Dual percept of movement and spatial periodicity in stroboscopically illuminated moving noise patterns

A new phenomenon is reported: stroboscopically displayed moving spatial noise patterns are subjectively described as a superposition of a smoothly moving noise pattern and a somewhat irregular grating. The periodicity of the grating depends on the displacement of the noise pattern between two successive stroboscopic presentations. The detection threshold for the ghost grating is similar to that for stationary sine-wave gratings. The visual system must integrate the stimulus for at least 0.5--1 s in order for such an illusory grating to appear. Yet the subjectively smoothly moving noise pattern looks sharp.