Can Observers Judge Future Circular Path Relative to a Target from Retinal Flow?

We investigated the ability of observers to judge whether they will pass left or right of a visible target from simulated motion along a circular path. Strategies based on optic flow would generally require compensation for pursuit eye movements. Wann & Swapp (2000) proposed an alternative strategy that requires only retinal flow. The experiments compared three conditions that provide the same retinal flow but different observer-relative optic flow. In the heading-relative view condition, simulated view direction rotated with change in heading, as naturally occurs when driving a car. In target-relative view condition, simulated view direction rotated to keep the direction of the target constant. In world-relative view condition, the simulated view direction was fixed relative to the environment. If an observer fixates the target, these conditions produce the same retinal flow. The initial heading direction of simulated motion was varied across trials, and responses were used to compute PSEs representing perceptual bias. Judgments were most accurate in the heading-relative condition. In the target-relative and world-relative view conditions, PSEs indicated large biases consistent with underestimation of path curvature. The large biases suggest that retinal flow is not sufficient to judge future circular path relative to a target.