Abstract
The various influences on posture of visual and somatosensory conditions were analyzed in 15 normal subjects with the use of the EquiTest and optokinetic stimulation. Two types of somatosensory input were used: a static support surface and a moving surface coordinated with the anterior-posterior (A-P) sway of the subject. Four types of visual input were applied in each somatosensory condition: (1) eyes open, (2) eyes closed, (3) movement of visual surroundings coordinated with A-P sway, (4) horizontal optokinetic stimulation. On the static support surface, the center of gravity (COG) was very stable with eyes open and with optokinetic stimulation. The COG was less stable when the eyes closed, and the least stable when the visual surroundings were moving. The FFT in the A-P direction under static somatosensory conditions showed characteristic changes when the visual surroundings were moving: a significant increase of power in the frequency range below 0.25 Hz, a decrease between 0.25 and 0.5 Hz. Although no significant change in lateral direction was observed, the power under optokinetic stimulation seemed to be 10% greater in the range of 0.25-0.5 Hz than it was under the three other visual conditions.
On a moving support surface, however, COG stability was severely disturbed under all visual conditions except with the eyes open. Furthermore, FFT showed no differences when the subject was standing on a moving support surface. We concluded that sensory conflict caused by moving visual surroundings affects postural stability more than do the other visual conditions tested in our experiment. However, sensory conflict caused by the reduction of somatosensory information results in more severe instability than that caused by any of the visual conditions. These findings confirm the importance of sensory information for postural stability in humans.
