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A. Fuzz, Qasim Zaidi; Perceptual asymmetry in solid shape perception. Journal of Vision 2002;2(7):306. doi: 10.1167/2.7.306.
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Iso-perspectival solids are physically distinct 3-D objects which project to the same image, e.g. a cube at eye height is iso-perspectival to a family of shapes created from it by applying a shearing affine transform along the line of sight. Griffiths & Zaidi (2000) found that cubes sheared away from the observer were misperceived as regular upright cubes, but cubes sheared towards the observer were perceived veridically. We hypothesized that the percepts were influenced by the ground plane, and we now investigate the effects of stimulus contact with neighboring surfaces.
Stimuli were sheared cubes, presented at eye level so that the closest edge to the observer was either vertical, or oriented towards or away from the observer. Stimuli were mounted either on top of a horizontal planar “floor”, underneath a “ceiling”, or were “free-floating” without a visible support. Gauge measurements of the perceived orientation of the closest edge of these stimuli were used as estimates of perceived shape.
Stimuli with vertical edges were perceived veridically. For other stimuli, the closest edges of free-floating stimuli were consistently perceived as vertical, independent of stimulus shape. Floor-mounted and ceiling-mounted stimuli exhibited bistable percepts in which the perceived edge orientations fluctuated between slants towards or away from the observer. Slant was underestimated by up to 50%. However, stimuli in which the mounted face was farther from the observer than the opposite face were more likely to be perceived veridically.
Under perspective projection, the edges of the physically closer top or bottom face of these sheared cubes will be wider in the image than those of the farther face. If the leading edge is inferred to be vertical, the perceived shape will be wider at the top or bottom. It appears that observers accept a prior assumption that edges are vertical for shapes which contact a surface at their widest edges, but reject it for the opposite configuration.
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