If you cross fuse the two left images (or diverently fuse the right two), you will see 5 black discs occluded by small, unconnected white surfaces (shown schematically in (a) below). However, if you cross fuse the two left images (or diverently fuse the right two), a connected, irregularly shaped black object is visible through 5 portholes (shown schematically in (b) below). The two images are indentical in both stereopairs, all that is changed is the eye that receives a given image (thereby inverting the disparity relationships). This has significant impact on theories of visual completion, and an paper on this topic can be found here.