Postural stability during visual stimulation and the contribution from the vestibular apparatus.

Conclusion: When combined with vestibular dysfunction, visual flow can exacerbate reductions in postural stability. This effect may be one of the mechanisms underlying visual vertigo, which can be evaluated using frequency analysis of body sway elicited by optokinetic stimulation (OKS).

Objective: To clarify the interaction between the postural responses to visual flow and to input from the vestibular apparatus as a mediator of visual vertigo.

Methods: Horizontal and vertical OKS with a stable fixation target were presented to 14 healthy subjects and 38 peripheral vestibular patients standing in a Romberg's posture, and the center of standing pressure was monitored using a force platform. The direction and amplitude of induced body sway were analyzed, along with the power spectra of the body mass fluctuations.

Results: Each directional optokinetic stimulus induced body sway that would compensate for the virtual inclination of the subject's gravitational reference frame. However, the amplitude of this body sway was not increased by vestibular dysfunction. Healthy subjects showed increased stability in response to downward OKS and decreased stability in response to upward OKS, whereas no specific changes were seen in response to horizontal OKS. This stability was greatly diminished in patients with vestibular dysfunction subjected to the same directional OKS.