Vernier acuity of illusory contours defined by motion.

We present here a series of experiments exploring a special class of visual completion that is strictly tied to the perception of apparent motion. The stimuli consist of sparse random-dot arrays, in which dots remain in place. Changes of luminance or color of the dots at leading and trailing edges of an apparently moving region are integrated over space and time to produce the perception of well-defined contours, shapes, and color. We test how Vernier acuity of apparent motion-defined illusory bars depends on speed, density, and stimulus configurations. We found that higher speed of apparent motion reduces the Vernier acuity thresholds. These thresholds also decrease with increasing density of dots, whose luminance changes provide the apparent motion signal required for the perception of illusory contours. In subsequent experiments, we showed that luminance-defined flankers could seamlessly integrate with and improve the perception of apparent motion-defined contours, reducing their Vernier thresholds.