why upright maintenance of posture is unique in human ?

We examined the maintenance of the upright posture under conditions of immersion of the tested subject in a 3D virtual visual environment, VVE. The latter consisted of two plans, the foreground looked like a window of the room, while the background was a fragment of the urban landscape. The software used allowed us to correlate body sway within the sagittal plane with shifts of the VVE foreground doing the latter mobile. The linkage in the course of testing could be either antiphase (APh) or synphase (SPh); the background always remained immobile. Each experiment was conventionally divided into two parts. In the first part, the subjects were not informed that body sways in some trials can be linked with shifts of the visual environment and were proposed to look at some object within the stable background. In the second part, the subjects were preliminarily informed of the existence of such a linkage and proposed to minimize body sways within the sagittal plane by tracking shifts of the foreground with respect to some, selected preliminarily, object within the background. Trials with a linkage between body sways and VVE shifts were altered by trials with standing of the subject facing an absolutely immobile visual pattern (IVP) and trials with the eyes closed (EC). The impact of visual control on the postural maintenance was estimated according to changes in the amplitude and frequency characteristics of two variables; i) shifts of the vertical projection of the center of gravity (CG) of the body and ii) difference between the position of the center of feet pressure (CFP) and CG projection. Changes in the latter were considered the main controlled variable in the postural maintenance, while the CFP-CG was interpreted as a variable containing information on changes in the resulting muscle/joint stiffness in the ankle joints related to activation of the shin muscles. The analysis of CG and CFP-CG oscillations demonstrated clear dependence of their spectra on the direction of linkage between body sways and oscillations of the foreground and on the instruction obtained by the subjects. In the first group of tests, RMSs of the spectra of oscillations of both variables at the SPh linkage were found to be within the same range as in standing with the EC, while at the APh linkage these oscillations were closer to the range typical of IVP conditions. In the second group of tests, the RMS of the spectra calculated for both variables were qualitatively different. In the case of APh relations between body sways and foreground shifts, these values were smaller than values in standing at IVP, while under SPh conditions they were smaller than those typical of standing with the EC. Introduction of the instruction of the second test group was also reflected in changes of the parameters of the CFP-CG variable. At the APh relation, the RMSs of the spectra for this variable decreased somewhat, and their median frequency increased significantly. The data obtained allow us to hypothesize that the improvement of stability standing after introduction of the instruction specifying visual control and concentrating the latter on one direction of body sway results from more active using of visual feedback under conditions of perception of the VVE unstable foreground. This should be considered additional proof in favor of effective involvement of vision in postural control upon disturbances in stability of the external environment.