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H A. Sedgwick, Barbara Gillam, Raj Shah; Stereoscopically perceived depth across surface discontinuities. Journal of Vision 2004;4(8):174. https://doi.org/10.1167/4.8.174.
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One way of perceiving the spatial relations between two objects is by relating them to a common underlying surface such as the ground plane. A simple case occurs when both objects are directly in contact with the common surface. In more complex scenes, there may be a hierarchy of contact relations linking the objects to the common surface. Meng and Sedgwick (2002), using monocular static displays of two spatially separated platforms resting on the ground plane, showed that a pointer resting on one platform could be adjusted to match the relative distance of a block resting on the other platform. The precision of the adjustment decreased, however, when the two platforms were at different heights. When the platforms were at different distances from the observer the settings showed a bias suggesting incomplete integration of local information relating each object to its platform with global information relating the two platforms to each other. With stereoscopic vision, there is now substantial evidence (Gillam and Sedgwick, 1996, among others) that the stereoscopically perceived depth of a probe can be mediated by a surface behind the probe, even though not in direct contact with it. In this study we used stereoscopic displays of two spatially separated frontal plane platforms located at differing depths from a frontal background surface. Observers adjusted the perceived depth of a small probe, located in front of one platform, so that its distance from the observer appeared equal to that of another probe, standing out in depth in front of the other platform. Variability of observers' settings increased as the depth separation of the two platforms increased, indicating that observers were using the background surfaces to relate the two probes to each other. For some observers the depth separation between the two platforms also produced a bias in the perceived equidistance of the two probes analogous to the bias found by Meng and Sedgwick using monocular displays.
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