Early observations by Leonardo da Vinci (c. 1508) noted that the two eyes can see different parts of the background at the edges of occluding surfaces. This is illustrated in Leonardo's drawing (Figure 3.1) and for two cases in Figure 3.2. In Figure 3.2a, an occluder hides the dotted region of background from both eyes, but there is a region on the right which only the right eye can see and a region on the left which only the left eye can see. Figure 3.2b shows a similar effect of looking through an aperture. In this case, a region on the left of the background seen through the aperture is visible to the right eye and vice versa. It is only since the early 1990s or so that there has been any serious investigation of the perceptual effects of such monocular occlusions, and a whole new set of binocular phenomena, involving the interaction of binocular and monocular elements in determining spatial layout, have been demonstrated and investigated (Harris and Wilcox, 2009). In this chapter, I shall concentrate on four novel phenomena that exemplify different ways in which unpaired regions influence binocular spatial layout.

1. (1) Da Vinci stereopsis. This will be defined as the perception of monocular targets in depth behind (or camouflaged against) a binocular surface according to constraints such as those shown in Figure 3.2.

2. (2) Monocular-gap stereopsis. In this case, monocular regions of background influence the perceived depth of binocular surfaces.

3. […]