Mechanisms of reference posture correction in the system of upright posture control

It was earlier shown that ultraslow tilts of the support under quiet standing conditions evoke an unusual response reflecting the operation of compensatory mechanisms: postural sway is a superposition of postural oscillations typical of quiet standing and greater, slower inclinations of the body caused by the tilt. This may be explained by the presence of two hierarchical levels of upright posture control: real-time control compensates for small deviations of the body from the reference posture prescribed by presetting control. Mathematical simulation methods have been used to study the mechanisms of reference posture control. The results are compared with available experimental data. It is demonstrated that the reference posture can be corrected according to the gravitational vertical with the use of a kinesthetic reference alone. It is hypothesized that, when correcting the reference posture, the nervous system “assumes” the support to be immobile. The afferent input from sole pressure receptors is an important factor in reference posture correction. The advantages of the putative two-level control over control based on an explicit internal model are discussed.