If you cross free fuse the left to columns (or divergently fuse the right two columns), you will perceive illusory windows that appear to cut around the top and bottom of the figure. These arise from the vertical offsets of some of the lines in the two eyes, which the visual system interprets as arising from occluding surfaces. The first report of this was in my 1994 Nature paper (no pdf yet), and a more detailed account of these phenomena was presented here. A quantitative model and data of these phenomena were presented here.