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Nikolaus Troje, Seamas Weech, Sophie Kenny; Can we perceive linear perspective in biological motion point-light displays?. Journal of Vision 2013;13(9):188. doi: https://doi.org/10.1167/13.9.188.
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Orthographically projected point-light walkers are ambiguous with respect to depth. For instance, fronto-parallel views from the front and from the back of a bilaterally symmetric walker result in identical projections. In a number of recent studies it was shown that the introduction of linear perspective can gradually disambiguate perceived facing direction. This observation seems to imply that observers are sensitive to linear perspective in point-light displays. We hypothesize that effects of using an approaching perspective camera have nothing to do with linear perspective per se, but with the fact that the camera looks down at the feet – at least when its vertical location is above ground level. The two hypotheses can be distinguished experimentally: If the effect is due to linear perspective, only distance between camera and walker matters. If it is a result of looking down at the feet, the camera elevation angle (height/distance) determines the effect. Using a staircase procedure, we measured PSE and slope of the psychometric function relating percentage of perceived facing direction to the amount of perspective (quantified in terms of field-of-view angle, i.e. the visual angle subtended by the walker) in 10 participants. Three vertical camera levels were used within-subjects: feet-level, half walker height, and head-level. As the camera is lowered from head- to mid-level, field-of-view angles at PSE increase from 3.8 deg to 6.7 deg, and the slope of the psychometric function decreases from 3.7 %/deg to 2.4 %/deg. If the camera is at floor level the field-of-view angle never converges to a stable value, reaching a nominal PSE of 43 deg and a slope of only 0.55 %/deg at the end of a 80-trial staircase. The data imply that it is not perspective, but the projection of the feet seen slightly from above that dis-ambiguates the perceived facing direction of the walker.
Meeting abstract presented at VSS 2013
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