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.
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’.
Koffka (1935) Principles of Gestalt Psychology. Lund Humphries, London [Chapter 1]
Demo SWF file from Ted Adelson’s site