The role of optical velocity in the control of stance

Two experiments were designed to investigate the use of optical flow for postural control as a function of its velocity, geometry, and retinal placement. In the first experiment, subjects were exposed to a room that moved at a velocity below the reported threshold for detection of point-light motion. In the second, the room was moved so that optical velocities were above this threshold. Compensatory sway was assessed for both lamellar and radial flow to central and peripheral areas of the retina. Compensatory sway was elicited with optical velocities below the point-light motion threshold, suggesting that this threshold is not relevant to the detection of egomotion. The results also indicate that the retinal periphery does not detect posture-relevant information from flow fields with a radial dynamic structure. Since the higher velocities used in the second experiment exceeded those generated by natural postural instabilities, it was concluded that radial flow in the retinal periphery is not used as a source of information for normal postural maintenance, and therefore that flow structure is an important factor in the detection and control of egomotion.