This is true, and so also does Biederman's Geon Theory consider a 3-D representation. And yet in both of these models the 3-D is represented exclusively in the abstract, each component geon or generalized cylinder being expressed NOT as a reified surface or volume, but as a set of abstract parametric variables, as suggested in Figure 1 (A). So what is missing both from Marr's and Biederman's models is a reified spatial representation, a representation encoding simultaneously every point on every perceived surface in depth, as suggested pictorially in Figure 1 (B) (to be imagined as a solid spatial percept). This is clearly evident in the subjective experience of perception, whose informational content resembles a 3-D rendition of Figure 1 (B) more than the abstractions in Figure 1 (A). The fact that this reviewer remained unclear on this central point highlights the difficulty of explaining exactly what is meant by the notion of spatial reification.

Figure 1.