When is visual information about obstacle size needed to step over an obstacle?

摘要

When humans walk over rough terrain, the trailing limb is moved over obstacles on the ground without immediate visual information about the obstacle. That is, by the time the trailing limb begins its step, the obstacle has already passed from the field of view. Nonetheless, the trajectory of the trailing foot is precisely tuned to the location and dimensions of an obstacle being crossed. In this study, we investigated when visual information about an obstaclea??s dimensions is needed during approach to successfully guide the trailing limb over the obstacle. The experiment was conducted in an immersive virtual environment (VE) viewed through a head-mounted display. The positions of the head and both feet were tracked by a motion capture system. Subjects walked toward a goal initially located 5 away, and stepped over a box-shaped virtual obstacle on the ground along their path. The obstacle was initially located 4 m away and varied in dimension, by height and depth. Throughout most of the approach, the obstacle was replaced by a narrow line on the ground at the front edge of the obstacle, providing information about its location but temporarily removing information about its dimension. Information about obstacle dimension was briefly made available at one point during the approach: before walking was initiated or when the subject was three steps, two steps, or one step in front of the obstacle. Analyses focused on the effects of the availability of information about obstacle dimension on measures of precision and performance, which reflect the sufficiency of the visual information presented with respect to obstacle crossing. Initial testing suggests trailing foot trajectories are best tuned to obstacle dimensions when the information is available after the approach begins but before one step ahead of the obstacle.