Figure 2.

Contrast recognition. (a) We perceive squares properly as black or white whether in or out of the shadow, but the circled squares are actually of equal brightness (see above the puzzle). (b) The center region looks lighter than the sides because of the sharp light-dark junctions. If you cover one or another of the junctions, you will see that the cores of the regions are identical. Image details: (a) © M F Marmor. (b) M F Marmor, from reference 2.