Defining the limits of flicker defined form: effect of stimulus size, eccentricity and number of random dots

We investigated the spatio-temporal limits of flicker defined form (FDF) phase contrast thresholds generated from the phantom contour illusion. Random dots (diameter 0.25 degrees, tapered edge) were used throughout the test field. FDF was generated using circular stimuli (temporal frequency 30 Hz, mean background luminance 50 cdm(-2)), the edges being defined by illusory borders generated from the out-of-phase dots within the display. Thresholds improved with increasing stimulus size and number of random dots at all eccentricities. For a constant threshold, fewer random dots were required with increasing eccentricity. Predictive mathematical relationships between contrast threshold, stimulus size and random dot number are discussed.