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D. Goren, P.T. Quaid, J.G. Flanagan; Perceived Spatial Frequency of a Temporally Defined Illusion: Flicker Defined Form (FDF) . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5659.
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Purpose: To determine the perceived spatial frequency (SF) of the illusory border created by flicker defined form (FDF). FDF, also known as the phantom contour illusion, is created by flickering two counterphase regions of random dots. The resulting illusory boundary is dependent on contrast, the number of random dots/degree, eccentricity and stimulus size (Quaid & Flanagan, 2004; in press). Methods: Two monitors were positioned perpendicular to one another at equal distances to the subject (48cm). The first, directly in front of the subject, displayed the FDF stimulus. The second displayed a suprathreshold sine wave grating which was superimposed on the first screen using a 50:50 beam splitter. The grating was positioned below the illusory border in the case of foveal testing, and at the centre of the screen for peripheral testing. FDF was created using random dots (0.25º diameter) flickering at 30Hz. Subjects viewed the stimulus monocularly, for an unlimited time. Using a method of adjustment, subjects were instructed to indicate which of five SFs most closely matched the border. This was repeated three times, each time the step size was reduced. The contrast of the random dots, dot number and eccentricity were manipulated. Results: At a suprathreshold contrast (0.401 log Michelson contrast %) and an average dot density of 3.5 dots/degree, subjects perceived the border to be approximately 1 cycle/degree. Contrast of the stimulus had minimal effect on the perceived SF. At the fovea, an increase in the number of random dots/degree caused a linear increase in the perceived SF. For dot densities of 3.5 dots/degree, increasing eccentricity lowered the perceived SF. Conclusions: Increases in the number of dots per degree caused a perceptual shift to higher SFs. Increasing eccentricity decreased the perceived SF of the illusory contour.
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