Flicker-defined form (FDF) is a temporally driven illusion within which randomly positioned background elements are flickered in counterphase to stimulus elements, creating the illusion of a contour in the region between the background and the stimulus dots. It has been proposed that FDF is dependent on the boundary region between the counterphase flickering dots. Is the stimulus area or the illusory contour itself (region between stimulus and background) paramount to the FDF percept? Circular stimuli were compared to ring stimuli to determine the relative importance of area and contour. The rings were tested in the following configurations: constant maximum diameter/variable area; constant area/variable contour; and constant contour/variable area. For rings with constant diameter, no effect of ring thickness was found. No effect of contour was found for rings of a constant area. For rings of constant contour, the smaller the area the greater the threshold. These results suggest a greater dependence on the area of a stimulus rather than its contour. Area dependence suggests that the theory of contour dependence by a fast extraction system is unlikely. This temporally defined magnocellular-dependent illusion is influenced by slow surface perception mechanisms of the parvocellular system.