Wertheimer-Koffka Ring

from Michael’s   Visual Phenomena & Optical Illusions

Warning: this is a subtle effect.

Compare the two halves of the ring. The left half looks subtly darker than the right one. Now move the mouse over them (or tap)…

You may need to change your observation distance to see a sizable effect.

Comment

In the presence of the bar, the brightness difference without luminance difference is a standard case of simultaneous contrast. This is probably due to retinal mechanisms. Without the bar, filling-in (a cortical phenomenon) ‘wins’.

Sources

Koffka (1935) Principles of Gestalt Psychology. Lund Humphries, London [Chapter 1]

Demo SWF file from Ted Adelson’s site