Suppression of false binocular matches by global contour fusion in stereopsis

Purpose: Vertical contours in random-dot stereograms usually have an irregular 'ragged' shape from falsely fused elements. We compared the appearance of these local false matches in dot stereograms with - and without monocularly visible global vertical contours. Methods: Subjects viewed random-dot stereograms featuring vertical objects of a limited width (<3 elements), which are known to have a high probability of false matches. Vertical contours of adjacent occluding objects were either monocularly or only binocularly visible. In a 2nd set of stereograms with a repeating pattern, local vertical edges were compressed (increased x/y ratio of the rectangular elements), resulting in gradually more distinct subjective vertical contours. Effect of global contour correspondence on local disparity coding was studied. Results: Stable fusion failed for the stereograms with only binocularly visible contours; global objects were not observed, while local false matches resulted in elements at oblique orientations and at irregular distances from the observer. Addition of monocularly visible object contours restored global correspondence; Subjects reported a regular smooth shape of the vertical contours and enclosed surfaces. In the 2nd set of stereograms global vertical contours were preferentially matched, suppressing the local correspondence of adjacent individual elements and edges. Conclusion: Global contour correspondence prevents false binocular matches by overruling the weaker incidental correspondence of monocular elements and local edges near occluding contours.