Time Constant of Forward Body Sway Induced by Downward Optokinetic Stimulus

Forward body sway induced by a downward optokinetic stimulus (OKS) was observed in 38 healthy subjects. The downward OKS was generated by a moving random dot pattern projected onto a half-pipe screen 2 m in diameter and was provided by a rectangular-like step function with velocities of 10, 20, 30 and 40 deg/sec. A fixation target was set by a red laser spot projected onto the screen in front of the subjects. As an additional stimulatory condition, 20 deg/sec of OKS with no fixation target was employed. The induced forward body sway was measured by a force-detecting platform as displacement of the center of gravity (COG). The gradual forward shift of COG followed by the plateau state was closely akin to a graphical representation of an equation of the first order lag time. As OKS velocities increased, the plateau level increased. Curve fitting analysis using a computer was employed to establish an approximation equation of the first order lag time. This analysis showed that the time constant of the optokinetic spinal response system was approximately 15 sec and the value of the time constant was independent of OKS velocity. Based on this long time constant, we concluded that the postural readjustment induced by OKS mainly plays a role in the low frequency range of human postural control.