Asymmetry between horizontal and vertical illusory lines in determining the depth of their embedded surface

To investigate how the visual system integrates disparity information from horizontal and vertical edges and conveys it to the regions without any depth cues, we introduce a new phenomenon of subjective surface formation in an Ehrenstein-style configuration with inducing elements at different depths, and without explicit monocular occlusion zones. Different sets of experiments by separate groups of subjects suggest that when a subjective (illusory) square forms, it is at the depth of vertical illusory sides rather than horizontal ones. When the vertical side inducers are stereoscopically behind the horizontal ones, subjective surface formation is less likely. In depth assignment, we interpret the dominance of vertical sides over horizontal ones geometrically: vertical orientation can convey the horizontal disparity--a critical factor for Wheatstone (classic) stereopsis--but horizontal orientation per se lacks horizontal disparity information. Therefore, in the disparity integration, vertical illusory sides play a dominant role and their depth information influences the embedded subjective surface as well as the horizontal illusory sides.