The Role of Extra-Retinal Cues in Velocity Constancy

To estimate the real world speed of an object the velocity of the retinal projection must be scaled by the perceived distance. If observers perceive objects travelling with the same speed at different distances from the eye as equally fast, they are said to exhibit velocity constancy. However, not all studies examining velocity constancy support the idea that observers can scale speeds for the viewing distance. In fact they suggest that subjects perceive angular rather than objective velocities (McKee Welch 1989). The degree to which velocity constancy is observed depends on the information provided by the stimulus and its surround (Wallach 1939, Epstein 1978, Zohary Sittig 1993). So far, studies on velocity constancy and distance have not considered the separate contribution of vergence as a cue to distance. Here, we specifically investigate whether eye vergence (as an extra-retinal cue to distance) contributes to velocity constancy. Subjects viewed two sequentially-presented rotating wire-frame spheres moving horizontally in the frontoparallel plane. They were required to report whether or not the speed of the second sphere exceeded the objective velocity of the first one. By varying the disparity of the second sphere with respect to the background plane, we could investigate the constancy of velocity judgments at different disparity defined distances. Under conditions of vergence to the plane of the presentation screen, observers produced data consistent with velocity constancy