Visual stabilization of posture. Physiological stimulus characteristics and clinical aspects.

Visual stabilization of posture is critically dependent on stimulus characteristics as well as on the performance of the visual system. The purpose of the present investigation was to obtain quantitative data in man by means of posturography of fore-aft and lateral body sway in relation to different visual stimulus characteristics. Visual acuity, when decreased logarithmically, causes a linearly increasing postural instability, twice as prominent for fore-aft than for lateral sway. Any measurable visual contribution for fore-aft sway ceases with an acuity lower than 0.03 and for lateral sway with an acuity lower than 0.01. The central area of the visual field as compared with the peripheral retina dominates postural control. The foveal region exhibits a powerful contribution, in particular for lateral sway. A partial but significant visual stabilization is preserved with a visual input rate between 1 to 4 Hz flicker frequency. As soon as continuous motion perception becomes involved with frequencies higher than 4 Hz, visual stabilization gradually improves with a saturation at frequencies higher than 16 Hz. Lateral body sway activity and eye-object distance are linearly related: body sway decreases with increasing distance corresponding to the linear decrease of net retinal displacement with increasing eye-object distance. Aspects of 'afferent' and 'efferent' visual motion perception, which involve fore-aft and lateral body sway differently, are evaluated. The clinical relevance is demonstrated in patients with oculomotor disturbances. The results are discussed with respect to the variety of related clinical disorders, which involve reduction in visual acuity, field defects, accommodation disturbances and ocular oscillations.