The Size of the Visual Feedback-Providing Object Affects the Vertical Posture Maintenance in Humans

Abstract

Of all information that permits a person to elaborate an idea about the spatial position of their body, visual signals about the structures and sizes of visible objects in the surrounding play an important role. A study was made to find out how a change in the size of an object in the field of vision affects the stability of standing in healthy people. Subjects stood in stereo glasses and a mask that limited the field of vision in a darkened room in front of a screen (silver screen, 2 × 1.5 m), and a three-dimensional image of a dark gray sphere was projected on the screen. The subjects saw only the virtual three-dimensional image of the sphere in these conditions. Four sphere sizes with diameters of 8.75, 17.5, 35, and 70 cm were used. Spheres with these sizes covered visual fields of 4.5, 9, 18, and 36 degrees, respectively. The sphere was motionless in control conditions, while displacements of the sphere in the anteroposterior and lateral directions were in phase or out of phase with oscillations of the body’s center of gravity (CoG) in test conditions. The amplitude of sphere oscillations was twice as high as the amplitude of CoG oscillations. The subjects performed 36 tests of 40 s each; the instruction was to calmly stay on a stabilographic platform and to look at the sphere. The tests were grouped into four blocks, each including three control tests with the immobile sphere and six tests with the sphere moving in phase or out of phase with CoG oscillations. The interval between tests was 1 min. The sphere had the same size in all tests of a block. After each block, the subject rested in a sitting position for 4–5 min. Blocks with different sphere sizes were performed in a random order. The amplitude-frequency characteristics of two elementary variables were calculated from the trajectories that the center of pressure (CoP) of the feet showed in the anteroposterior and lateral directions. The variables were the trajectory of the CoG projection on the support (CoG variable) and the difference between the CoP and CoG trajectories. Sphere displacements destabilized the vertical posture in the test conditions. Destabilization increased with the increasing sphere size. Quality of standing worsened because of the changes in both amplitude and frequency characteristics of the CoG and CoP–CoG variables. An increase in sphere size exerted an opposite effect in control tests; i.e., body oscillations decreased. Changes in the stability of the vertical posture with increasing size of the sphere were presumably associated with the peculiarities of using visual information that comes from the central and peripheral fields of view.