Masked figures are hidden behind an irrelevant ground.
(Please view the illustrations at the top of the page in the graphic version.)
Above, in addition to the famous masked figure, is also a less dramatic but more germane figure-ground task. A screen is placed before the item. For you and I, the screen makes us work, perhaps a little harder and it may take a fraction of a second longer to identify the figure (a vacuum). But it is still easy to ignore the screen (ground) and concentrate on the significant lines.
But for individuals with figure-ground discrimination problems, this could be an almost impossible task. They would look as much at the hash lines as the figure itself. The result is just a confusing meaningless morass of lines.