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Songjoo Oh, Maggie Shiffrar; Multistability of point-light gait is resolved by the optical flow of the ground. Journal of Vision 2005;5(8):25. doi: 10.1167/5.8.25.
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Two dimensional point-light walker displays are ambiguous in depth (Proffitt et al., 1984; Vanrie et al., 2004). Yet, observers are frequently unaware of this ambiguity (Bulthoff et al., 1998). What cues does the visual system use to disambiguate human motion? Object perception is strongly dependent upon ground cues. We therefore tested whether the ground influences the perceived direction of a point-light walker's gait. Across three studies, naïve observers viewed computer-generated displays of a point-light walker and reported whether the point-light person walked towards or away from them. In Exp 1, the point-light person was positioned above 10 points in a trapezoidal arrangement depicting a rigidly translating surface. When the points defining this ground surface translated away from observers, the point-light person appeared to walk towards the observers (82% “toward” and 18% “away” responses). Conversely, when this ground translated toward observers, the point-light person appeared to walk away from the observers (70% “away”). Thus, ground motion significantly influences the perception of gait direction. In Exp. 2, the point-light defined surface was positioned above the point-light walker's head. When positioned as a ceiling, the translating surface had significantly less impact on the perceived direction of the walker's gait. In Exp. 3, the ceiling and ground motions were placed in conflict. That is, inward ceiling motion was paired with outward ground motion and visa versa. When an ambiguous point-light walker was placed in between conflicting ceiling and floor motions, the floor motion was significantly more likely to capture the perceived direction of gait. Taken together, these results suggest that the visual interpretation of human motion depends upon the physical restrictions on that motion. Specifically, the ground constrains human motion (Gibson, 1979) and visual analyses appear to take that into account.
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