Evidence for relative disparity matching in the perception of an ambiguous stereogram

To compute depth from binocular disparity, the visual system must correctly link corresponding points between two images, given multiple possible correspondences. Typically, model solutions to this problem use some form of local spatial smoothing, with many physiologically inspired models doing so implicitly, through the use of local cross-correlation-like procedures. In this paper we show that implicit smoothing, without the explicit consideration of relative disparity, cannot account for biases in the perception of a novel ambiguous stereo stimulus. Observers viewed a stereogram consisting of multiple strips of periodic random-dot patterns, perceived as either a slanted surface, or a triangular wedge in depth, and reported their perception in a 4AFC task. Biases in the perception of this stimulus are shown to depend upon the stimulus configuration in its entirety, and cannot be accounted for by low-level preferences for disparity sign. Such results are not consistent with local smoothing effects arising solely at the level of cross-correlation-like absolute disparity detectors. Instead, our results suggest the presence of smoothing constraints that consider the differences in disparity between neighboring image regions. These results further suggest that such smoothing generally biases matching toward solutions that minimize relative disparity, regardless of the presence of changes in disparity sign.