An image such as the duck-rabbit, Mach illusion, Necker cube, reversible goblet, Schröder staircase, Scripture's blocks, or young girl/old woman figure, the perception of which appears to oscillate or flip involuntarily between (usually) two alternatives, one perception or interpretation completely excluding the other while it persists. Although such figures are not strictly illusory, they are almost invariably included in discussions of visual illusions. Also called an oscillating figure or a reversible figure. See also figure-ground reversal, perspective reversal, top-down processing, visual illusion. Compare impossible figure.