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Brett R. Fajen, William H. Warren; Interception of moving objects on foot. Journal of Vision 2001;1(3):187. doi: 10.1167/1.3.187.
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How do people walk to intercept a moving target? Several visual control strategies can be identified. (a) Current direction: walk in the current egocentric direction of the target. (b) Background flow: keep the heading specified by background flow on the target. These two predict curved paths with heading angles of 0 deg. (c) Local expansion: keep the target's local focus of expansion on the target. (d) Constant bearing: keep the target at a constant angle with respect to the locomotor axis. (e) Constant heading: keep the target at a constant angle with respect to the heading direction specified by background optic flow. These three predict straight paths with constant heading angles greater than 0 deg. Participants walked in a 40 × 40 ft virtual environment toward a moving target. To examine the role of local expansion, the target was either a post (expansion) or a thin line (no expansion). To examine the role of background flow, the virtual environment consisted of a black background (No Room), a room with textured walls, floor, and ceiling (Stationary Room), or a room that moved with the same velocity as the target, providing conflicting background flow (Moving Room). If observers rely upon the egocentric direction of the goal, behavior should be unaffected by background flow. We analyzed observers' paths and time series of the angle between direction of locomotion and the goal (heading angle). Observers walked so as to achieve and maintain a constant heading angle between ∼ 8° and 18°, ruling out the first two strategies. Heading angle was unaffected by target type, undermining the third strategy. However, heading angle was smaller in the No Room and Moving Room conditions than in the Stationary Room condition. The results suggest that observers intercept a moving target by traveling at a constant angle to the goal, consistent with the Constant Bearing strategy, with an influence of background optic flow.
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