Figure 4.

(A) The hexagon on the left is clearly visible. The very same hexagon is hard to find in the shape in the middle because adding further elements leads to a very different perceptual organization. A simpler overall shape is perceived and the hexagon, as a sub-shape, is rendered invisible (solution on the right, shape in red). Adopted from Aydin et al. (2011). (B) Either a face or a vase is perceived. It is impossible to see both interpretations at the same time (Rubin, 1915). (C) The retina codes the external world by a 2-D representation. Hence, the brain needs to infer the third dimension. For example, there are infinitely many pens (only two are shown) that give rise to the same retinal image. To infer which pen is presented, the human brain needs to estimate two unknowns, namely the size and distance of the pen. This is an ill-posed problem, i.e., there is no unique mathematical solution, since only one value is available, namely the retinal size of the projection of the pen.