Velocity constancy is the ability to equate physical speeds of objects placed at different depths, despite object's angular speeds on the retina changes proportionally with depth. Multiple studies have shown that size cues play a central role in the achievement of velocity constancy. On the other hand, some studies have provided evidence showing that depth cues are unnecessary for velocity constancy. However, since retinal size is linearly related to depth, it is reasonable to hypothesize that both cues should affect the perception of speed. We present here results of two experiments in which we measure the bias of perceived speed and size as a function of depth for rotational motion. We use this type of motion to avoid the effect of the frame on the perceived speed since the reference in rotational motion is its own center. We introduce binocular disparity to produce depth perception. The stimulus consisted of 16 dots (0.15 deg size) located 2 deg away from the center of rotation, undergoing rotational motion. 6 observers, the authors and 3 others naives as to the purpose of this study took part in the experiment. Results show that observers overestimate dot speed and pattern size of further stimuli but perceiving angular velocity as invariant. This result shows that the visual system would re-scale dot speed when the apparent radius increases so as to maintain angular velocity constant. However, the bias in perceived size is much larger than that of speed, which suggests that such re-scaling is not linear.
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