Flicker Augmented Contrast
With Alan Ho
A gray cross looks apparently dark gray on a white surround, and light gray on a white surround. But a black/white flickering cross looks almost black on a white surround, and almost white on a black surround.
Reason: The visual system selects the phase that has the higher contrast on the surround, and ignores the other phase.
A spatially uniform bar looks apparently graded from light gray to dark gray on the graded surround. But a black/white flickering bar looks almost white at one end and almost black at the other end.
Two upper crosses are identical, flickering between blue & yellow. Black surround emphasises the light yellow, and white surround emphasises the dark blue, so the two crosses look different colors. Same is true for two green/magenta flickering crosses below.