IS JUDGING TIME-TO-CONTACT BASED ON TAU

An investigation was undertaken into whether judgments of time-to-cont act between a laterally moving object and a bar are based on the direc t perception of an optical variable (tau), or on the ratio between the perceived distance and perceived velocity of the object. A moving bac kground was used to induce changes in the perceived velocities without changing the optical variables that specify time-to-contact. Backgrou nd motion induced large systematic errors in the estimated time-to-con tact. It is concluded that the judgment of time-to-contact is primaril y based on the ratio between the perceived distance and the perceived velocity, and not on tau.