[Influence of visual cues on upright postural control: differentiated effects of eyelids closure]

In most protocols aimed at testing balance abilities, patients are generally required to close their eyes in order to gain insight about proprioceptive cues and the way the central nervous system (CNS) uses this information. However, one should not exclude possible interaction with the physiological mechanisms involved in eyelid closure, thus leading to a biased neurological evaluation. To assess this possible involvement, 15 healthy adults were required to keep their eyes open in the dark (YOn), to close normally (YF) and forcibly their eyelids (YFF), respectively in random order. The analysis was focused on elementary motions computed from the complex center of pressure (CP) trajectories, i.e. the horizontal motions of the center of gravity (CG(h)) and the difference between the CP and the vertical projection of the center of gravity (CP-CG(v)). The former is recognized as the main variable in postural control whilst several interesting features can be extracted from the latter: their link with the horizontal accelerations communicated to the CG, the level of muscular activity and the implied ankle stiffness expressed by their frequential distribution. The results indicate that the amplitudes of the CP-CG(v) spectra are statistically reduced in YOn when compared to the YF condition, especially in the antero-posterior direction. On the other hand, no shift in the frequential bandwidth was observed on these spectra, signifying a constancy in ankle stiffness over all conditions. Interestingly, the CNS does not really seem to gain from these reduced horizontal accelerations at the CG level since the CG(h) amplitudes are only slightly reduced. However, it is important to emphasize that the CP trajectories alone are not able to demonstrate any statistical trend. It can thus be hypothesized that, despite the fact that visual information is still unavailable, the proprioceptive cues nonetheless continue to play a minor role in the detection-correction process aimed at controlling body sway. Past studies on the physiology of eyes blinking have suggested the probability that the cerebellar cortex or brain stem structures (such as the reticular formation) are involved in these commands. Because of its dual facilitating-inhibiting function, the latter is indeed a fair candidate for modulating the descending command operating through the postural muscles. These data are of interest for the practician in order to assess with precision the role played by proprioceptive and visual cues in possible balance disfunctioning.