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D. Goren, J.G. Flanagan; Is It Really the Edge in Flicker Defined Form? . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5406.
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© ARVO (1962-2015); The Authors (2016-present)
Flicker defined form (FDF) is an illusory contour defined by high temporal modulations of luminance across boundaries. Whether it truly is the contour or the patches of counterphase flickering dots that creates the illusion is unknown.
Subjects were seated 32cm from a monitor which displayed the 5° circular FDF target. The stimulus was composed of 0.34° randomly positioned dots. Dots inside and outside of the patch were modulated in luminance according to 16.7Hz square wave flicker. A simple detection task was applied, along with a 4–2–1 staircase procedure to determine a phase contrast threshold. Four eccentricities were tested (0°, 4°, 13°, 21°). To determine the effect of contour versus patch three separate experiments were performed. The first used a common diameter and adjusted the proportion of the circle which was in counterphase to the background. The second used a common area, and adjusted both the diameter of the ring and the amount of contour presented. The third used a common amount of contour, and adjusted both the diameter of the ring and the amount of area which was in counterphase to the background.
Adjusting the amount of the circle which flickers in counterphase affects phase contrast thresholds. Thicker rings create lower thresholds, particularly the smallest ring, which differs up to 3 fold from all other ring sizes when at fixation. No effect of contour was found provided area was kept constant. Changing the area of the target while maintaining contour shows decreasing thresholds for larger areas up to 10 fold differences.
Reducing the amount of area which flickers out of phase degrades perception of the illusory border. Reducing or increasing the amount of contour, which means more area where there is a spatial difference in phase, has no effect on perception of the illusion. These results suggest that although the presence of a phase difference at the border is important to perception of the illusion, it is not the key to increasing its salience. This lends favor to the theory that the area within the patch is the greatest contributor to perception of this illusion.
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