Surface discontinuity is critical in a moving observer's perception of objects' depth order and relative motion from retinal image motion

The visual system perceptually decomposes retinal image motion into three b asic components that are ecologically significant for the human observer: o bject depth, object motion, and self motion. Using this conceptual framewor k, we explored the relationship between them by examining perception of obj ects' depth order and relative motion during self motion. We found that the visual system obeyed what we call the parallax-sign constraint, but in dif ferent ways depending on whether the retinal image motion contained velocit y discontinuity or not. When velocity discontinuity existed leg in dynamic occlusion, transparent motion), the subject perceptually interpreted image motion as relative motion between surfaces with stable depth order. When ve locity discontinuity did not exist, he/she perceived depth-order reversal b ut no relative motion. The results suggest that the existence of surface di scontinuity or of multiple surfaces indexed by velocity discontinuity inhib its the reversal of global depth order.