From: The World In Your Head by Steven Lehar
Fig. 4.6 (A) The three-dimensional field of influence due to a two-dimensional Y vertex projected into the depth dimension of the volumetric matrix. (B) Each field of influence—for example, the one due to the vertical edge—stimulates a folded surface percept. The folded surface intersects the other fields of influence due to the other two edges, thereby tending to produce a percept of a single corner. (C) One of many possible emergent surface percepts in response to that stimulus, in the form of a convex trihedral surface percept. (D) The percept can also be of a concave trihedral corner, as seen sometimes at the center of this bistable figure.