Gepshtein, S. & Cooperman, A. (1998). Vision Research 38 (19) pp. 2913-32.

Abstract (extended for this page)

Binocular perception of complex visual scenes is inherently ambiguous because elements in one eye's view can often find multiple matches in the other eye's view. It has been suggested that to resolve these ambiguities the brain adopts a continuity constraint assuming that binocular disparities change smoothly with eccentricity. Stereoscopic transparency is characterized by abrupt changes of binocular disparity across retinal locations. The focus of the present study is how the brain uses the continuity constraint in the perception of stereoscopic transparency despite the presence of abrupt disparity changes.

Observers viewed random-dot stereograms of overlapping transparent plane and cylindrical surfaces and had to distinguish between two orientations of the cylindrical surface under conditions of strictly controlled depth fixation. We found that maximal dot density of the transparent plane at which perception was still veridical dramatically decreased as depth separation between the surfaces grew. In other words, stereoscopic masking by the front surface increased as the inter-surface distance in depth grew. This relationship persisted when the dots of different surfaces had opposite contrast polarity, to be handled by presumably separate ON and OFF contrast channels. This fact suggests at least two stages in the underlying computation: (a) binocular matching, (b) inter-surface interactions. We also measured the robustness of this phenomenon (i) as contrast difference between the two surfaces varied, and (ii) when the two surfaces were swapped in depth (near-far asymmetry).

We show that the observed phenomena cannot be accounted for either by higher difficulty of matching with high dot densities or by the ability of the denser surface to pull vergence to its depth. We propose a model of competitive interactions between dissimilar disparities where the lateral extent of inhibitory interactions grows as a function of depth separation.