Warning: this is somewhat subtle.
What to do & see
On the right there is a simplified version of the painting “Nu bleu II” by Henri Matisse. The original is a medium blue on a light yellow background. Here, the background brightness slowly oscillates and passes a point where the luminance of the background equals that of the figure.
When the brightness of fore- and background is identical, the gestalt becomes ill-defined, it loses cohesion. The exact point of this so-called ‘isoluminance’ or ‘equiluminance’ depends strongly on your visual display unit and on your eyes.
You can narrow down the effect by manually adjusting the brightness with the slider. Also you can try different background colours – for instance the same hue as the figure.
I dimly recall the first demonstration I saw of gestalt perception breaking down at equilumiance was by Richard Gregory.
Thanks to Geoff Arden for many inspirations, and namely pointing me to this picture to demonstrate the equiluminance effect.