2008 年 67 巻 1 号 p. 24-33
Many studies have shown that optokinetic stimulation induces postural sway and that vision, in addition to vestibular and somatosensory inputs, plays an important role in postural control. In the present study, we examined individual differences in the postural sway induced by movement of visual images back and forth and the effect of the response to the projected image. Three kinds of visual images were used: "normal direction" (image moving backward), "reverse direction" (image moving forward), and "static" images. Subjects received visual stimulation via a head-mounted display while standing on a stabilometer with their feet aligned in the Romberg position, and the postural sway was measured. The subjects initially maintained the standing position at rest for one minute (a static image was presented) and were then given visual stimulation for two minutes, followed by a 2-minute presentation of the static image. The period of visual stimulation is referred to as "during stimulation" and the 2-minute period of presentation of the static image is referred to as "after stimulation". The postural sway was measured over time. In Experiment 1, the measurement was conducted 5 times under each stimulation condition. In many subjects, postural sway was induced in the direction of visual stimulation "during stimulation" and in the opposite direction "after stimulation" (normal type); however, some subjects showed the opposite postural sway (reverse type) or showed no induced postural sway at all (poor response type). Most subjects showed similar changes for each of the five stimulations. Individual differences in postural sway were considered to be induced by the effect of vision on the postural control system. In Experiment 2, the measurement was conducted using the same visual stimulation under two kinds of mental states: a sub-conscious condition, in which the subjects were instructed to watch the image without any assumption regarding their own motion, and a conscious condition, in which the subjects were instructed to assume that that they were moving in response to the projected image (although no actual walking motion occurred). Many subjects showed postural sway in the opposite direction when the state was changed, despite the image remaining the same. These results suggest that posture is smoothly controlled using visual information to allow adaptation to various environments.
