Image velocity, not tau, explains arrival-time judgments from global optical flow

The time-to-passage (TTP; i.e., the time) until an object passes an observe r is optically specified by global tau, a variable that operates on the exp ansion late of the angle subtended by an object relative to the observer's heading, M. K. Kaiser and L. Mowafy (1993) provided evidence for observers' sensitivity to global tan in a 3-D cloud of point lights. This interpretat ion is challenged, and it is suggested that TTP judgments are based on a re lated but much simpler variable, the image velocity of the object. The pres ent study reexamined several factors that are relevant for the extraction o f global tau. When global tan and image velocity were brought into conflict by varying the lateral offsets of the targets, observers showed a strong t endency to rely on the latter variable. Other factors that are supposed to affect TTP judgments only if observers relied on global tau, such as how-he ld density and gaze-movement angle, did nor affect performance.