Specific and Nonspecific Visual Influences on the Stability of the Vertical Posture in Humans

Abstract

We studied physiological mechanisms of vision-related stabilization of the vertical posture in humans using a stabilographic technique; spontaneous deviations of the projection of the center of gravity during quiet stance and magnitudes of the postural response to vibratory stimulation of proprioceptors of the lower leg muscles under varied conditions of visual control were measured. The stability of quiet stance, as estimated according to the root mean square value of the sagittal component of the stabilogram, was the best with eyes open. Vibration-induced postural responses were the smallest also under these conditions. Spontaneous postural sway and the amplitude of response to vibratory stimulation increased when only a central sector of visual field (20 ang. deg) was preserved and, especially, under conditions of closed eyes and horizontal inversion of visual perception using prismatic spectacles. Parallel changes in the quantitative stabilographic indices and amplitude of vibration-induced postural responses show that the intensity of the latter is probably determined by the background stiffness of the musculoskeletal system. We tried to estimate separately the contributions of the stiffness factor, on the one hand, and specific visual influences, on the other hand, by testing the parameters of quiet stance and postural responses under conditions of standing while lightly touching a support with the index finger. We found that the influence of the conditions of visual control on the stability of quiet stance while touching the support was eliminated. At the same time, the magnitude of postural responses to vibratory stimulation decreased but, nonetheless, changed with visual conditions in the same manner as when standing without additional support. We conclude that vision performs a dual function in the control of the vertical posture; it forms the basis for the spatial reference system and serves the source of information on the movements of one's body.