The role of retinal flow in walking.

Both visual direction and optic flow have been implicated in walking toward a goal in experiments that use prisms to alter visual direction. However, prisms bias retinal flow patterns in ways that might affect the path chosen. We used a prism manipulation in virtual reality for goal-directed walking, where we manipulated the location of texture-defined flow as well as the direction of gaze. The addition of texture straightened paths, but primarily when it provided flow on the ground plane. Texture confined to the ground straightened paths similar to full-scene texture, whereas wall-only texture produced little straightening. This finding reveals a disproportionate contribution of motion generated on the ground plane to control locomotion, potentially reflecting higher retinal speeds and the prism-induced rotations in the lower visual field. This suggests that, when visual direction is manipulated by a prism, trajectories might result from unexpected motion patterns, leading to compensatory foot placement. Thus trajectories may be shaped by complex interactions between goal direction, gaze, and local retinal motion patterns.