Hyperglobal Interactions in Perception of Stereoscopic Transparency

Abstract

A method has been developed (1) to measure limitations of human observers in perception of visual scenes containing a transparent surface, and (2) to study computational stages underlying stereoscopic transparency perception. Observers viewed random-dot stereograms of overlapping plane and cylindrical surfaces and had to distinguish between two orientations of the cylindrical surface under conditions of strict depth fixation control. Dot density of the transparent plane was increased at various intersurface depth separations until identification of cylinder orientation became random (limiting density). Limiting density dramatically decreased as depth separation between the surfaces grew, and this basic relationship could not be accounted for either by higher severity of matching with larger dot densities or by the ability of a denser surface to pull vergence to its depth. The basic relationship persisted with opposite contrast polarities of plane and cylinder dots, which suggests that competitive interactions between the surfaces occur as a separate process from binocular matching at each surface [Harris and Parker, 1995 Nature (London) 374 808 – 811]. Thus, successive computational stages of matching and intersurface interaction characterising stereoscopic hyperglobality may be distinguished. We also found a facilitating role of intersurface brightness difference and a near — far asymmetry of the basic limiting density versus depth separation relationship with reversed surface positioning. We suggest a scheme of cortical inhibitory connections that agrees with the concept of disparity gradient limit and predicts performance similar to that reported. In the scheme, each activated unit produces a suppression zone in the disparity projection field so that lateral extension of the inhibition grows with binocular disparity.