Postural sway during optokinetic stimulation - The relationship between posture and vection during optokinetic stimulation

A stationary observer, while viewing a large visual scene rotating around his line of sight, experiences a sensation of self-motion opposite in direction to that of the pattern motion and a limited degree of body tilt and displacement of the visual vertical. The present study in 12 healthy volanteers showed that contrary to the general belief, these phenomena are not continuous, but in most cases, occurs intermittently. In addition, the present study also demonstrated that the tilt of the body, as revealed by postrurographic and videographic recordings, exhibited a close temporal relationship with vection. While the observer does not experience vection, no or only a limited degree of body tilt was observed. The moment the observer experienced vection, however, the body tilted in the direction of optikinetic stimulation. The close relationship between vection and posture as seen in the present study seemed to be the consequence of reciprocal inhibitory visual-vestibular interaction. While the observer experiences vection, the visual input presumably dominates in the fomation of the internal representation of the surrounding space, leading to the body tilt in the direction of the optokinetic stimulation. This direction of body tilt coincided with the tilt of the subjective visual vertical. When vection is not sensed by the observer, on the other hand, the body returned to the upright position, which indicated that proprioceptive and otolith inputs dominate in the internal representation of the surrounding space during the vection-free period.