Effects of Destabilization of Visual Environment Perception on the Maintenance of Upright Stance by Humans on Different Support Surfaces

Abstract

We compared characteristics of the maintenance of human upright stance under conditions of a real visual environment (VE) and “immersion” into a virtual visual environment (VVE). The foreground of the latter corresponded to the window in the room, while the background was a view of the aqueduct with the adjacent terrain. Destabilization of the VVE was created by “coupling” of the foreground position with oscillations of the subject’s body within the sagittal plane. We measured elementary variables calculated according to the trajectory of the center of feet pressure (CFP); these variables were: (i) displacement of the vertical projection of the center of gravity (CGv) and (ii) difference between the positions of the CFP and CGv (variables CGv and CFP – CGv). When standing on a rigid support surface, the root mean square (RMS) of the spectra of oscillations of both variables decreased in the case of an antiphase relation between displacements of the VE foreground with oscillations of the body and increased in the case of an inphase relation between these variables, as compared with the RMS in the maintenance of upright stance under conditions of an immobile VE (ImVE). Under conditions of the inphase relation, however, there were no dramatic disorders in the vertical stance; maximum oscillations of the body in this case did not exceed values typical of the upright stance with the eyes closed (EC). When the upright stance was maintained on a squeezable support, body oscillations increased significantly under all visual conditions, and the difference between the RMS of the CGv spectra obtained for the conditions of the inphase relation and EC became statistically significant. In the case of standing on a squeezable support, RMSs of the CFP – CGv variable at the antiphase relation of the VVE foreground were greater than those at the inphase relation. At the same time, the RMS of the CGv spectra were, vice versa, greater at the inphase relation. Thus, upon variation of the conditions for the vertical stance maintenance, the amplitude characteristics of elementary variables (CGv and CFP – CGv) determining the CFP on a support can vary in both a parallel and an independent manner. These variables can be controlled not only by coupled but also by independent (uncoupled) mechanisms controlling their amplitude/frequency parameters.