CHARACTERISTICS OF VOLUNTARY VISUAL SAMPLING OF THE ENVIRONMENT FOR SAFE LOCOMOTION OVER DIFFERENT TERRAINS

The characteristics of visual sampling required for successful locomot ion over various terrains is the focus of this work. In the first expe riment we directly address the role of continuous visual monitoring of the environment in guiding locomotion by allowing the subjects to cho ose when and where to take a visual sample of the terrain and examine the effects of different terrains on characteristics of visual samplin g. Young subjects walked over travel paths of varying difficulties whi le wearing opaque liquid crystal eyeglasses and pressed a hand-held sw itch to make the glasses transparent when they needed to sample the en vironment. Travel time and visual sampling characteristics were record ed. Results show that intermittent sampling (less than 50%) of the env ironment is adequate for safe locomotion, even over a novel travel pat h. The frequency, duration and timing of visual samples are dependent on terrain characteristics. Visual sampling of the environment is unaf fected by preview restriction of the travel path and is increased when specific foot placement is required and there is a potential hazard i n the travel path. In the second experiment we dissociated steering co ntrol and obstacle avoidance from specific foot placement and examined visual sampling demands prior to the initiation of the swing phase an d during the swing phase. The results show that steering control and o bstacle avoidance do influence the visual sampling time, which is scal ed to the magnitude of change. Vision was used in a feedforward contro l mode to plan for and initiate appropriate changes in the swing limb trajectory: its use during the swing phase to provide on-line control was minimal.