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Songjoo Oh, Maggie Shiffrar; Ground cues influence the visual perception of rolling. Journal of Vision 2006;6(6):1039. doi: https://doi.org/10.1167/6.6.1039.
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Traditional theories assume that the visual system solves the correspondence problem by defaulting to the shortest path connecting any two points. As a result, two points in apparent motion appear as a single point translated back and forth along the shortest trajectory. Yet, outside of the laboratory, object motion is constrained by the environment. Psychophysical studies of the perceived motion of a circle were used to compare the relative impact of environmental constraints and shortest-path constraints on the perception of object motion. Observers viewed computer generated displays of a circle undergoing smooth displacement. Given the circle's homogeneous structure, its displacement could be interpreted as rolling (orientation change)or translating (no orientation change). Across experiments, observers viewed various displays and reported whether the circle appeared to roll or translate. In Experiment 1, a single horizontal line was added to the display. When the line was located directly below the displacing circle, the circle appeared to roll along this “ground”. When the line was located directly above the circle, the circle was more likely to appear to translate. In Experiment 2, the line was tilted. When the circle moved from the top to the bottom of the tilted line, it appeared to roll down a sloped “ground”. When the line was vertical, the circle appeared to translate. These results suggest that cues supporting the presence of a ground surface significantly impact motion percepts. This conclusion supports theories that suggest that visual perception reflects an internalization of universal environmental constants (Shepard, 1982).
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