p159fig04.35 Spatial impenetrability prevents grouping between the pac-men figures in the left figure, but not in the figure on the right.<BR>|| p158c2h0.75 "... In the image shown in the left panel, the horizontal boundaries of the background squares interfere with vertical boundary completion by vertically-oriented bipole cells, again by spatial impenetrability.  In contrast, the vertical boundaries of the background squares are collinear with the vertical pac-man inducers, thereby supporting formation of the square boundaries. Finer aspects of these percepts, such as why the square ... (right panel) appears to lie in front of four partially occuded circular discs, as regularly occurs when the Kanizsa square can form (eg <A HREF="http://www.BillHowell.ca/ProjMini/TrNNs_ART/images- Grossberg 2021/p156fig04.33 emergent boundary groupings can segregate textured regions.png">Figure 3.3</a>), can be understood using FACADE theory mechanism that will shown below to explain many figure-ground percepts using natural extensions to the three dimensional world of boundary and and surface mechanisms that we have already discussed. ..."