Simultaneous Contrast: White's Illusion, V-variant

The applet illustrates what is known as White's illusion. The gray rectangle are of the same shade. The ones that appear lighter are drawn within the darker bands. The interesting observation is that the same illusion appears even when their longer sides are in the contact with the lighter bands. Somehow the background seems to stretch from left to right rather than vertically. The present configuration is a modification of the original one where the parallel dark and light bands are rather horizontal. The effect seems to persist.

Edward Adelson gives the following explanation for the horizontal configuration: "This effect has been interpreted in terms of the T-junctions. Patches straddling the stem of a T are grouped together for the lightness computation, and the cross-bar of the T serves as an atmospheric boundary. ... the action of T-junctions can be so strong that it overpowers traditional grouping cues such as coplanarity. Therefore the grouping rules for the lightness computation evidently differ from those underlying subjective belongingness."

It appears to me that bringing up the T-junction consideration is irrelevant to the present, vertical configuration What works here behind the scenes, is the perception of a single item occluded by the longer and darker bands that lengthwise consists of 6 rectangles, not 3. The lightness of the perceived bar (i.e. the stem of a T previously) is obtained by averaging the visible parts and the occluding pieces. The cross-bar of a T does not appear to play any role in the effect.

References