Maintenance of the Upright Posture in Humans upon Manipulating the Direction and Delay of Visual Feedback

Abstract

We examined the maintenance of the upright posture in subjects immersed in a 3D virtual visual environment, VVE. The latter consisted of two plans; the VVE foreground represented the image of a room window, while the background looked like an outside urban landscape. The position of the foreground (window) was linked, using the respective software, with body sways in the sagittal plane and shifted in accordance with body movements, while the background always remained immobile. By manipulating the direction and time delay of the relationship between the body sways and shifts of the VVE foreground, we tried to estimate the contribution of visual signals to postural control. For this purpose, we used the technique of frequency filtration of displacements of the center of feet pressure (CFP) and, in such a way, detected signals proportional to movements of the general center of gravity (CG) of the body (CG variable). After this, we calculated the difference between the CFP and CG shifts (CFP-CG-variable). The latter variable is known to be proportional to the horizontal acceleration; this is why it was used for estimation of the muscular torques correcting CG shifts. Analysis of changes in the above variables (CG and CFP-CG) demonstrated a clear dependence between the root mean square (RMS) of their amplitude spectra on the direction of feedback between body sways and shifts of the VVE foreground. With a synphase (SPh) pattern of the link between shifts of the body and those of the VVE foreground, the RMS values were closer to the range typical of standing with the eyes closed, while with an antiphase (APh) link, they were closer to the zone of values observed under conditions of normal vision. Introduction of time delays in feedback between sagittal body sways and shifts of the VVE foreground exerted nearly similar effects on the CG and CFP-CG variables. In both SPh and APh relations, 0.2- and 0.5-sec-long time delays resulted in increases in the RMS of the sway spectrum, while 0.8- and 1.0-sec-long delays led to RMS decreases. Thus, our findings show that the maintenance of upright stance under conditions of manipulations of the direction and time delay of visual feedback is performed, at least partly, with the use of an unstable VVE foreground as the reference. This can be considered proof of a special (frequently, dominating) role of the visual signals in postural control.