It is only now beginning to be studied. This last is perhaps the most crucial feature of environmental layout for, unlike the others, it involves optical occlusion and yields the seeming paradox of perceiving one thing behind another, as noted above. Continuous gradients of density or parallax do not hold edges. The four puzzles of sensationless perception were all said to be cases of the continuity or coherence of a surface that is partly occluded . This problem, however, is inseparable from that of the perception of a surface that is temporarily occluded.
Partial occlusions are always temporary in life.
To put it another way, the continuity of a layout extended in space implies the continuity of existence of the layout in time. Rigid extendedness and rigid persistence go together. The outcome of reversible transformations for Case 1 has been studied by Michotte in the tunnel phenomenon . The outcome of reversible transformations for the cases has not yet been studied. This is the question.