Abstract
Binocular fusion relies on matching points in the two eyes that correspond to the same physical feature in the world. Not all features, however, create binocularly similar, or even binocularly paired, retinal projections. In particular, features without binocular projections frequently occur at depth edges. While unmatched features can impede fusion and result in interocular competition, the monocular features at depth edges are often perceptually stable. Previous work has shown that this visual stability breaks down if the stimulus is not geometrically plausible, suggesting that the visual system exploits geometric regularities when determining whether features are binocularly or monocularly visible. We investigated the hypothesis that natural depth edges contain not just geometric regularities, but also luminance regularities between neighboring monocularly and binocularly visible regions. We sampled a large set of stereoscopic image patches containing depth edges from a natural image database with co-registered distance measurements. Monocular regions in these patches tended to belong to the background and to share visual features with the adjacent binocular background, but not with the adjacent binocular foreground. In a perceptual experiment, observers rated the visual stability of each patch, which was used as a measure for lack of interocular competition. Stability was well predicted by the magnitude of the depth discontinuity and, more interestingly, by the magnitude of the luminance change across the edge. A large luminance change where the monocular region adjoined the binocular foreground predicted higher stability, whereas a large change where the monocular region adjoined the binocular background predicted lower stability. In other words, perception was more stable when the visual context was statistically likely. Together, these results suggest that the visual system uses ecologically-valid assumptions in determining whether features are monocularly or binocularly visible. This strategy may reduce interocular competition during natural vision and facilitate stable perception of depth edges.