The Perception of an Optical Flow Projected on the Ground Surface

Abstract

In most experiments in which the importance of visual control on postural stability is studied, optical stimuli attached to vertical surfaces are used. Analyses of long-term standing readjustments generally involve back-and-forth movements of a visual scene or its projection on vertical circular screens. In a natural environment, however, visual information is largely available from the ground. The aim of the experiment reported was to assess the effect of a flow pattern simulating an open outdoor setting on motion perception and postural control. Subjects were presented with an optical texture projected onto the ground. Periods of motionless texture alternated with equivalent durations of unidirectional flows. The change of position of the subject's centre of gravity over time was recorded on a force platform. Results show that the direction of body sway corresponded with that of texture motion. Important aftereffects, as shown in linear vection experiments, were also observed. However, the latency of postural responses was much shorter than with prolonged unidirectional flows produced in other locations of the visual environment. The hypothesis of an ecological specificity of the flows perceived on the ground during terrestrial displacements is discussed.

Walking in an environment of moving ground texture

“…This point is of importance, as it has been established that flow structure is a determining factor in perceived (or induced) motion (Stoffregen, 1985). We have, in particular, shown that flows produced on the ground, such as those present during most common terrestrial displacements, may have a different perceptual status from textures flowing on vertical surfaces (Flückiger & Baumberger, 1988). In summary, it can be assumed that the visual characteristics of the scene produced by the TFG are very close to the natural geometrical optical structure encountered by a walker.…”

“…The TFG was used to project a pattern of luminous dots on the ground (see Flückiger & Baumberger, 1988). The TFG was fixed on the ceiling and a number of circular holes were punched at regular intervals in a circular belt.…”

“…This dissymmetry has also been noted by Perrone (1986), who showed that threedimensional rigidity is perceived much faster when the visual scene is approaching than when it is receding. Other studies on postural adjustments (Flückiger & Baumberger, 1988;Lestienne, Soechting, & Berthoz, 1977) have provided evidence of smaller postural readjustments in a receding visual texture than what is observed in an approaching visual environment. Using a mobile room set-up in experiments on locomotion, Konczak (1994) also found more pronounced effects on locomotor speed with an approaching visual flow.…”

“…If so, the optical velocity of the flow could provide an absolute reference in itself for the control of locomotion. Therefore, as we have previously suggested, for terrestrial species the position of the flow on the ground holds a privileged status from a perceptual point of view (Flückiger & Baumberger, 1988), and humans may have developed a particular sensitivity to the motion of textures that are projected mainly on the upper retina during most displacements over the ground.…”

Effect of visual surrounding motion on body sway in a three-dimensional environment

“…Real visual environments used to study this phenomenon have been tilting rooms (Bles, Kapteyn, Brandt, & Arnold, 1980), rotating disks (Dichgans, Held, Young, & Brandt, 1972), and translating rooms (Bronstein, 1986;Lee & Lishman, 1975). Projected visual displays have simulated moving walls, tunnels, floors, and ceilings (Dijkstra, Schöner, & Gielen, 1994;Fluckiger & Baumberger, 1988;Lestienne, Schoechting, & Berthoz, 1977;van Asten, Gielen, & Diener van der Gon, 1988). Those experiments have shown that motion of a single surface background (in the simplest case, two dimensional) induces a codirectional postural sway.…”