Depth without disparity in random-dot stereograms

Depth can be perceived in random-dot stereograms in which dots are binocularly uncorrelated, in the absence of overall positional disparity (e.g., Julesz, 1960). This phenomenon, which we have calledrivaldepth, persists over a wide range of luminances, stereogram dot densities, and dot sizes. Rivaldepth is also observed with interocular uncorrelation provided by complementation and orthogonal rival contours, and in both static and dynamic random-dot stereograms. Like conventional stereopsis, rivaldepth magnitude grows with increasing viewing distance, but unlike conventional stereopsis its direction does not reverse when half-images are interchanged. In a survey of 52 individuals, about half consistently reported far rivaldepth; they were also more likely to exhibit overconvergence on a test of fixation disparity. The remaining subjects reported near rivaldepth and tended to show underconvergences. A second experiment, however, showed that eye movements measured immediately after offset of rivaldepth stereograms were in the opposite direction (e.g., convergences were associated with near depth). Evidently, once rivaldepth is established, subjects attempt to fuse the “object” projecting the uncorrelated images. Rivaldepth magnitude is similar to depth produced by stereograms with disparities too large to be fused. The pattern of eye movements and rivaldepth magnitude suggest that the binocular visual system treats uncorrelation as a potentially correlated disparity beyond the fusion limit.