EFFECTS OF THE SPATIOTEMPORAL STRUCTURE OF OPTICAL-FLOW ON POSTURAL READJUSTMENTS IN MAN

How does the spatio-temporal structure of an oscillating radial optica l flow affect postural stability? In order to investigate this problem , two different types of stimulus pattern were presented to human subj ects. These stimuli were generated either with a constant spa tial fre quency or with a spatial frequency gradient providing monocular depth cues. When the stimulation was set in motion, the gain response of the antero-posterior postural changes depended upon the oscillation frequ ency of the visual scene. The amplitude of the postural response did n ot change with the amplitude of the visual scene motion. The spatial o rientation of the postural sway (major axis of sway) depended strictly and solely on the structure of the visual scene. In static conditions , depth information resulting from the presence of a spatial frequency gradient enhanced postural stability. When set in motion, a visual sc ene with a spatial frequency gradient induced an organization of postu ral sway in the direction of the visual motion. Considering visual dyn amic cues, postural instability depended linearly both on the logarith m of the velocity and on the logarithm of the temporal frequency. A no nlinear relationship existed between the amplitude of the fore-aft pos tural sway at the driving frequency and the temporal frequency, with a peak around 2-4 Hz. These results are discussed in terms of their imp lications for the separation of visual and biomechanical factors influ encing visuo-postural control.