A depth illusion seen differently by different viewers

Since almost all visual illusions seem to represent faults in the human visual system that affect all normal viewers similarly, it is surprising that the illusion seen below should produce stable diametrically opposed perceptions in different viewers. The figure consists simply of a blue background onto which randomly placed small circles are scattered rather densely. In the middle of the resulting texture a lighter circle (this time with a yellow background) is placed. About 70% of viewers see the light circle as floating above the background. Another 20% see the reverse depth effect: the light circle appears as a hole cut into the darker background. A final 10% get no perception of depth at all.

The illusion intensifies as one starts to move back from the screen. At a distance of roughly 6 feet the hole perceived (if a hole rather than a raised circle is seen) can seem to be almost an inch deep. Similarly, a cursor moving over the textured background can seem to be floating slightly above (or sunken slightly into) it.

Other combinations of texture and color produce the same illusion,but the texture shown seems to bring it out particularly strongly.