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Abstract

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 [1]. The degree to which velocity constancy is observed depends on the information provided by the stimulus and its surround [2,3,4]. So far, studies on velocity constancy and distance have not considered the separate contribution of vergence as a cue to distance. Here, we investigate whether eye vergence (as an extra-retinal cue to distance) and
horizontal disparity (as an retinal distance cue) contribute to Velocity Constancy. Subjects viewed two sequentially-presented dots or 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. The eye vergence was manipulated using a haploscope. Varying the disparity of the second sphere with respect to the background plane, the constancy of the subject’s velocity judgments at different disparities was investigated. Vergence defined
distances did not influence the subject’s velocity percept, whereas the distances defined by disparity did.