Résumé
Gibson argued that the movement of an observer through a stationary environment is specified by the optical flow field. For postural control, one property of the flow field is usually emphasized: radial expansion. Motion of a single marker fixed to the participant’s neck was used to determine the direction and amplitude of body sway. Marker position was recorded with a two-camera elite system at a sampling rate of 100 Hz. The mean direction of sway, mean peak-to-peak amplitude in that direction, phase of sway with respect to the driver, and correlation with the driver were determined by fitting the time series for each trial with a sinusoidal function at the driver frequency. Displays of an oscillating 3D “post-it” cloud were presented to an observer walking on a treadmill, while compensatory sway was recorded. Because expansion and parallax were congruent, we expected postural responses to be directionally specific and isotropic in amplitude.