Testing the Tau Hypothesis against Alternative Information Sources used for Collision Avoidance

Research on the visual information used in collision avoidance has inappropriately attributed adequate task performance to the use of the visual variable tau (τ). We constructed a collision avoidance experiment where the τ hypothesis was tested against the use of other potential information sources, including expansion rate. Subjects viewed a head-on flight towards one or two barriers. During the first half of trials, a single barrier was present and subjects were instructed to approach it as closely as possible before ascending over it. During the second half of the trials, a second barrier was placed above the first and subjects flew through the resulting gap. The ascent dynamics were manipulated between subjects, such that a time-to-contact margin demarcated the collision boundary with one group, but a constant distance-to-contact margin with another group. Subjects in the time-relevant group responded in a manner more consistent with the use of expansion rate information, but neither a single critical expansion rate or τ strategy adequately explains performance in the two-barrier condition. Subjects in the distance-relevant group responded in a manner consistent with the use of relative distance information.