclusion Our results confirm that the CoP velocity has a crucial role in postural control through its synchronization with sensorimotor cortex oscillations. The efferent information predominance suggests that posture is partly controlled by the sensorimotor cortex by a mechanism named corticopostural coherence. Our results show that this corticopostural coherence could represent a mechanism for controlling balance during quiet standing.